NOTES/SOLVED EXERCISES / Key book/Guide: GENERAL SCIENCE-7 PTB PUNJAB TEXT BOOK BOARD LAHORE PAKISTAN 2017
NOTES/SOLVED EXERCISES: GENERAL SCIENCE-7 PTB PUNJABTEXT BOOK BOARD LAHORE PAKISTAN 2015-16 (new course)
CH:1 HUMAN ORGAN SYSTEMS P:3-13
3- GIVE SHORT ANSWERS:
QUESTION No. i- How does carbon dioxide produce in our
ANSWER: Production of carbon dioxide: Carbon
dioxide is the waste gas that is produced
when carbon is combined with oxygen as
part of the body’s energy-making process
QUESTION No. ii. How is small intestine important in our
ANSWER: Importance of Small Intestine: Small
intestine is important in our digestive system
because final digestion of carbohydrates,
fats and proteins occur here.
QUESTION No. iii. Why are we provided with teeth?
ANSWER: We are provided with the teeth because
our teeth are the main tools we use to break
own food in the mouth.
QUESTION No. iv. What are alveoli?
ANSWER: Alveoli: Alveoli are tiny sacs in lungs
where gases exchange takes place.
QUESTION No. v. Briefly describe the mechanism of breathing?
ANSWER. Mechanism of breathing:
Two types of
muscles work during the breathing process;
the intercostal muscles of ribs and the
dome-shaped diaphragm. The process of
breathing is completed in two steps:
1) inhaling: When the inter costal muscles
pull our ribs outward and the diaphragm
contracts the air enters the lungs. It is called
2) Exhaling: When the inter costal muscles
and diaphragm muscles relax, the air moves
out of the lungs. It is called exhaling
QUESTION No. vi. What measures can one take to prevent
ANSWER: Measures -to prevent diarrhea: We can
prevent diarrhea by following these
measures. Always wash hands with soap
after using the toilet. Wash all fruit and
vegetables before cooking or eating.
Do not eat uncooked meat and eggs.
QUESTION No. 4. Explain the process of digestion of food in the mouth and the stomach?
ANSWER: Process of digestion of food in Mouth:
The process of digestion be inside mouth. Our teeth break the food into small pieces by cutting and grinding The tongue mixes food with saliva which is secreted by salivary glands. Saliva starts the digestion of carbohydrates (starch and sugar).
After sometime the food in the mouth becomes soft and moist. The tongue pushes this food if to the back of our mouth.
Process of digestion of food in Stomach:
Our stomach is a large J-shaped muscular bag. It mixes the food with digestive juice.
The digestive juice-begins the digestion of proteins (meat, egg, milk, pulses, etc.). The
food spends about four hours in the stomach. The digestive juice in the stomach also contains an acid. The acid kills the germs present in our food. lt also helps in the digestion of proteins.
QUESTION No. 5. Describe the human respiratory system.
Answer: Respiration: it is the process by which living organisms use oxygen of air and food
produce energy. Carbon dioxide is also produced during this process.
Respiratory System: The parts of body used in the process of breathing form the respiratory system.
Parts of Respiratory System: Our respiratory system consists of the nose and throat, the
wind pipe (trachea), the breathing muscles and the lungs.
Nose and Throat: The air enters through our nose or mouth. Our nose has hair and
mucous to clean, moisten and warm the air.
Mucous is a sticky liquid. Dust particles and germs present in the air stick to the mucous.
The air enters the throat and passes through the larynx. Our vocal cords are present in the larynx to produce sound.
Our throat contains two pipes. One for the food and the other for breathing. It is the
Epiglottis which allows things to go down the right way. When we eat or drink something, the epiglottis ‘covers the windpipe.
Trachea (Windpipe): Air passes from the into the trachea or windpipe.. Our windpipe is made of C-shaped rings of cartilage. These rings keep our windpipe open. Mucous and tiny hairs in the trachea also filter the air;
Bronchi and Lungs: The trachea divides into two branches called bronchi (singular
bronchus). Bronchi carry air into the lungs.
Our lungs are the most important organs of the respiratory system. ln each lung, the bronchus divides into smaller tubes called
At the end of each bronchiole, tiny air sacs called alveoli are present.
Q:6 WRITE A NOTE ON THE FOLLOWING
Being constipated means your bowel movements are tough or happen less often than normal. Almost everyone goes through it sooner or later.
Although it’s not usually serious, you’ll feel much better when your body is back on track.
The normal length of time between bowel movements varies widely from person to person. Some people have them three times a day. Others have them only once or twice a week.
Going longer than 3 or more days without one, though, is usually too long. After 3 days, the stool or feces become harder and more difficult to pass.
Pneumonia is a bacterial or viral infection of the lungs. Symptoms can include fever, chills, shortness of breath, coughing that produces phlegm, and chest pain. Pneumonia can usually be treated successfully at home with antibiotics but some cases may require hospitalization and can result in death. Vaccines are available against some of the more common infectious agents that cause pneumonia.
CH:2 TRANSPORT IN HUMAN AND PLANTS P: 14-24
3- GIVE SHORT ANSWERS:
QUESTION No. i. What do you mean by oxygenated blood?
ANSWER: Oxygenated blood: Oxygenated blood is the blood in which oxygen is added to the hemoglobin molecules within the red blood cells in ide the lungs.
QUESTION No.ii. What is the estimated size of our heart?
ANSWER: Estimated size of our heart: The estimated size of our heart is about the size of our fist.
QUESTION No. iii. Which arteries carry deoxygenated blood from heart to the lungs?
ANSWER: Pulmonary arteries carry deoxygenated
food from heart to the lungs.
QUESTION No. iv. Name at least two diseases that can damage our kidneys.
ANSWER: Two diseases: High blood pressure and diabetes can damage our kidneys.
QUESTION No.v. Which tissue does transport water in plants?
ANSWER: Xylem tissue transports water in plants.
QUESTION No. vi. Name three parts of the circulatory system.
ANSWER: Parts of circulatory system: These are the parts of circulatory system:
- Blood Vessels
QUESTION No. vii. Explain why it is important to circulate blood through the body.
ANSWER: lt is important to circulate blood through the body because the food we eat is carried through the blood in our whole body. Oxygen which enters in our body during respiration is also carried through the blood.
QUESTION No.viii. How many chambers does the human heart have? Give names.
ANSWER: Number of Chambers: There are four chambers in the human heart; two upper chambers called atria (singular atrium) and two lower chambers called ventricles.
QUESTION No. 4. Describe the structure of human heart.
