Air around us...

Excercise questions.

1. What is the composition of air?

Solution:

Air comprises water vapour, Oxygen, Nitrogen, Carbon dioxide, dust and smoke.

2. Which gas in the atmosphere is essential for respiration?

Solution:

Oxygen in the atmosphere is essential for respiration.

3. How will you prove that air supports burning?

Solution:

Place two candles of the same length on a table. Light both the candles. Cover one of the candles with an inverted glass tumbler. We observe that the candle covered with a glass tumbler got extinguished after some time, whereas the other candle continued burning. The candle gets extinguished because the component inside of the glass tumbler, which supports burning, is limited. Most of the component is used up by the burning candle. However, the other candle is getting continued supply of air. This component of air, which supports burning, is known as oxygen.

4. How will you show that air is dissolved in water?

Solution:

Take some water in a container. Heat it slowly on a tripod stand. Before water begins to boil, look at the inner surface of the container. We observe tiny bubbles inside. 

These bubbles come from the air dissolved in water. When you heat the water, to begin with, the air dissolved in it escapes. This experiment concludes air is present in the water.

5. Why does a lump of cotton wool shrink in water?

Solution:

The lump of cotton wool shrink in water because the air inside the cotton lumps are replaced by water which makes the layer stick together.

6. The layer of air around the earth is known as ___________.

Solution:

The layer of air around the earth is known as the atmosphere.

7. The component of air used by green plants to make their food, is ___________.

Solution:

The component of air used by green plants to make their food is carbon dioxide.

8. List five activities that are possible due to the presence of air.

Solution:

Activities that are possible due to air are:

Photosynthesis

Cloud formation

Respiration

Transpiration

Winnowing

9. How do plants and animals help each other in the exchange of gases in the atmosphere?

Solution:

During the process of respiration, animals and plants consume oxygen from the air and release carbon dioxide gas in the air. But green plants also release oxygen gas by utilizing carbon dioxide during the process of photosynthesis. Hence, in this way, plants and animals help each other in the exchange of gases in the atmosphere.

some natural phenomena

 NOTES

Static Electricity: The chemical charge generated by rubbing is called static electricity because these charges do not transmit.

There are two types of charges-positive charge and negative charge.

When we rub two objects, made of different substances, together the charge they acquire are opposite to each other.

Electric Current: When charges move they constitute an electric current.

Earthing: The process of transfer of charges from a charged object to the Earth is called earthing.

Earthing is provided in electrical wiring in building to protect us from electrical shocks, in case of any leakage of electrical current.

The process of electric discharge between clouds and the earth or between different clouds causes lightning.

Lightning strike could destroy life and property.

Electric discharge: When negative charges from the clouds and positive charges on the ground meet, a huge amount of energy is produced as bright light and sound, which we see as lightning. The process is called electric discharge.

Richter Scale: The power of an earthquake is expressed in terms of magnitudes on a scale called the Richter Scale.

Seismograph: The seismic waves are recorded by an instrument in the form of a graph called the seismograph.

Exercise Questions

Select the correct option in Questions 1 and 2.

1. Which of the following cannot be charged easily by friction?

(a) A plastic scale

(b) A copper rod

(c) An inflated balloon

(d) A woollen cloth.

Soln:

Answer is (b) A copper rod

Explanation:

Only non-conducting materials can be easily charged by friction. Copper is a highly conducting materials. Therefore, a copper rod cannot be charged easily by friction.

2. When a glass rod is rubbed with a piece of silk cloth the rod

(a) and the cloth both acquire positive charge.

(b) becomes positively charged while the cloth has a negative charge.

(c) and the cloth both acquire negative charge.

(d) becomes negatively charged while the cloth has a positive charge.

Soln:

Answer is (b) becomes positively charged while the cloth has a negative charge.

Explanation:

When two objects are rubbed against each other, they acquire opposite charges. By the law of convention, it is known that the rod acquires the positive charge and the cloth is acquiring the negative charge.

