NCERT Exemplar Solutions For Class 9 Science (Chemistry

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NCERT Exemplar Solutions For Class 9

Science (Chemistry)

Chapter 1–Matter in our Surrounding

Multiple Choice Questions

1. Which one of the following sets of phenomena would show an increase on

increasing the temperature?

(a) Diffusion, evaporation, compression of gases

(b) Evaporation, compression of gases, solubility

(c) Evaporation, diffusion, expansion of gases

(d) Evaporation, solubility, diffusion, compression of gases

Ans: (c) Evaporation, diffusion, expansion of gases

Explanation:

(1) Evaporation is the process of conversion of liquid state into gaseous state. It

happens due to the absorption of heat energy by the liquid molecules from the

surrounding. Increasing temperature results in more heat energy that in turn

increases the rate of evaporation of a liquid.

(2) Increased temperature leads to more kinetic energy in the liquid particles,

which also increases the rate of diffusion (movement of particles from a region

of higher concentration to a region of lower concentration).

(3) Increase in temperature increases the kinetic energy of gas particles which

causes the gases to expand when temperature is increased.

(4) Solubility of solute particles is also increased on increasing the temperature.

(5) Increase in pressure and decrease in temperature would result in the

compression of gases.

2. Seema visited a Natural Gas Compressing Unit. She found that a gas can

be liquefied under specific conditions of temperature and pressure. While

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sharing her experience with her friends, she got confused. Can you help her

to identify the correct set of conditions?

(a) Low temperature and low pressure

(b) High temperature and low pressure

(c) Low temperature and high pressure

(d) High temperature and high pressure

Ans: (c) Low temperature and high pressure

Explanation:

High pressure leads to increased inter-molecular forces of attraction which

reduces the inter-molecular spaces between the gas particles. Low temperature

reduces the kinetic energy of the gas particles. Thus, low temperature and high

pressure are the correct set of conditions for liquefaction of a gas.

3. Fluids have a unique property of flowing. Which of the following

statements is correct in this respect?

(a) Only gases behave like fluids.

(b) Gases and solids - both behave like fluids.

(c) Gases and liquids – both behave like fluids.

(d) Only liquids are fluids.

Ans: (c) Gases and liquids – both behave like fluids.

Explanation: Both gases and liquids show the properties of fluids as they have

the apt intermolecular force of attraction and intermolecular spaces in between

the particles that facilitate the flow of these states of matter.

4. During summers, water kept in an earthen pot becomes cooler after some

time. Which phenomenon leads to the cooling of water in an earthen pot?

(a) Diffusion

(b) Transpiration

(c) Osmosis



(d) Evaporation

Ans: (d) Evaporation

Explanation: The evaporation of the water through the pores of the earthen pot

absorbs the heat energy from the surroundings reducing the temperature of the

surroundings. Due to this, water in the earthen pot becomes cooler after some

time.

5. Which of the following represents the correct arrangement of substances

arranged in an increasing order of “forces of attraction” between their

particles?

(a) Water, air, wind.

(b) Air, sugar, oil.

(c) Oxygen, water, sugar.

(d) Salt, juice, air.

Ans: (c) Oxygen, water, sugar

Explanation:

Intermolecular forces of attraction refers to the forces of attraction in between the

particles of a state. The gases have the weakest intermolecular forces (such as

oxygen or air) and solids have the strongest intermolecular forces of attraction

(such as sugar or salt). In liquids, the forces of attraction are moderate and lie

between that of gases and solids.

6. When converted to Kelvin scale, what will be the correct sequence of the

temperatures 25 C , 38 C and 66 C ?

(a) 298K , 311K and 339K

(b) 298K , 300K and 338K

(c) 273K , 278K and 543K

(d) 298K , 310K and 338K

Ans: (a) 298K , 311K and 339K



Explanation:

The conversion between the Kelvin scale and Celsius scale can be done by adding

273 to each Celsius measurement to convert it to a Kelvin measurement.

So, we have, 25 273 298 C K , 38 273 311 C K , and 66 273 339 C K .

7. Choose the correct statement for change of states of matter :-

(a) Conversion of solid into vapours without passing through the liquid state

is called vaporization.

(b) Conversion of vapours into solid without passing through the liquid state

is called sublimation.

(c) Conversion of vapours into solid without passing through the liquid state

is called freezing.

(d) Conversion of solid into liquid is called sublimation.