ANSWER: Structure of human heart: There are four chambers in heart; two upper chambers called atria (singular atrium) and two lower
chambers called ventricles.
The ventricles of heart are larger than the atria. Both atria contract at the same time, and so the ventricles. The blood passes from
the atria into the ventricles. There is a valve keep the blood flowing in one direction.
Deoxygenated blood from the body enters the rig t atrium and oxygenated blood from the lungs enters the left atrium of our heart.
The right ventricle pushes the blood to the lungs and the left ventricle pushes the blood to the body.
QUESTION No. 5. Compare the structures and functions of blood vessels.
ANSWER: Blood Vessels: The blood travels throughout the body through blood vessels.
Types of Blood Vessels:
The three types of blood vessels are:
Arteries: Arteries are the blood vessels that carry blood away from the heart. Arteries have thick and flexible walls. Most of the arteries carry oxygenated blood but pulmonary arteries car deoxygenated blood to the lungs. Arteries divide many times to smaller tubes, called capillaries
Capillaries: Capillaries are the smallest blood vessels in the body. They are so small that red blood cells flow through them one cell at a time. Food and oxygen from the blood of capillaries diffuse into the cells.
Waste materials and carbon dioxide from the cells diffuse into the blood of capillaries.
Capillaries again join to form the larger blood vessels called veins.
Veins: Veins are the blood vessels that bring blood back to the heart. Most of the veins bring deoxygenated blood black to the heart, but pulmonary veins bring oxygenated blood from the lungs to the heart.
QUESTION No. 6. Describe that structure of a plant’s root permits the movement of materials.
ANSWER: Structure of Root: From the moment a seed grows, its root starts to search water and minerals in the soil. Roots have branches that play an important role in absorption of water. The root hairs on the roots absorb water and minerals from the soil. Roots have xylem tissues to move water and minerals from the soil up through the stems, to the leaves. Phloem tissues in roots to help to transport food.
- Label the diagram.
CH:3 REPRODUCTION IN PLANTS P:25-33
3- GIVE SHORT ANSWERS:.
QUESTION No. i. Differentiate between asexual and sexual reproduction.
ANSWER: Difference between asexual and sexual reproduction: The type of reproduction in which a cell from only one parent develops; info offspring is called asexual reproduction
whereas, when two gametes one from each parent combine to form a zygote, the process is called sexual reproduction.
QUESTION No. ii. What are the male and female parts of a flower?
ANSWER: Male Parts: Stamens are the male pa of a flower.
Female Parts: Carpels are the female parts of a flower.
QUESTION No. iii. Define pollination.
ANSWER: Pollination: The transfer of pollen grains from the anther of a flower to the stigma of the carpel called pollination.
QUESTION No. iv. Name a few pollinators.
ANSWER: Pollinators: Wind, water, insects, birds and bats, etc. are a few pollinators.
QUESTION No. v. What is a sperm and an egg?
ANSWER: Sperm: Male gametes or male sex cell is called sperm.
Egg: Female gametes or female sex cell IS called egg.
QUESTION No. vi. Which part of a seed develops into the first root?
ANSWER. Radicle of the embryo develops into the first root of the new plan .
QUESTION No. 4. Write a detailed note on pollination in plants.
ANSWER: Pollination: The transfer of pollen grains from the anther of a flower to the
stigma of the carpel is called pollination. With the help of this process, the male sex
cell (sperm)’reaches to the female sex cell (egg). Sex cells are also called gametes.
Agents For Pollination: Wind, insects, animals and water are the agents for pollination in different plants.
Kinds of Pollination: There are two kinds of pollination:
- Self-pollination ii. Cross-pollination
(i) Self pollination: The transfer of pollen grains from the anther to the stigma‘ of the
same flower or another flower on the same plant is called self-pollinated. Pea, tomato,
rice plants, etc. are self-pollinated.
(ii) Cross-Pollination: The transfer of pollen grains from the anther of a flower to the stigma of a flower on another plant of the same kind is called cross-pollination. Poplar, willow, apple, papaya trees, etc. are cross-pollinated plants. For cross-pollination, the plants must grow
flowers at the same time. Cross-pollinated usually happens in plants near each other.
Cross-pollination produces stronger plants
As compared to the self-pollination.
Some flowers have special features that favour cross-pollination, e.g. coloured‘
petals, long and sticky stigmas, nectar and‘ fragrance.
QUESTION No. 5. Explain fertilization in plants.
ANSWER: Fertilization in Plants: The surface of the stigma in a flower is sticky and pollen; grains stick to it. Here, a pollen tube grows out from each pollen grain. Two sperms are ;
present in this pollen tube. The tube grown downward through the style and enters the
ovary. Pollen tube finally enters an ovule and ; releases its sperms in it. One of the sperms
combines with the egg to form zygote. The other sperm combines with another cell to
make the store of food.
Fertilization: The process of fusion of sperm with the egg is called fertilization. 1
The male gamete (sperm) in the pollen grain combines with the female gamete (egg) in
Changes after Fertilization: After fertilization, several changes take place in the flower. The 1,
sepals, petals and stamens dry up and fall off. The fertilized egg inside the ovary
develops into embryo. Ovules become‘ seeds. The ovary grows large and develops T.
into a fruit. The fruit protects the seed or seeds.
QUESTION No. 6. Describe the structure of a seed.
ANSWER: Structure of Seeds: After fertilization, an ovule becomes a seed. The embryo and
its store of food are covered by a tough seed coat. The most important part of a seed is its
embryo. Embryo grows into anew plant. Parts of Embryo: The embryo consists of the
Radicle: This part of the embryo develops into the first root of the new plant.
Plumule: This part of the embryo develops into the first shoot (stem) of the new plant.
Cotyledons: This part of the embryo supplied food to the growing young plant.
QUESTION No. 7. Write a note on fruit.
ANSWER: Fruit: The ripened ovary is called a fruit.
The ovary wall forms the fruit wall, called the pericarp lnside the ovary, ovules develop
Into seed the matured fruit may contain single or many seeds
The pericarp has three layers in most fruits like peaches and mangoes. The outer layer is skin, the middle layer is fleshy and the inner layer is tough or hand. Some fruits have hard and dry pericarp, nuts.
CH: 4 ENVIRONMENT AND FEEDING RELATIONSHIPS P:34-44
- GIVE SHORT ANSWERS.
QUESTION No. i. What kinds of organisms are at the start of most food chains?
answer. Producers are at the start of most food chains.