3. Write T against true and F against false in the following statements.

(a) Like charges attract each other (T/F)

(b) A charged glass rod attract a charged plastic straw (T/F)

(c) Lightning conductor cannot protect a building from lightning (T/F)

(d) Earthquakes can be predicted in advance (T/F)

Soln:

a) False-Unlike charges attract each other while the like charges repel each other.

b) True-A charged plastic straw has a negative charge on its surface while the glass rod has positive charges on its surface. Unlike charges attract each other so, they both attract each other.

c) False -When lightning occurs, the atmospheric charges are transferred to the earth directly by a lightning conductor. Therefore the building is protected from lightning.

d) False-Even though the earthquake causes are known, there are no instruments that are invented to detect them in advance. Therefore earthquakes cannot be predicted in advance.

4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.

Soln:

When we take out sweater, Woolen sweater gets charged due to friction between the sweater and the body. This results in crackling sound.

5. Explain why a charged body loses its charge if we touch it with our hand.

Soln:

The charges get conducted to the earth through our body when we touch it and the conductor loses its charge. This phenomenon is known as electric discharge.

6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?

Soln:

Richter scale is used to measure the destructive energy of an earthquake. The scale has a reading from 1 to 10.

An earthquake measuring 3 would be recorded by a seismograph.

The magnitude of scale 3 would not cause much damage. An Earthquake of magnitude 5 is considered destructive in nature.

7. Suggest three measures to protect ourselves from lightning

Soln:

Various ways to protect ourselves from lightning are

(i) Always remain in a closed place and if you are in a car stay there until the lightning is over and keep the windows closed.

(ii) Never touch any electrical wires, telephone cables, metal pipes.

(iii) Never bathe in running water, this may cause electric shock.

8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon.

Soln:

The surface charge on the balloons are of the same nature hence they get repelled. When a charged balloon is brought near an uncharged balloon due to the induction of charges, it acquires charges which are opposite in nature with that of a charged balloon. As unlike charges attract each other, the uncharged balloon gets attracted by the charged balloon.

9. Describe with the help of a diagram an instrument which can be used to detect a charged body.

Soln:

It consists of a metal rod on which two leaves of aluminium foil are fixed to one end and a metal disc at the other end. The leaves are kept inside a conical flask and it is corked to isolate it from the atmospheric air.

When a charged body comes in contact with the metal disc, the aluminium leaves move away from each other because some charges get transferred to aluminium leaves through the metal rod. This process is called charging by conduction. The charges on the leaves and the charged body are of same in nature and thus the leaves of aluminium repel each other. If the body is not charged then they would attract each other.

10. List three states in India where earthquakes are more likely to strike.

Soln:

Gujarat, Assam and Jammu & Kashmir are the three states where an earthquake is more likely to strike.

11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?

Soln:

The following precautions should be taken when earthquake strikes

(a) Find and go to an open field and stay away from buildings, trees, electric wire and poles.

(b) If you are in a car, then drive to an open field and do not come out of your car.

12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain.

Soln:

No, one should not carry an umbrella during a thunderstorm. The thunderstorm is accompanied by lightning and the charges might travel from the cloud to the metal rod on the umbrella and might cause an electric shock to the person carrying it. So, it is not safe to carry an umbrella during lightning.

CHEMICAL EFFECTS OF ELECTRIC CURRENT

 NOTES

LED- light emitting diode

chemical Effects of Electric Current: The passage of an electric current through a conducting solution causes the chemical reaction. The resulting effects are called chemical effects of current.

Electroplating is an example of a chemical effect of current.

Electrode: It is the metallic rod/conductor through which electricity enters or leaves an electrolyte.

Current – Conducting Liquids

Conducting Liquid

• Liquids conduct electricity too when there are salts dissolved in the liquid.
• Most liquids that conduct electricity are solutions of acids, bases or salts.

Acids, bases and salts

• Acids and bases are chemical substances that dissociate to form ions when dissolved in a solution. They are a good conductor of electricity because of the presence of the ions.
• Salts when dissolved in water also conduct as they release positive/negatively charged ions.

Electroplating

Electroplating

• The process of depositing a layer of desired metal on another material by means of electricity is known as electroplating.
• Example: Using Copper Sulphate solution as electrolyte and copper electrodes. Copper is electroplated on the negative electrode. The Cu in the solution is replenished due to the addition of copper ions from the positive electrode.