Ans: (b) Conversion of vapours into solid without passing through the liquid state

is called sublimation.

Explanation:

Sublimation is the change of matter from solid phase to gaseous phase directly or

vice versa without going through the intermediate liquid phase.

8. Boiling points of diethyl ether, acetone and n-butyl alcohol are 35 C , 56 C

and 118 C respectively. Which of the following set correctly represents the

boiling points on the Kelvin scale?

(a) 306K , 329K , 391K

(b) 308K , 329K , 392K

(c) 308K , 329K , 391K

(d) 329K , 392K , 308K

Ans: (c) 308K , 329K , 391K



Explanation: The Celsius measurements can be converted to the corresponding

Kelvin measurements by adding 273 . So, 35 273 308 C K , 56 273 329 C K ,

and 118 273 391 C K .

9. Identify the condition that will increase the evaporation of water?

(a) Increase in the temperature of water.

(b) Decrease in the temperature of water.

(c) Less exposed surface area of water.

(d) Adding common salt to water.

Ans: (a) Increase in the temperature of water

Explanation: Increasing the temperature will provide more heat energy to the

water molecules. More heat energy will lead to increased rate of evaporation.

Rate of evaporation of water is directly proportional to the temperature of water

and the exposed surface area. The other mentioned conditions such as decrease

in temperature and reducing exposed surface area will reduce the rate of

evaporation of water.

10. Identify the conditions where the distance between the molecules of

hydrogen gas will increase?

(i) Increasing the pressure on hydrogen gas contained in a closed container.

(ii) Leaking some hydrogen out of the container.

(iii) Increasing the volume of the container.

(iv) Addition of hydrogen gas to the container without increasing the volume

of the container.

(a) (i) and (iii)

(b) (i) and (iv)

(c) (ii) and (iii)

(d) (ii) and (iv)

Ans: (c) (ii) and (iii)



Explanation: Leaking some hydrogen gas out of the container will reduce the

number of hydrogen molecules in the container and increase the intermolecular

spaces. Increasing the volume of the container would result in reduced pressure

inside the container and increase in intermolecular spaces.

Short Answer Questions

11. A sample of water under study was found to boil at 102 C at normal

pressure. Is the water pure? Will this water freeze at 0 C ? Comment.

Ans: The boiling point of pure water at 1 atm is 100 C . The sample of water

provided to us boils at 102 C . Therefore, the sample provided to us is not pure

water. The freezing point of pure water is 0 C . But the sample is not pure. So, it

will not freeze at 0 C . It will freeze at a temperature below its freezing point, 0 C

.

12. A student heats a beaker containing ice and water. He measures the

temperature of the content of the beaker as a function of time. Which of the

following (Fig. 1.1) would correctly represent the result? Justify your choice.

a)

(b)



(c)

(d)

Ans: Option (d) is the correct answer. Initially, the temperature of the mixture

would be zero as the ice and water will be in equilibrium. When the student starts

heating the mixture, the heat supplied (equivalent of latent heat of fusion) is

initially utilized in the process of melting of ice. So, the temperature does not

increase till the ice in the beaker melts and converts to water. This explains the

constant temperature initially. When the student continues to heat the mixture

further, an increase in temperature is observed. This explains slope of the graph

(increasing temperature).

13. Fill in the blanks:-

(a) Evaporation of a liquid at room temperature leads to a ______ effect.

(b) At room temperature, the forces of attraction between the particles of

solid substances are _______ than those which exist in the gaseous state.

(c) The arrangement of particles is less ordered in the _______ state.

However, there is no order in the _______ state.

(d) ______ is the change of gaseous state directly to solid state without going

through the ______ state.



(e) The phenomenon of change of a liquid into the gaseous state at any

temperature below its boiling point is called _______.

Ans: (a) cooling

(b) stronger

(c) liquid, gaseous

(d) sublimation, liquid

(e) evaporation

14. Match the physical quantities given in column A to their SI units given

in column B

Column A Column B

(a) Pressure (i) cubic metre

(b) Temperature (ii) kilogram

(c) Density (iii) pascal

(d) Mass (iv) kelvin

(e) Volume (v) kilogram per cubic metre

Ans: (a) — (iii) The SI unit of pressure is “pascal”. Pascal is denoted by P.

(b) — (iv) The SI unit of temperature is “kelvin”. Kelvin is denoted by K.

(c) — (v) The SI unit of density is kilogram per cubic metre denoted by 3

kg

m.