QUESTION No. ii. Name biotic factors of an ecosystem.
ANSWER: (i) Plants Animals
(iii) Fungi (iv) Microorganisms
QUESTION No. iii. How are producers, consumers and decomposers related to each other?
ANSWER: Producers produce their own food.
Consumers use that food which is produced by producers and decomposers decompose the producers and consumers.
QUESTION No. iv. Define an ecosystem.
ANSWER: Ecosystem: A system formed by the interaction of living organisms and non-living things in an environment is called an ecosystem.
QUESTION No. v. What do you mean by community in an; ecosystem?
ANSWER: Community: All the populations of different kinds of organisms living together in an area make a community.
QUESTION No. vi. Name the ways by which we can save our natural resources;
ANSWER: Reducing, reusing and recycling are they ways to save natural resources.
QUESTION No. 4. What is a habitat? Describe its few kinds.
ANSWER: Habitat: The place w ere an animal or plant lives and reproduces is called its; habitat. A habitat provides the things an.
organism need, i.e. food, water, shelter, etc.
Man); populations of organisms live in each habitat.
Kinds of Habitats Organisms live in di
kinds of habitats. An organism has special . features to live in its habitat.
1) The Grassland Habitat: Grassland is a grassy, windy, partly-dry area. These areas receive a medium amount of rain. The soil found here is very fertile. Grasses are the producers in a grassland habitat. Mostly grazing animals like the sheep, goats, cows, antelopes, buffaloes, and deer are a few examples that are found in a grassland. A few flesh-eaters like cheetahs, foxes,
wolves and few birds like owls, eagles, hawks, etc. are also found in this habitat.
Many kinds of insects are also found in grasslands.
2) The Pond Habitat: A pond is an aquatic habitat which is rich in life. Plants like algae, duckweed, water lily, etc. are found in water.
The animals like fishes, pond skaters, woIf spiders, snails, frogs and microscopic
organisms also found in the pond habitat.
3) The Desert Habitat: Deserts are the driest land areas. They receive very little rainfall.
Rainwater quickly drains away due to the sandy soil. Some plants and animals have adapted to the limited supply of water. Cacti, euphorbia, lizards, snakes, kangaroo rats, camels, etc. are found in a desert habitat.
4) The Rainforest Habitat: Rainforests are always wet. They receive rain the whole
years. A large number of plant types (herbs, shrubs and trees) is found here. Several varieties of butterflies, snakes, lizards, frogs, parrots, cockatoos, humming birds, cats and jaguars are also found in this habitat.
QUESTION No. 5. Describe factors that can bring daily and yearly changes in the habitat.
ANSWER: The Factors Causing Changes Habitat: We know that light, temperature, air, soil and water and abiotic factors of the environment. Changes in these factors bring changes in the populations of a habitat
Some other natural factors and humans also cause changes in habitats are:
- Sunlight ii. Temperature
iii. Water iv. Migration
Sunlight: Sunlight is the basic source energy on the Earth. Plants use light energy to make their own food. All forms of life on the Earth depend directly or indirectly on green plants for food. They also need light for their survival. Light intensity affects he number of plants in a habitat. Increase in i number of plants may result in the decrease of animals number in the habitat.
- Temperature: Temperature can also bring of a habitat. Any extraordinary rise or fall in temperature may b disturb the habitat. For example, warm water contains less oxygen.
iii. Water: Water is essential for life. Where there is more water, more organisms are found there. Availability of water in the habitat can greatly influence its organisms.
- Migration: Migration is another factor that changes the size of populations of habitat.
Where a few individuals come to an area, it increases the size of the population in that area. Organisms migrate in search of better living places.
Natural Disasters: Natural disasters such as; droughts, floods, earthquakes, etc. can bring changes in habitats.
Drought: A drought is a period when there is no rain for a long time in an area. The ponds or streams may dry up during a drought.
Most pond plants die and animals die or more to other ponds. Some crops do not
grow in the area affected by a drought. flood: When an area gets a lot of rain for a
long time, there may be a flood in that area.
Many plants die and many animals die or move to other drier places during a flood.
Lightning: Sometimes lightning strikes a tree in a forest, causing forest fires. Plants and tree is are burned and destroyed. Some animals die, others may move to safer places. it takes many years for a forest to grow back.
EARTHQUAKES: Earthquakes are sudden shocks of the Earth’s surface. Earthquakes
can change a habitat very quickly. On October 8, 2005, a massive earthquake damaged a widespread area across Pakistan. Over 70,000 people lost their lives.
A lar e number of animals and plants were also destroyed.
QUESTION No. 6. Describe adaptations of some aquatic animals to live in their habitat.
ANSWER: Aquatic Habitats; Animals and plants living in -aquatic habitats have such body parts that help them to live in water.
- Streamlined body shape is an important adaptation for animals to move easily
through water. Webbed feet of ducks, seagulls and frogs work likes ores to help move in water. Floating plants like water hyacinth, duckweed and water lettuce have ﬂoating leaves and submerged roots. Their bodies contain air spaces. Their leaves have waxy covering to prevent water from collecting on them.
QUESTION No. 7. Explain a food chain and a food web with examples.
ANSWER: Food Chain: Animals cannot make their own food. They eat plants or other
animals that eat plants. Thus, animals are the consumers. A consumer may be a
primary consumer (herbivore), a secondary consumer or a tertiary consumer. i Organisms eat organisms and are in turn being eaten by others. This feeding relationship among organisms is called a food chain. Most food chains start-with producers like:
grass -> zebra —> lion
eaves -> caterpillar —> bird —> hawk
Green plants are producers so, they are the first in most food chains.
Primary Consumer: Animals that feed on plants are the second in a food chain. They are called primary consumers.
Secondary Consumer:- Animals that eat primary consumers are called secondary
consumers. Secondary consumers may be eaten by tertiary consumers.
Examples of food chains:
- In a grassland habitat a grasshopper eats grass. A snake may eat this grasshopper.
This snake may become the food of a hawk. Are chain while others are consumers.
Food Web: An organism can be part of many food chains. Several food chains in an ecosystem overlap to form a network called food web.
Example 1: A lion does not feed entirely on deer but it also hunts cows and goats.
Similarly, owl and hawk may also take different organisms as their food. So, most
animals feed on one or more than one kind animals. Therefore, many food chains form a kind of network or a food webs.
Example 2: A snake does not feed on frog alone. It also eats birds, rats and even
rabbits. Birds eat grains. They also eat insects, spiders and worms. lf we arrange
food chains in an ecosystem, it takes the form of a web.