Applications of Electroplating

• Coating zinc on the iron to prevent corrosion and rust.
• Coating silver and gold for jewellery.
• Coating tin onto iron for cans as tin is less reactive than iron.
• Chromium coating for car parts, bath fittings as it has a shiny appearance.

Exercise Questions

1. Fill in the blanks.

(a) Most liquids that conduct electricity are solutions of , ______________ and ______________.

(b) The passage of an electric current through a solution causes ______________ effects.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ___________terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called _________.

Soln:

(a) Most liquids that conduct electricity are solutions of acids, bases and salts.

(b) The passage of an electric current through a solution causes chemical effects.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the negative terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.

2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Soln:

The compass needle shows a deflection which concludes that current is flowing through the wire. The circuit becomes complete as the free ends of the tester are immersed inside the solution. So, the solution is conducting solution hence deflection is obtained in the compass needle.

3. Name three liquids, which when tested in the manner shown in Fig.14.9, may cause the magnetic needle to deflect.

Soln:

I. Saltwater

II. Lemon juice

III. Vegetable oil

These liquids can be taken in a beaker to show the passage of electricity as they will show a deflection in the magnetic needle.

4. The bulb does not glow in the setup shown in Fig.14.10. List the possible reasons. Explain your answer.

Soln:

The possibility of the bulb not glowing maybe because of the following reasons:

a. The liquid may be non-conducting. In this case, the circuit is incomplete and the current does not pass through the liquid.

b. Electric current may be weak for the circuit is made up of a material which is not a good conductor of electricity or there is insufficient energy in the battery to generate electricity.

5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that

(i) liquid A is a better conductor than liquid B.

(ii) liquid B is a better conductor than liquid A.

(iii) both liquids are equally conducting.

(iv) conducting properties of liquid cannot be compared in this manner.

Soln:

Liquid A is a better conductor than liquid B.

The conductivity of the solution determines the amount of current flowing through the solution. Greater the conductivity, greater will be the quantity current passing through the solution and lesser the conductivity, the quantity of current passing through will be correspondingly less. So, the conductivity of liquid A is more than the conductivity of liquid B.

6. Does pure water conduct electricity? If not, what can we do to make it conducting?

Soln:

Pure water does not conduct electricity as it does not contain any type of salts. Adding, a small amount of Common salt (Sodium Chloride ie,.NaCl) will turn the water to a conducting medium.

7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Soln:

In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area because water sprayed from the hose might conduct electricity which may come in contact with the electrical appliances which increases the chance of electricity passing through wire. This may hurt the fireman.

8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Soln:

The amount of dissolved salts present in the seawater is more than that of the drinking water. So, the sea water will be a better conductor than the drinking water. That is the reason behind the increased deflection of the needle in the seawater when compared with the drinking water..

9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Soln:

No. It is not safe to repair electrical appliances outdoors during a heavy downpour. Rainwater is composed of a certain percentage of dissolved salts making it conductive. This may cause electric shocks and harm the electrician while working outdoors during heavy downpours.

10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons?

Soln:

Rainwater is composed of a certain percentage of dissolved salts making it conductive. This makes the deflection in the compass.

11. Prepare a list of objects around you that are electroplated.

Soln:

Chromium plating: This is done on exterior parts of automobiles in order to obtain a shiny appearance.

Gold Plating: Silver ornaments are coated with a thin layer of gold and the product are called Gold-plated Ornaments.

Zinc Plating: Iron used for Construction are coated with a Zinc layer in order to protect them from corrosion and rusting.

12. The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transfered to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?

Soln:

The thick rod of impure copper plate is to be attached to the positive terminal of the battery because when electric current is passed through the copper sulphate solution, it gets dissociated into copper and sulphate. The free copper, being positively charged, gets drawn to the negative terminal of the battery and gets deposited on it. On the other hand the loss of copper from the solution is regained from the impure copper rod which is attached to the positive terminal of the battery.

WATER : A PRECIOUS RESOURCE

 NOTES 

The upper limit of groundwater is called water table. It represents the depth of water filled area at a given place. The water table rises and falls depending upon the amount of rainwater that seeps into the groundwater and how much groundwater is drawn out for irrigation and industry. The seeping down of rainwater into the groundwater is called infiltration. The groundwater thus gets recharged by this process.

