NCERT Exemplar Problems Class 9 Science – Matter In Our Surroundings

Multiple Choice Questions (MCQs)

Question 1:
Which one of the following sets of phenomena would increases on raising 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
Answer:
(c) Evaporation, diffusion and expansion of gases increase on raising the temperature.Evaporation rate increases because on increasing temperature, kinetic energy of molecules increases, so the molecules present at the surface of the liquid leave the surface quickly and go into the vapour state. Diffusion and expansion of gases also increase as the molecules move more rapidly and try to occupy more space.

Question 2:
Seema visited a Natural Gas Compressing Unit and found that the gas can be liquefied under specific conditions of temperature and pressure. While sharing her experience with friends she got confused. Help her to identify the correct set of conditions.
(a) Low temperature, low pressure
(b) High temperature, low pressure
(c) Low temperature, high pressure
(d) High temperature, high pressure
Answer:
(c) Low temperature and high pressure are required to liquefy gases to liquids. There is a lot of space between the particles of a gas. On applying high pressure, the particles of gas move get so close that they start attracting each other sufficiently forming a liquid.
When gas is compressed too much, heat is produced, so it is necessary to cool it. Cooling lowers the temperature of compressed gas and helps in liquefying it. Hence, a gas can be liquefied by applying high pressure and lowering the temperature (cooling).

Question 3:
The property to flow is unique to fluids. Which one of the following statements is correct?
(a) Only gases behave like fluids             (b) Gases and solids behave like fluids
(c) Gases and liquids behave like fluids (d) Only liquids are fluids
Answer:
(c) Gases and liquids behave like fluids. Both gases and liquids tend to flow due to less force of attraction between their particles. Also, they require vessel to keep them. Solids do not flow.

Question 4:
During summer, water kept in an earthen pot becomes cool because of the phenomenon of
(a) diffusion    (b) transpiration   (c) osmosis    (d) evaporation
Answer:
(d) During summer, water kept in an earthen pot becomes cool because of the phenomenon of evaporation. Earthen pot has a large number of tiny pores in its walls and some of the water molecules continuously keep seeping through these pores to outside the pot.
This water evaporates continuously and take the latent heat required for vaporisation from the remaining water. In this way, the remaining water loses heat and gets cooled.

Question 5:
A few substances are arranged in the increasing order of ‘forces of attraction’ between their particles. Which one of the following represents a correct arrangement?
(a) Water, air, wind              (b) Air, sugar, oil
(c) Oxygen, water, sugar     (d) Salt, juice, air
Answer:
(c) The correct order of increasing ‘force of attraction’ between their particles is
Oxygen < Water < Sugar
It is because the force of attraction increases in the order i.e., Gas < Liquid < Solid

Question 6:
On converting 25°C, 38°C and 66°C to kelvin scale, the correct sequence of temperature will be
(a) 298 K, 311 K and 339 K    (b) 298 K, 300 K and    338 K
(c) 273 K, 278 K and 543 K    (d) 298 K, 310 K and    338 K
Answer:
(a) Therefore, the correct sequence of temperature will be 298 K, 311 K and 339 K.
On converting 25°C, 38°C and 66°C, to kelvin scale, we get the following temperatures
25°C + 273 = 298 K => 38°C + 273 = 311 K => 66°C + 273 = 339 K
Therefore, the correct sequence of temperature will be 298 K, 311 K and 339 K.

Question 7:
Choose the correct statement of the following.
(a) Conversion of solid into vapours without passing through the liquid state is called vaporisation.
(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.
Answer:
(b) Conversion of solid into vapours on heating or vapours into solid on cooling without undergoing in liquid state is called sublimation.
The conversion of liquid into gas (vapour) is called vaporisation.
The conversion of liquid into solid is called freezing.
The conversion of solid into liquid is called melting.