Several food chains are presented in a food at the food web and the questions given below.
- Name the producer in the food web.
- From where the producer gets energy?
ANSWER: Producer gets energy from sunlight
iii. Name three consumers in the food web.
ANSWER: Goat Rabbit Rat
- Write down two food chains in this food
2) Seaweed ——)”G0aiI > Jackal > Lion
ANSWER: Two food chains:
1) Seaweed» Rabbit _> Jackal—> Lion
8- Look at the following food web and answer the questions given below
CH: 5 WATER P:46-55
3- GIVE SHORT ANSWERS:
QUESTION No. i Why is fresh water important?
QUESTION No. ii Where is most of the fresh water found?
QUESTION No. iii Why is the most of the Earth’s water salty?
QUESTION No. iv lf water runs downhill to the seas, what are some ways that fresh water becomes polluted?
QUESTION No. v Why clear water is not necessarily safe to drink?
QUESTION No. 4- Explain how water is the most essential part of life?
QUESTION No. 5- What makes our water impure?
QUESTION No. 6- How can we preserve water?
QUESTION No. 7- Describe a few methods to clean water;
QUESTION No. 8- Complete the concept map given below
CH: 6 STRUCTURE OF AN ATOM P:56-67
- GIVE SHORT ANSWERS.
QUESTION No. i. What does give the positive charge to the
nucleus of an atom?
ans: The presence of protons give the
positive charge to the nucleus of an atom.
QUESTION No. ii. Define a cation and an anion?
ANSWER: Cation: When an atom releases it one r
more electrons from the outermost shell, the
number of protons increases. It becomes ta
positive ion or cation.
Anion: When an atom absorbs one or more
electrons in its outermost shell, the number
of electrons increases. It becomes
negative ion or anion.
QUESTION No. iii. What is chemical formula?
ANSWER: Chemical Formula: Describing
molecule in the form of symbols and
valencies is called the chemical formula. The
chemical formula of a molecule shows:
kinds of elements in the molecule
for number of atoms of each element
QUESTION No. iv. List the names, charges and location of
three kinds of particles that make up an
Name fl: Charges ‘Z15 Location fl
Electron Negative Around the nucleus
Proton Positive In the nucleus
M4 =4 tﬁ
Neutron No charge ln the nucleus
QUESTION No. v. Flow are the isotopes of an element alike
and how are they different?
ANSWER: Isotopes of an element alike because
they have the same number of protons, but
they have different number of neutrons.
changing the overall weight of the element.
QUESTION No. vi. A chlorine atom has 17 protons and 18
neutrons. What is its mass number? What is
it atomic number?
ANSWER: Mass number ﬂJl.= Number of protons 1
+ Number of neutrons ;
Mass number /Jl. = 17 +18 l
Mass number /J} = 35 ‘ pf
= Number of protons ﬂpl 735/ p
QUESTION No. vii. Why the electrical charge on an atom
zero, or neutral?
ANSWER: The electrical charge on an atom is zero .
or neutral because in an atom, the number of
protons is equal to the number of electrons.
So the total positive charge of protons‘
balance the total negative charge of
QUESTION No. 4. Describe the structure of an atom.
ANSWER: Structure of an atom: Atoms are made
of even smaller particles called electrons,
protons and neutrons.
Nucleus: The central part of the atom is
called the nucleus. Protons and neutrons are
present in the nucleus.
Electrons: Electrons revolve around the
nucleus. An electron has negative charge. Its
mass is extremely small.
Protron: A protron has positive charge. The
number of protons in an atom is equal to the
number of electrons revolving around the
nucleus. lt has a mass 1837 times greater
than that of electron.
Neutron: A neutron has no charge. This
neutral particle is also found in the nucleus
of an atom. The mass of a neutron is almost
equal to the mass of a proton.
QUESTION No. 5. What is an lion? How ions are formed?
ANSWER: lon: An atom with positive or negative
charge is called an ion. For example, sodium
ion (Na*), chloride ion ((;|- ), oxide ion (Q*),
copper ion (cult ), etc.
Formation of ion: When an atom releases its
one or more electrons from the outermost
shell, the number of protons increases. It
becomes a positive ion or cation. When an
atom absorbs one or more electrons in its
outermost shell, the number of electrons
increases. lt becomes a positive ion or
cation. When an atom absorbs one or more
electrons in its outermost shell, the number:
of electrons increases. it becomes ax
negative ion or anion. Positive Ions and;
negative ions attract each other to form;
QUESTION No. 6. Define the term isotope. Write about the
application of isotopes in the fields of
medicine and agriculture.
ANSWER: Isotope: The atoms of the same element
having same atomic number but different
mass number are called isotopes.
Examples: Hydrogen (H) has three isotopes.
An atom of hydrogen may have zero, one or
Two neutrons in its nucleus. Protium (H),
Deuterium (H) and Tritium (H) are three
isotopes of hydrogen. A
Carbon (c) has three isotopes, i.e.
isotopes are of great important in the
fields of medicine and agriculture.
- Carbon-14 can be used to calculate the age
- Nitrogen-15 can be used to study the
effects of nitrogenous fertilizers in plants.
iii. Sodium-24 can be used to study
circulation of blood.
- Phosphorus-32 can be used in treatment
of blood a cerand one diseases
- Chromium-51 can be used to study red
blot? cells in patients with blood efficiency.
- Iron-59 can be used to study absorption
lofi oni human body.
vii. Cobalt-60 can be used in cancer
iii. Iodine-131 can be used to treat a disease called goiter.
State ‘The Law of Constant Composition
land give examples.
ANSWER: Law of Constant Composition: i
:Composition refers to the make up of a
substance. In the late 1700, a French
“t 1[ scientist Joseph Proust studied the chemical compounds and presented The Law of
Constant Composition. The law states that the composition of a compound is always
or ‘E the same, regardless of how the compound
I)’ lt was made or obtained.
Examples: Water can be obtained from
‘many sources (river, well, sea, etc.), but its
‘; composition is always the same. There are 2
fatoms of hydrogen and 1 atom of oxygen
1 present in a molecule of water (H2O).
- Carbon dioxide (CO2) is produced in a number of ways, but its one molecule always consists of one carbon atom and two oxygen atoms.
QUESTION No. 8. Using the table 6.1, make formulate of the
3- GIVE SHORT ANSWERS:
CH: 7 PHYSICAL AND CHEMICAL CHANGES AND PROCESSES p:68-76
- GIVE SHORT ANSWERS.