At some places, the groundwater is stored between layers of hard rock below the water table. This is aquifer which contains the groundwater usable by tubewells and handpumps. These sources however, have limited sources of water and may get exhausted if used excessively.

Exercise Questions

1. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) The freshwater stored in the ground is much more than that present in the rivers and lakes of the world. (T/F)

(b) Water shortage is a problem faced only by people living in rural areas. (T/F)

(c) Water from rivers is the only source for irrigation in the fields. (T/F)

(d) Rain is the ultimate source of water. (T/F)

Solution:

a) True

b) False

c) False

d) False

2. Explain how groundwater is recharged.

Solution:

The rainwater and water from other sources such as rivers and ponds seep through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets recharged by this process.

3. There are ten tubewells in a lane of fifty houses. What could be the long term impact on the water table?

Solution:

If ten tube-wells in a lane of fifty houses are situated, the groundwater will get used for domestic purposes. This results in depletion of groundwater level and water table would go down.

4. You have been asked to maintain a garden. How will you minimise the use of water?

Solution:

We can reduce the usage of water for gardening by adopting drip irrigation method. In this method, water will reach the roots of the plant drop by drop, which will reduce the wastage of water.

5. Explain the factors responsible for the depletion of water table.

Solution:

Factors responsible for the depletion of the water table are as follows:

Increase in population; Increase in population leads to increased demand for water for domestic and drinking purpose. This reduces the water table.

Industrialisation: Industries need a lot of water for manufacturing purposes. This reduces the water table.

Agriculture: Agriculture requires a lot of water for growing crops; this results in depletion of water table.

6. Fill in the blanks with the appropriate answers:

(a) People obtain groundwater through________ and ________.

(b) Three forms of water are ________, ________ and ________.

(c) The water bearing layer of the earth is ________.

(d) The process of water seepage into the ground is called ________.

Solution:

(a) People obtain groundwater through tube wells and hand pumps.

(b) Three forms of water are ice, water and vapour.

(c) The water bearing layer of the earth is Hydrosphere.

(d) The process of water seepage into the ground is called infiltration.

7. Which one of the following is not responsible for water shortage?

(i) Rapid growth of industries

(ii) Increasing population

(iii) Heavy rainfall

(iv) Mismanagement of water resources

Solution:

The answer is (iii) Heavy rainfall

8. Choose the correct option. The total water

(i) in the lakes and rivers of the world remains constant.

(ii) under the ground remains constant.

(iii) in the seas and oceans of the world remains constant.

(iv) of the world remains constant.

Solution:

The answer is (iv) of the world remains constant.

9. Make a sketch showing groundwater and water table. Label it.

Solution:

LIGHT

 NOTES

There are two types of images:
Real image: It is an image which can be obtained on a screen, e.g. the image formed on a cinema screen. When the light rays coming from an object actually meet at a point after reflection from the mirror, then it results in the formation of a real image.

Virtual image: It is an image which cannot be obtained on a screen, e.g. image formed by a plane mirror. When the light rays coming from an object appear to meet after reflection from the mirror, then it results in the formation of virtual image. It is not possible to form a virtual image on the screen because light rays actually do not pass the screen or cannot be received on a screeN

 LATERAL INVERSION- the right side of our body becomes the left side in the image while the left side of our body becomes the right side of the image. It appears as if our image has been ‘reversed side ways’ with respect to your body. The effect of reversing the sides of an object and its image is called lateral inversion.

Dispersion of Light
In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colours. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen. The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is known as a spectrum of white light. The seven colours of the spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet.

So, dispersion of light is the phenomenon of splitting up of white light into seven colours on passing through a transparent medium like a glass prism. The formation of a spectrum of seven colours indicates that white light is a mixture of seven colours. White light can be sunlight. So, now we can say that sunlight consists of seven colours.

Exercise Questions

1. Fill in the blanks:

(a) An image that cannot be obtained on a screen is called ____________.

(b) Image formed by a convex __________ is always virtual and smaller in size.

(c) An image formed by a __________ mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a _________ image.