(d) — (ii) The SI unit of mass is kilogram (kg).

(e) — (i) The SI unit of volume is 3m .

15. The non-SI and SI units of some physical quantities are given in column

A and column B respectively. Match the units belonging to the same physical

quantity:

Column A Column B

(a) degree Celsius (i) kilogram

(b) centimetre (ii) pascal

(c) gram per centimetre cube (iii) metre

(d) bar (iv) kelvin

(e) milligram (v) kilogram per metre cube



Ans: (a) — (iv) Degree Celsius and kelvin are the units of temperature.

(b) — (iii) Centimetre and metre are the units of length.

(c) — (v) Gram per centimetre cube and kilogram per metre cube are the units of

density.

(d) — (ii) Bar and pascal are the units of pressure.

(e) — (i) Milligram and kilogram are the units of mass.

16. ‘Osmosis is a special kind of diffusion’. Explain.

Ans: Both the processes, osmosis and diffusion have a lot in common. In both

the phenomena, there is movement of particles to region of lower concentration

from region of higher concentration. However, in the case of osmosis, the

movement of solvent particles take place from the area of less concentration to

area of more concentration through a semi-permeable membrane.

17. Classify the following into osmosis/diffusion:-

(a) Swelling up of a raisin on keeping in water.

Ans: Osmosis; The water enters into the raisin through the process of osmosis as

the solution outside the raisin has a higher concentration of water molecules and

the outer membrane of the cells of the raisin acts as a semi-permeable membrane.

Hence, the raisin swells up when kept in water.

(b) Spreading of virus on sneezing.

Ans: Diffusion. The microscopic virus particles diffuse in the air through the

process of diffusion.

(c) Earthworm dying on coming in contact with common salt.

Ans: Osmosis; Earthworms die on coming in contact with common salt because

the aqueous common salt has a lower concentration of water molecules in

comparison to the earthworm’s body.

(d) Shrinking of grapes kept in thick sugar syrup.

Ans: Osmosis; The grapes kept in a thick sugar syrup shrink due to the process

of osmosis as the thick sugar syrup is highly concentrated and has lower



concentration of water molecules as compared to inside the grapes. So, the water

molecules move out from the grapes where the outer membrane of grape cells act

as semi-permeable membrane.

(e) Preserving pickles in salt.

Ans: Osmosis; Pickles are preserved in salt because fruits and vegetable

commonly used in pickles have high natural water content which has to be

eliminated in order to increase the shelf life. So, salt is added to prevent the

oxidation of the fruits and vegetables as it absorbs the excess moisture from them

through the process of osmosis.

(f) Spreading of smell of cake being baked throughout the house.

Ans: Diffusion; the smell of a baking cake gets spread in the house through the

diffusion of particles in the air.

(g) Aquatic animals using oxygen dissolved in water during respiration.

Ans: Diffusion; the oxygen from the atmosphere diffuses in the water and this

diffused oxygen is used up by the aquatic animals during respiration.

18. Water as ice has a cooling effect, whereas water as steam may cause

severe burns. Explain these observations.

Ans: In case of steam, particles carry high heat energy due to which it may cause

severe burns. On the other hand, the particles carry very low energy. In the case

of ice, the molecules absorb energy from the surroundings and thus produce a

cooling effect.

19. Alka was making tea in a kettle. Suddenly she felt intense heat from the

puff of steam gushing out of the spout of the kettle. She wondered whether

the temperature of the steam was higher than that of the water boiling in the

kettle. Comment.

Ans: The temperature of the water boiling in the kettle and the temperature of the

steam gushing out of the kettle is exactly same. The high energy carried by the

steam leads Alka to wonder that the temperature of steam is higher than that of

boiling water. The steam and the boiling water co-exist at equilibrium at the



boiling point of water, 100 C . However, the steam has a higher energy due to the

latent heat of vaporization.

20. A glass tumbler containing hot water is kept in the freezer compartment

of a refrigerator (temperature < 0 °C). If you could measure the temperature

of the content of the tumbler, which of the following graphs (Fig. 1.2) would

correctly represent the change in its temperature as a function of time.

(a)

(b)

(c)



(d)

Ans: Option (a) is the correct answer as graph (a) represents the change in the

temperature of the contents of the tumbler correctly. The water will cool till it

reaches the freezing point of water i.e. 0 C . At the freezing point, the ice and

water co-exist at equilibrium. The temperature will remain constant till all the

water freezes to ice. After this point, the temperature would decrease again.