- What is meant by a physical change?
ANSWER: Physical Change: A physical change is
lone in which only the physical properties of colour, etc. are the physical properties of a
a substance change and its chemical
composition remains the same. Size, shape,
- Define a chemical change.
ANSWER: Chemical Change: A chemical change
is one in which the physical and the
chemical properties of a substance change.
Question Give an example to show that people
change the environment.
ANSWER: Example: People grow plants change
their environment. Plants provide oxygen
land take in carbon dioxide. The more you
1’ §l plant, the more oxygen you aid inputting
into the air and the more carbon dioxide you
aid in removing from the air.
- What is hydrogenation?
ANSWER: Hydrogenation: The process in which
hydrogen chemically combines with
vegetable oil to change it into fat.
- What is a plastic?
ANSWER: Plastic: A plastic is any material that
can be moulded into any form.
Question 4. Explain with examples that a chemical
change brings change in the properties of a
ANSWER: A chemical change brings change in
the properties of a substance.
Formation of Yogurt from Milk: Yogurt is
formed as a result of bacterial action on
milk. When certain bacterial are added to
warm milk and it is allowed to ferment and
lactic acid is formed. The formation of lactic
acid causes the milk curdle. lt is a chemical
change because a new substance is formed;
Burning of Natural Gas: Natural gas is
composed of methane (CH4). The burning of
natural gas on a stove is an example of
combustion reaction. Methane reacts with
oxygen in the atmosphere to form carbon
dioxide (CO) and water (H20). Since it
involves the formation of new products, it is
a chemical change.
QUESTION No. 5. WRITE BRIEF NOTES ON:
ANSWER: i. Plastics: Plastics are also the result
of chemical changes. A plastic is anyl
material that can be moulded into any form. l
Plastics are very large molecules made from l
many smaller molecules called monomers.
That is why plastics are also called polymers 5
(long molecules ‘made from smaller:
molecules). Monomers are obtained from
crude oil. Polyethylene, polyvinyl chloride,
(PVC), etc. are some examples of plastics. ,
By heating, plastics can be mould into a
number of shapes, we can find it in toys,
cups, bottles, utensils, etc. Plastics do not.
decay. They are a cause of pollution.
Recycling is the best method to manage
- Change of vegetable oil into fat:
A chemical process called hydrogenetion
changes vegetable oil into solid fat (Banaspati ghee). When hydrogen is passed
through vegetable oil in the presence of
nickel, it converts into solid fat. This process
‘is called hydrogenation. Vegetable oil is
liquid while fat (ghee) is solid at room
temperature. A large amount of heat is used
to bring about this chemical change.
Vegetable oil + Hydrogen
Banaspati ghee (fat),
vegetable oil combines with hydrogen and .
onverts into solid fat or ghee
Question 6. HOW ARE FERTILIZERS USEFUL AND HARMFUL FOR
ANSWER: Usefulness of Fertilizers? Farmers use
certain substances to increase the fertility of
the soil. A substance which adds minerals to the soil is called a fertilizer. It may be a natural fertilizers or a chemical fertilizer.
Harmful Effects of improper Use of
Fertilizers: In case of excess spreading,
some fertilizers are not absorbed by the
plants. These fertilizers may reach into
canals and rivers causing water pollution
and encourage the growth of algae. During
the manufacture of chemical fertilizers a lot
of fossil fuel, coal and natural gas is used,
due to which our fuel reserves are reducing
Manure: Waste materials of plants and
animals are called manure. Manure is rich in
nutrients needed by the soil. Chemical
changes in manure increase the production
Question 7. EXPLAIN REVERSIBLE AND IRREVERSIBLE
CHANGES WITH EXAMPLES.
ANSWER: Reversible Changes: A change that can go
forwards or backwards is called a reversible
change. lt is a temporary changer
Examples: We can get the same thing again.
Melting ice into liquid water, switching on
the tube light, increases of heartbeat during
running, mixing of salt in water, wetting a dry
cloth, etc. are reversible changes.
Irreversible Change: A change that cannot
go back is called a irreversible change. It is a
permanent change. We cannot again get the
thing in its original form.
Examples: Turning of milk into yogurt,
mixing of plaster of Paris with water, burning
of paper and wood, rotting egg or fruit. etc.
are examples of irreversible changes.
CH: 8 TRANSMISSION OF HEAT P:77-90
- GIVE SHORT ANSWERS.
Question i. Why do we use cooking pots made of metals?
ANSWER: Metals are good conductors of heat.
When the bottom of the pot is heated, the
heat is conducted through the metal of the
pot into the food inside, cooking it. it means,
heat move through it very quickly and easily
so that we cook food easily.
Question ii. What is a convection current.
ANSWER The upward and downward movement
of molecules of water or air is called a
- Which surfaces absorb maximum heat?
ANSWER: Black surfaces absorb maximum heat.
- Why do we use woolen clothes and
blankets during winter days?
ANSWER: We use woolen clothes and blankets
During winter clays to escape from coolness.
Woolen clothes acts insulator.
- What is the advantage of gliding might for
The advantage of gliding flight for a
bird is that. during a gliding flight a bird does
not move its wings but glides on wind
currents and saves it a lot of energy.
- What is convection? How does it occur?
ANSWER: convection The transfer of heat in
which molecules of a medium actually move
to the source of heat energy to absorb heat
and the move away from it, is called
Occurrence of convection: Convection
occurs in liquids and gasses only because
their molecules can move freely. Their molecules of a solid are held closely together
together. they cannot move freely there ore
convection is not possible in solids.
- Write a few everyday application of
conduction of heat.
ANSWER: everyday Applications of Conduction of
Heat: Conduction plays an important role in
- Cooking utensils, electric kettles, irons,
soldering irons, etc. are mode of metals to
conduct heat quickly. Their handles are
made of plastic or wood which are bad
- Birds have feathers which keep their
bodies warm because feathers bad
conductor of heat.
- Woolen clothes and blankets slow down
the transfer of heat. lt happens so because
the wool traps air in it. The air is a bad
conductor of heat.
- Ice is covered with jute rugs to reduce it
‘melting speed. Jute is a bad conductor of heat
- An insulating material (e.g. syrofoam ) is
filled between the double walls of ‘a
refrigerator. it reduces the transfer of heat
across the walls of refrigerator.
- Double pan windows are used in buildings
to slow the transfer of heat. Air between ‘the
two layers of glass acts as an insulator.
- Thermos bottles use air or a vacuum to
slow the transfer of heat by conduction.