(e) An image formed by a concave ___________ cannot be obtained on a screen.

Solution:

(a) An image that cannot be obtained on a screen is called virtual image.

(b) Image formed by a convex mirror is always virtual and smaller in size.

(c) An image formed by a plane mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a real image.

(e) An image formed by a concave lens cannot be obtained on a screen.

2. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)

(b) A concave lens always form a virtual image. (T/F)

(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)

(d) A real image cannot be obtained on a screen. (T/F)

(e) A concave mirror always form a real image. (T/F)

Solution:

a) False

b) True

c) True

d) False

e) False

3. Match the items given in Column I with one or more items of Column II.

Column-I	Column-II
(a) A plane mirror	(i) Used as a magnifying glass
(b) A convex mirror	(ii) Can form image of objects spread over a large area.
(c) A convex lens	(iii) Used by dentists to see enlarged image of teeth.
(d) A concave mirror	(iv) The image is always inverted and magnified
(e) A concave lens	(v) The image is erect and of the same size as the object.
	(vi) The image is erect and smaller in size than the object.

Solution:

Column-I	Column-II
(a) A plane mirror	(v) The image is erect and of the same size as the object.
(b) A convex mirror	(ii) Can form an image of objects spread over a large area.
(c) A convex lens	(i) Used as a magnifying glass
(d) A concave mirror	(iii) Used by dentists to see an enlarged image of teeth.
(e) A concave lens	(vi) The image is erect and smaller in size than the object.

4. State the characteristics of the image formed by a plane mirror

Solution:

Characteristics of the image formed by a plane mirror are as follows:

• Image distance and object distance are equal
• Size of object and image are equal
• The image formed is erect and virtual
• Images are laterally inverted

• 5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

  Solution:

  A, H, I, M, O, T, U, V, W, X, Y alphabets form images in a plane mirror exactly like the letter itself because these alphabets are laterally symmetric.

6. What is a virtual image? Give one situation where a virtual image is formed.

Solution:

The image that cannot be obtained on a screen is called a virtual image. The image formed by a plane mirror is virtual.

7. State two differences between a convex and a concave lens.

Solution:

Convex Lens	Concave Lens
Thick in the middle and thin at the edge	Thin in the middle and thick at the edge
Image formed is real or virtual	Image formed is virtual

8. Give one use each of a concave and a convex mirror.

Solution:

Concave mirrors are used in the headlights of cars and scooters.

Convex mirrors are used as side-view mirrors in vehicles.

. Which type of mirror can form a real image?

Solution:

The concave mirror can form a real image.

10. Which type of lens forms always a virtual image?

Solution:

Concave lens form a virtual image.

Choose the correct option in questions 11–13

11. A virtual image larger than the object can be produced by a

(i) concave lens (ii) concave mirror

(iii) convex mirror (iv) plane mirror

Solution:

The answer is (ii) concave mirror

12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be

(i) 3 m (ii) 5 m

(iii) 6 m (iv) 8 m

Solution:

The answer is (iii) 6 m

13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be

(i) 1 m/s (ii) 2 m/s

(iii) 4 m/s (iv) 8 m/s

Solution:

The answer is (iii) 4 m/s

WATER

 NOTES

Water cycle: It is circulation of water through the process of evaporation or condensation as rain or snowfall. Water cycle is like a ring. In nature, the water cycle takes place from sea to land and back to sea again.

Evaporation

• The process of changing water to its vapour form is known as evaporation.
• Evaporation takes place from open surfaces of water all the time—day and night.
• Evaporation of water takes place continuously from oceans, rivers, lakes, wells and soil. Oceans, seas, rivers, lakes, ponds and wells together are often known as water bodies.
• During the day time, sunlight falls on the water in oceans, rivers, lakes. The fields, roads, rooftops and other land areas also receive sunlight. The sunlight also carries heat with it. As a result, water from oceans, rivers, lakes and the soil, and other land areas gets continuously changed into vapour.
• Thus, water vapour gets continuously added to air due to evaporation.

Transpiration

• Transpiration: It is the loss of water from the leaves of plants in the form of vapour.
• Every plant, whether it is in a crop field, a forest, on the roadside or in a kitchen garden, transpires to give off water vapour. The amount of water vapour that goes into air through the process of transpiration is very huge.