21. Suggest which of the vessels A, B, C or D in Fig. 1.3 will have the highest

rate of evaporation? Explain.

Ans: Vessel (c) will have the highest rate of evaporation. The rate of evaporation

increases with an increase in the surface area as evaporation is a surface

phenomenon. The moving fan will decrease the humidity and increase agitation

in the top layer of molecules, hence increasing the rate of evaporation. Vessel (d)

is closed and hence offers no surface area for evaporation. Vessel (c) has the

largest surface area and thus has the highest rate of evaporation.

22. (a) Conversion of solid to vapours is called sublimation. Name the term

used to denote the conversion of vapours to solid.



Ans: Sublimation.

Sublimation is the process in which a substance directly converts from solid state

to gaseous state or vice versa without passing through the intermediary liquid

state.

(b) Conversion of solid state to liquid state is called fusion; what is meant by

latent heat of fusion?

Ans: The amount of heat energy that is required to convert one kilogram of any

substance in solid state into liquid state at its melting point at one atmosphere

pressure is known as the latent heat of fusion of that substance.

23. You are provided with a mixture of naphthalene and ammonium chloride

by your teacher. Suggest an activity to separate them with a well-labelled

diagram.

Ans: Naphthalene, being an organic compound, is insoluble in water but soluble

in ether (an organic solvent) whereas ammonium chloride is soluble in water.

Also, naphthalene is volatile at room temperature whereas ammonium chloride is

volatile at a higher temperature. So, the mixture can be easily separated through

decantation process. Naphthalene remains insoluble in water and can be collected

in a funnel whereas the filtrate of ammonium chloride can be evaporated to obtain

dry ammonium chloride.

24. It is a hot summer day, Priyanshi and Ali are wearing cotton and nylon

clothes respectively. Who do you think would be more comfortable and why?

Ans: Priyanshi would be more comfortable in a cotton shirt as cotton is a soft

natural fibre whereas Ali wearing a nylon shirt may not be that comfortable.

Cotton absorbs the moisture better as compared to nylon. Thus, cotton helps in

faster absorption of sweat and thus leads to faster evaporation of sweat. Also,

faster evaporation of sweat from cotton shirt leads to cooling effect.

Long Answer Questions

25. You want to wear your favourite shirt to a party, but the problem is that

it is still wet after a wash. What steps would you take to dry it faster?



Ans: Conditions that can help in increasing the rate of evaporation of water from

the shirt are as follows:-

(a) Increase the surface area of the shirt exposed to the atmosphere – Evaporation

is a surface phenomenon. If we increase the surface area of the shirt exposed to

the atmosphere by spreading it on a cloth hangline, the rate of evaporation will

increase.

(b) Increase in temperature – The rate of evaporation is directly related to the

temperature. If we put the shirt in direct sunlight, then the shirt will dry out faster.

Ironing the shirt will also help in drying it faster.

(c) Reduction in humidity – rate of evaporation is inversely proportional to the

humidity level. If we reduce the humidity by increasing the wind speed, the shirt

can be dried out faster.

26. Comment on the following statements:-

(a) Evaporation produces cooling.

Ans: In the process of evaporation, the molecules present at the surface of a liquid

gain the heat energy from the surroundings to overcome the intermolecular forces

of attraction and change into vapours thereby producing a cooling effect.

(b) Rate of evaporation of an aqueous solution decreases with increase in

humidity.

Ans: If the surrounding air has more humidity level, then the capacity of air to

hold more vapours would decrease and hence the rate of evaporation would

decrease. Hence, the rate of evaporation decreases with increase in humidity.

(c) Sponge though compressible is a solid.

Ans: Sponge is a solid but is compressible because of the presence of minute

holes in it. These holes trap the air in between them. When we squeeze the sponge,

the air trapped in the holes gets expelled out and thus sponge is solid but can be

compressed.

27. Why does the temperature of a substance remain constant during its

melting point or boiling point?



Ans: The temperature of a substance remains unchanged at its melting and

boiling points until all the substance gets melted or boiled because the heat

supplied is continuously used up in overcoming the forces of attraction between

the particles of the substance as the physical state is being changed. This is known

as the latent heat of fusion at melting point and the latent heat of vaporization at

boiling point. Thus, heat energy is absorbed by the substance without showing

any rise in temperature.


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NCERT Exemplar Solutions For Class 9 Science (Chemistry