- Write a note on vacuum flask.
ANSWER: The Vacuum Flask: The vacuum flask is
a container which can keep hot things hot
and cold things cold. The vacuum flask
reduces the transfer of heat by conduction, convection and radiation.
;The vacuum flask (thermos flask) is actually
two thin glass or metal bottles, one inside
:;the other. Air between the glass walls is
removed to create vacuum. The vacuum
prevents the transfer of heat by conduction
land convection. The walls of both bottles are
coated with aluminum on the vacuum side.
These silvered (like a mirror) and smooth
glass walls prevent transfer of‘ heat by
radiation. The mouth of the flask is made
from bad conductor such as cork or plastic.
A little amount of heat is lost by conduction
through the mouth. The thin walled glass bottle is protected by keeping it in a metal or plastic container.
- Write brief notes on:
- Ocean currents and winds
- Gliding flight of birds
ANSWER: Ocean currents and winds: We know
that convection is the transfer of heat by the
actual movement the heated material. Winds
land ocean currents are examples of effects
i of convection.
The heat of the Sun heats up the surface of
the Earth and the air near it also gets hot.
The air expands and gets lighter. So, it rises
up leaving its space empty.
,Colder air from the neighbouring regions
moves in to fill this empty space. “The rising
warm air reaches the colder layers of the air
and gets cold. Getting heavier, it sings to the
Earth in cold regions to below again to warm
areas to take the place of the rising air. Thus,
convection currents are setup in nature and
the wind system goes on.
Ocean currents are also set up due to
‘convection of heat. Water of the hot regions of
the oceans gets hot, it expands and gets
lighter, but water in the colder regions remains
cold and heavy. Hot water moves along the‘
surface of the ocean towards the colder;
regions. The cold water flows below the
surface of the ocean towards the hot regions.
ln this way, ocean currents are set up.
Convection and Gliding Flight Birds:
Convection currents also take place in
atmosphere. The heat from the Sun warms
the air near ground. The warm air expands
and becomes lighter in weight. As warm air
rises, colder air rushes in to fill its place
near the ground. This process continues.
Birds like eagles,” hawks, vultures and gulls
take advantage of this phenomenon. hey
enjoy gliding. During gliding flight a bird
does not move its wings, but. glides on air,
currents lot of energy of a bird is saved
CH: 9 DISPERSION OF LIGHT P:91-104
- Give short answers.
- What happens, when light moves from
glass into air at an angle?
ANSWER: When light moves from glass into air at
an angle it bend away from the normal.
- What is refractive index?
ANSWER: The speed of light varies in different
mediums. Some mediums cause light to
bend more than others when it passes
through them. The degree to which a
medium can bend light is given by its.
Refractive index is the ratio of the speed. of
light vacuum to its speed in the medium.
Refractive Index of the medium
Speed of light in vacuum ,
Speed of light in the medium
Question iii How can we calculate the refractive index
ANSWER: Refractive index of water
= Speed of light in vacuum
Speed of light in the water =
Question iv. What happens when the primary colour
of light are mixed in equal proportions?
ANSWER: When primary colour of light are
mixed in equal proportion then secondary
colours are obtained.
- Why do we see colours of different objects?
ANSWER: We see colours of different objects they
reflect specific colours and absorb other
- Define critical angle.
ANSWER: Critical Angle: The angle of incidence
for which the angle of refraction is 90°
called the critical angle.
vii. State the laws of refraction.
ANSWER: Laws of Refraction: There are two laws
of refraction: 1.The incident ray, the
refracted ray and the normal at the point of
incidence, all lie in the same plane.
- The ratio of the speed of light in vacuum
to its speed In another medium is always
- Define refraction of light. Discuss the
effects of refraction with examples.
ANSWER: Refraction: Light does not need a
material medium to travel. Light travels the
fastest through the vacuum. Light travels at
different speeds in different mediums.
When light passes from one transparent
medium to another, it changes speed and
direction (or bends). This bending of light is .
called fraction. But, when light falls
perpendicular to the surface of the medium,
it does not change its direction.
A light beam bends as it travels rom air ).
glass and also from glass into air.
Effects of Refraction: Whenever we open our
eyes, we observe the refraction of light.
The lens in our eye refracts light to form
image in the retina of our eye
A pencil in a glass of water looks as it has
been broken at the water line. It is because
of refraction of light.
- Refraction causes the formation of the
- Refraction takes place in lenses used in.
spectacles, telescopes, magnifying glasses, etc.
- Define total internal refraction. Explain the
phenomenon of mirage.
ANSWER: Total internal Reflection: When light
passes from glass or water to air (denser to
rarer medium), it bends away from they
normal. But when angle of incidence (i) is w
greater than the critical angle the it
rays reflect in the same denser medium.
phenomenon is called total internal
ii Total internal reflection takes place i
Light passes from a denser medium
(water or glass) to a rare medium (air).
. The angle of incidence of all rays must
use greater than the critical angle of that
Mirage: A Mirage is an image of some
distant object which appears to us due to the
W .1 refraction and total internal reflection of
M The air higher up is cooler than the air near
‘the road. Light travels faster when it reaches
the warmer air. The light rays bend as they
travel downward due to refraction. Near the
ground where air is even warmer, the light
rays travel almost parallel to the ground but
.internal reflection). When we see these
bending light rays, Our brain assumes that
the rays have travelled in a straight line.
These rays seem to us as reflecting from
water. As a result we see a mirage. Desert
travelers often observe mirages.
- WHAT IS DISPERSION OF LIGHT? WHY DOES
IS WHITE LIGHT GET DISPERSED?
ANSWER: DISPERSION OF LIGHT: Sunlight is often called white light, although it is combination
of different colour. We can see these
colours rainbow.1 These colours are red,
continue to bend in other direction (total
orange, yellow, green, blue, indigo and
We can also split white light into its colours by
passing it through the prism. The band of seven
colours obtained is called spectrum of white
light. The splitting of white light into its
component colours is called dispersion of light.
White Light get Dispersed: When a beam of light enters a prism, all the colours of white
light refract at different angles; it causes the
white light to split into its component
colours. Red light bends the least. Violet
light bends the most and refracts by the
disperses into its component colours.
- EXPLAIN THE PHENOMENON IN THE FOLLOWING.
Ans: When a beam of light enters a prism, all
the colours of white light refract at different .
angles; it causes the white light to split into
its component colours. Red light bends least
and violet light bends most and refracts by
the largest angle.