Condensation

• Condensation: The process of conversion of vapour into liquid form of water is called condensation.
• The process of condensation is opposite to evaporation.
• Cloud formation: The climate close to the earth’s surface is warm. It gets cooled as one goes up in atmosphere. Water vapour being lighter, rises up in the atmosphere. At the upper layer of atmosphere, where the temperature is lower, the vapour gets condensed into tiny water droplets and forms clouds.

Precipitation

• Precipitation: Clouds carry small droplets of water in them. It may so happen that many droplets of water come together to form larger sized drops of water. Such drops of water may become so heavy that they begin to fall. Falling of water drops is called precipitation.
• Rain: If the water during precipitation remains liquid till it reaches the surface of the earth, we have rains.
• Hail/Snow: Sometimes precipitation may be in the form of hail or snow. Water in a hail or snow is in its frozen or solid form.
• Dew: Many times, especially during winter nights, the air near the surface becomes quite cool. As a result, the water vapour present in it condenses to form water droplets. These water droplets appear as dew.
• Water table: If you take soil from ground, it has air as well as water. As you go down, the amount of water increases and air decreases. A level below surface, where it is only water, is called the water table.

Rainwater harvesting: The process of collecting and storing rainwater from roofs or a surface catchment is called rainwater harvesting.

Exercise Questions

1. Fill up the blanks in the following:

(a) The process of changing of water into its vapour is called _________________.

(b) The process of changing water vapour into water is called _________________.

(c) No rainfall for a year or more may lead to _________________ in that region.

(d) Excessive rains may cause _________________.

Solution:

(a) The process of changing of water into its vapour is called evaporation.

(b) The process of changing water vapour into water is called condensation.

(c) No rainfall for a year or more may lead to drought in that region.

(d) Excessive rains may cause floods.

2. State for each of the following whether it is due to evaporation or condensation:

(a) Water drops appear on the outer surface of a glass containing cold water.

(b) Steam rising from wet clothes while they are ironed.

(c) Fog appearing on a cold winter morning.

(d) Blackboard dries up after wiping it.

(e) Steam rising from a hot girdle when water is sprinkled on it.

Solution:

(a) Condensation

(b) Evaporation

(c) Condensation

(d) Evaporation

(e) Evaporation

3. Which of the following statements are “true” ?

(a) Water vapour is present in air only during the monsoon. ( )

(b) Water evaporates into air from oceans, rivers and lakes but not from the soil.( )

(c) The process of water changing into its vapour, is called evaporation.( )

(d) The evaporation of water takes place only in sunlight.( )

(e) Water vapour condenses to form tiny droplets of water in the upper layers of air where it is cooler.( )

Solution:

a) False

b) False

c) True

d) False

e) True

4. Suppose you want to dry your school uniform quickly. Would spreading it near an anghiti or heater help? If yes, how?

Solution:

Spreading uniform near an anghiti or heater will help because it increases the rate of evaporation due to heat.

5. Take out a cooled bottle of water from refrigerator and keep it on a table. After some time you notice a droplets of water around it. Why?

Solution:

This is because the surface of the air around the bottle cools down and air condenses around the bottle.

6. To clean their spectacles, people often breathe out on glasses to make them wet. Explain why the glasses become wet.

Solution:

The air we breathe out contains water vapour which condenses on the surface of the spectacles. So, the glass becomes wet and with the help of a small amount of water molecules, it becomes easier to clean the spectacles.

7. How are clouds formed?

Solution:

The process of condensation plays an important role in bringing water back to the surface of the earth. As we go higher from the surface of the earth, it gets cooler. When the air moves up, it gets cooler and cooler. At sufficient heights, the air becomes so cool that the water vapour present in it condenses to form tiny drops of water called droplets. It is these tiny droplets that remain floating in the air and appear to us like clouds.

8. When does a drought occur?

Solution:

If it does not rain for two or more years, water is lost from the soil due to evaporation and transpiration. Since it is not being brought back by rain, the soil becomes dry. The level of water in ponds and wells of the region goes down and some of them may even dry up. The ground water may also become scarce. This may lead to drought.