CH: 10 SOUND WAVES P:
3-GIVE SHORT ANSWERS:
CH: 11 CIRCUITS AND CURRENT P:105-116
3-GIVE SHORT ANSWERS.
- WHAT IS AN ELECTRIC CURRENT?
ANSWER: Electric current: The flow of charges through a conductor is called electric current. Charges travel from one pole to the other pole of an electrical source (battery).
- WHAT CAUSES THE CURRENT TO FLOW IN AN
ANSWER: A voltage acts like a force on electrons
causes the current to flow in an electrical
iii. WHAT ARE THE FUNCTIONS OF AN ELCB?
ANSWER: ELCB (Earth Leakage Circuit Breaker)
An earth leakage circuit beaker (ELCB) is a
safety device used in electrical installations.
to prevent a shock. An ELCB is ant;
electromagnetic switch. It quickly turns off‘
the power when the current flowing through‘:
the earth wire exceeds the limit. lf some one
tries to use a faulty electric appliance, an
ELCB breaks the circuit at once.
- COMPARE FUSES AND CIRCUIT BREAKER
WHICH IS EASIER TO USE?
ANSWER: A fuse is a piece of thin conducting
wire connected in the path of a live wire
while circuit breaker is an electromagnet
switch that works like a fuse but it does not
blow out. Circuit breaker is easy to use.
- WHAT IS THE MAIN DIFFERENCE BETWEEN A
SERIES CIRCUIT AND A PARALLEL CIRCUIT?
|Series Circuit||Parallel Circuit|
|1- if all the components are
connected one after another loops, then it serious circuit.
in a single is a series circuit.
2. In a serious circuit,
there is only one path for the current to one path for the
flow current to flow.
|1- if all the components are connected in two
connected one after or more loops, then it parallel circuit.
2. ln a parallel circuit, there are more than one path for the
current to flow.
- The amount of current which flows through each component (bulb) of circuit is the same.
- The current, flowing through different branches of
a parallel circuit may be the same or different. But the current in each branch is less than the total current flowing out from the electrical source (battery).
vi- HOW DOES INCREASING THE POTENTIAL
DIFFERENCE AFFECT THE CURRENT?
ANSWER: Increasing Potential Difference Affect
If potential difference increased, current is
also increased because potential difference
is directly proportional to current.
- DESCRIBE SERIES AND PARALLEL CIRCUITS,
DISADVANTAGES OF A SERIES CIRCUITS AND
ADVANTAGES OF A PARRELL CIRCUIT.
ANSWER: Series Circuits: lf all the components
are connected one after another in a single
loop, then it is a series circuit. In a series
circuit, there is only one path for the current
to flow. The amount of current which flows
through each component (bulb) of the circuit
is the same.
Disadvantages of the Series Circuits:
There are few disadvantages of the series
- There is only one path for current to flow;
A break at any part of the circuit stops the
flow of current in the whole circuit.
- The light of bulbs in a series circuit
becomes dim as more bulbs are added.
Parallel Circuits: If the components are
connected in two or more loops, then it is a
parallel circuit. In a parallel circuit, there is
more than one path or the current top flow.
The current flowing through different
branches of a parallel circuit may be the
same or different. But the current in each
branch is less than the total current flowing
out from the electrical source (battery).
In a parallel circuit there is more than one
path for the current to flow.
Advantages of the Parallel Circuits:
A parallel circuit has some advantages over ll
a series circuit.
Ii There are more than one path for the;
current to flow. A break in any branch of the;
circuit stops the current flowing through that
branch on y.
- EXPLAIN HEATING AND CHEMICAL EFFECTS ON
ANSWER: Heating Effect of the Current: When
electric current flow through a metal wire, it
makes it hot. Light is also produced when a
wire becomes very hot. We use many
appliances in our homes that convert
electric current to heat.
Chemical Effect of the Current: An electric;
current‘ can chemically affect the materials
particularly in molten or solution form. When
currents flows through a solution, it can.
break up the solution into its components.
This process is called electrolysis,
Electricity is also used to coat a metal object
with a thin layer of another metal. This
process is called electroplating. ‘The rims of:
bicycles are nikle-plated. T
6- WRITE NOTES ON:
i- Resistance ii- Electrical Power
iii- MCBs (Miniature Circuit Breakers)
ANSWER: i. RESISTANCE: Electric current flows through
some things better than others do. The measurement of how well something
conducts electricity is its resistance.
Resistance is the hindrance to the flow of current. During its journey through an electric circuit, the charges collide countless
times with atoms within the conductor (wire).
These collisions result in the hindrance to
the flow of the current (resistance)
The resistance of a wire depends on length
of the wire and thickness of the wire.
ii- ELECTRICAL POWER: All electrical devices
lSUCh as fans, blenders, computers etc.
convert electrical energy into other forms of
energy. Electrical power is the rate at which
.a device converts electrical energy into
another form of energy. its unit is watt (W).
- MCBs (Miniature Circuit Breakers‘):
Replacing the fuse again and again is nota
pleasant experience. So, engineers have
developed the alternatives of fuses, i.e.
iii- MINIATURE CIRCUIT BREAKERS (MCBs). An MCB
is a small electromagnet switch that works
like a fuse but it does blow out. it just breaks
the circuit by tripping when a current more
than its rating passes through it.
CH:12 INVESTIGATING THE SPACE P:116-127
3- GIVE SHORT ANSWERS.
i- ARE BLUE STARS YOUNG OLD? HOW CAN YOU
ANSWER: Blue stars are the hottest and most
massive stars and burn themselves not
quickly. So all blue stars are young.
- NAME ONE OBSERVATION THAT SUPPORTS THE
BIG BANG THEORY.
ANSWER: Edwin Hubble found experimental
evidence to support The Big Bang Theory.
He found that distant galaxies in every
direction are going away from is with a very
high speed. This observation is acceptable if
the universe begin in a huge explosion.
iii. LIST IN ORDER, THE FOUR STAGES IN THE LIFE
YCLE OF A LOW-MASS STAR.
ANSWER: Four stages in the life cycle of a
) Birth of a star
- i) Death of a star
ii)’ Red giant stage
- v) Dwarf stage
- HOW DO CONSTELLATIONS DIFFER FROM
Ans. Constellation is a group of stars with a
definite pattern or arrangement while a
galaxy is a massive collection of stars,
nebulae, gases, dust and planets. .
- HOW DO SCIENTISTS THINK THE UNIVERSE
ANSWER: Scientists have been Presenting
different theories of creation of he universe
from time to time. One of these theories is
Big Bang Theory. According to this
About 20 billion years ago, the universe
was packed into one giant fireball. Then
tremendous explosion started the expansion
of the universe. This extraordinary explosion
is known as the Big Bang. This explosion
hurled matter and energy in all direction.
After the Big Bang, the universe assumed
and contracting gases. With the passage of
time , the matter cooled: the force of gravity
Pulled together the particles of mater to
form stars and galaxies.
The Big Bang Theory was first proposed in
1927 a priest, George Lamaitre of
Belgium. This story was supported by the
discoveries of Edwin Hub e and nobel
Prize-winning scientists Arno Penzias and
- WHAT TYPE OF STARS ENDS ITS LIFE CYCLE AS A
BLACK HOLE .
ANSWER: Black hole is the last stage of the life
cycle of a massive star.
vii. FOR HOW MANY YEARS WILL THE SUN BE A RED
Ans. For 500 million years, Sun will be a red
viii. ON WHICH FACTORS DOES THE BRIGHTNESS OF
A STAR DEPEND?
ANSWER: Brightness of Stars: ‘The brightness of a
star depends on two factors:
- Distance of the star from the Earth
- Amount of energy the star emits
- WHAT IS A LIGHT-YEAR?
ANSWER: Light-year: A light-year is a measure of
distance that light covers in one year with a
speed of 300,000 kilometres per second.
- WHAT GALAXY OUR SUN BELONGS TO?
ANSWER: Our Sun belongs to milky Way galaxy.
- EXPLAIN THE BIG BANG THEORY OF THE ORIGIN
OF THE UNIVERSE.
ANSWER: The Big Bang Theory: According to
Islam and o her lbrahimic religions, universe
was created by Allah Almighty. According to
the Holy Quran, Allah Almighty said
and the universe was created. Scientists
have been presenting different theories of
creation of the universe from time to time.
One of these theories is The Big Bang
Theory. According to this theory:
A out 10 to 20 billion years ago, the
universe was packed into one giant fireball.
Then a tremendous explosion started the
expansion of the universe. This
extraordinary explosion is known as the Big
Bang. This explosion hurled matter an
energy in all direction. After the Big Bang,
the universe assumed and contracting
gases. With the passage of time , the matter
cooled: the force of gravity pulled together
the particles of matter to form stars and
he Big Bang theory was first proposed in
1927 by. a priest, George Lamaitre of
Belgium. This story was supported W the
discoveries of Edwin Hub e and nobel
Prize-winning scientists Arno Penzias
- -Edwin Hubble found experimental
evidence to support The Big Bang Theory.
He found that distant galaxies in every
direction are going away rom is with a very
high speed. T is observation is acceptable if
the universe begin in a huge explosion.
- The Big Bang Theory also predicts the
existence of cosmic background radiation
the glow left over from the explosion in 1964
Arno Penzias and Robe, Wilson. They
later won the Nobel Prize for this discovery.
- DESCRIBE THE LIFE CYCLE OF A LOW-MASS STAR.
ANSWER: Stars more than six times as massive
as your Sun are called massive stars. A
massive star has short lifespan than the Sun
or other low-mass stars. hydrogen in the
core of a massive star is used up with a
much fast speed.
Supergiant: After only 50 to 100 million
years, no hydrogen is left in the core of
massive star. At is time, the core collapses
and the star becomes 1000 times greater
than its orignal size. lt is now called a
Supernova: With the passage of time the
supergiant becomes so dense that it cannot
bear the pressure of outer layers. The outer
layers crash inward with a tremendous
Neutron Star: At the time of supernova, the
light of the star becomes much more than all
other stars of the galaxy. Great shells of
gases fly off the star. Only the tiny core of
the star remains left. This core contains only
neutrons, so it is called a neutron star. It is
extremely dense. Some times after the
supernove explosion the massive star
becomes a black hole.
A black hole is so dense that nothing can
escape from it due to its very strong gravity.
Even light cannot escape from a black hole
and it is no more glowing. In fact the black
hole is the last stage of the life cycle of a
- DESCRIBE THE THREE MAIN TYPES OF GALAXIES.
ANSWER: Three main types of Galaxies: There are
many types of galaxies in the universe.
Scientists classify galaxies in three main
Types on the basis of shape.
(i) Spiral Galaxies: A galaxy that has a flat
dislike shape with a bulge in the centre is
called a spiral galaxy. Spiral galaxies may
have a few or many spiral or curved arms. A
large amount of dust and gases is present in
these galaxies. The Milky Way and
Andromeda are spiral galaxies. The Milky
Way galaxy contains 100 to 200 billion stars.
The Sun is about 30,000 light-years away;
from its centre. The Milky Way galaxy is
moving with a speed of 2,200,000 kilometers
per hour space.
Andromeda is about 2,250,000 light-years
Away from the Milky Way galaxy. lt is our
(ii) Elliptical Galaxies: These are oval shaped
galaxies. These galaxies do not rotate as
spiral galaxies around“ their axis. An elliptical
galaxy contains less amounts of dust and
gases as compared to a spiral galaxy.
Trillions of stars may be present in an
elliptical galaxy. New stars cannot form in
most elliptical galaxies. Most of them;
contain only old stars.
(iii) irregular Galaxies: These galaxies have
noldefinite shape. The stars in an irregular?
galaxy do not appear to be grouped in any
set shape. These galaxies have many shapes
and sizes. The Clouds of Magellan, is an‘
irregular galaxy. it is a very small galaxy near
the Milky Way. These galaxies are not very
- WRITE NOTES ON:
- STAR DISTANCE ZULU
- SAFETY METHODS TO USE WHEN OBSERVING THE
ANSWER: Star Distances: The stars are very far
away from us. They are also at great
distances from each other. Distances
between stars are so great that they cannot,
be measures in kilometres. Instead, we use
light-years to express the distance in the
A light-year is a measure of distance that‘
light covers in one year with a speed of
300,000 kilometres per second. It seems that
a light-year is a very long distance. The Sun‘
is our closest star in our galaxy. The next
closest star Proxima Centauri is 4.2
light-years away from us. We can also say
that light of this star will take 4.2 years to
reach the Earth
safety Tips For Observing the Sun:
The Sun emits dangerous radiation.
Viewing directly into the Sun can damage
our eye sight. Make sure the safety of your
eye before viewing the Sun.
- A pinhole or small opening is used to view
the image of the Sun on a screen placed a
half metre or more beyond the opening.
- Use two or three sheets of X-Rays film for
viewing the Sun.