Question 8:
The boiling points of diethyl ether, acetone and n-butyl alcohol are 35°C, 56°C and 118°C, respectively. Which one of the following correctly represents their boiling points in kelvin scale?
(a) 306 K, 329 K, 391 K    (b) 308 K, 329 K, 392 K
(c) 308 K, 329 K, 391 K    (d) 329 K, 392 K, 308 K
Answer:
(c) The correct order of boiling points of diethyl ether, acetone and n-butyl alcohol in kelvin scale is
308 K, 329 K and 391 K, which can be explained as (v T°C + 273 = 7K).
Boiling point of diethyl ether = 35°C + 273 = 308 K
Boiling point of acetone = 56°C + 273 = 329 K
Boiling point of n-butyl alcohol = 118°C + 273 = 391 K

Question 9:
Which condition out of the following will increase the evaporation of water?
(a) Increase in temperature of water
(b) Decrease in temperature of water
(c) Less exposed surface area of water
(d) Adding common salt to water
Answer:
(a) Increase in temperature of water will increase the evaporation of water. It is because, on increasing the temperature, kinetic energy of water molecules increases and more particles get enough kinetic energy to go into the vapour state.
This increases the rate of evaporation. Evaporation is the surface phenomenon so it depends upon the exposed surface area. Higher the exposed surface area of water,higher will be the evaporation. When common salt is added to water then surface is occupied by the solvent as well as non-volatile solute particles.
So, the escaping tendency of solvent particles decreases and thus the evaporation of water decreases. Therefore, other options (b), (c) and (d) will decrease the evaporation of water.

Question 10:
In which of the following conditions, the distance between the molecules of hydrogen gas would increase?
(i)   Increasing pressure on hydrogen contained in a closed container.
(ii)  Some hydrogen gas leaking out of the container.
(iii) Increasing the volume of the container of hydrogen gas.
(iv) Adding more hydrogen gas to the container without increasing the volume of the container.
(a) (i) and (ii)                                    (b) (i) and (iv)
(c) (ii) and (iii)                                  (d) (ii) and (iv)
Answer:
(c) (ii) and (iii) are the correct options because
In option (ii) hydrogen gas leaking from the container leaves some vacant space inside the container. So, hydrogen gas molecules inside the container occupy all the space available and the distance between the molecules of hydrogen gas will be increased.
In option (iii) on increasing the volume of the container of hydrogen gas, more space will be available inside the container and hydrogen gas molecules will occupy all the space available and hence distance between the molecules will be increased.
In option (i) on increasing pressure, hydrogen molecules will come closer and the distance between them will be decreased.
In option (iv) more hydrogen gas molecules are available in less volume, so the distance between them will be decreased.

Short Answer Type Questions

Question 11:
A sample of water under study was found to boil at 102°C at normal temperature and pressure. Is the water pure? Will this water freeze at 0°C? Comment.
Answer:
No, the water is not pure. It is because, the boiling point of pure water is 100°C but the given sample boils at 102°C. It indicates that it has some dissolved impurities.
No, the water will not freeze at 0°C. Instead it will freeze below 0°C as it has impurities dissolved in it.

Question 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 would correctly represent the result? Justify your choice.

Answer:
Figure (d) would correctly represent the result. Because when heat is provided to the mixture of water and ice at 0°C, the ice absorbs this heat and converts it into the water at 0°C. During this period, there is no rise in temperature. On further heating, the temperature starts rising.
This is because, in ice (solid), the particles attract one another with strong forces. The heat which we supply to ice during melting is all used up to overcome the forces of attraction between ice particles, so that they become loose and form liquid water.
This heat does not increase the kinetic energy of particles and hence no rise in temperature takes place during the melting of ice. When all the ice has melted forming water, further heating increases the kinetic energy of water, due to which the temperature of water starts rising sharply.

Question 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 ……………
Answer:
(a) Evaporation of a liquid at room temperature leads to a cooling effect.
(b) At room temperature the forces of attraction between the particles of solid substances are stronger   than those which exist in the gaseous state.
(c) The arrangement of paritcles is less ordered in the liquid state. However, there is no order in the gaseous state.
(d) Sublimation is the change of gaseous state directly to solid state without going through the liquid   state.
(e) The phenomenon of change of a liquid into the gaseous state at any temperature below its boiling point is called evaporation.

Question 14:
Match the physical quantities given in Column A to their SI units given in Column B.

Answer:
The correct matching is

Question 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.

Answer:
The correct matching is

Question 16:
‘Osmosis is a special kind of diffusion’ Comment.
Answer:
Diffusion is the process in which molecules of a substance move from the place of their higher concentration to the place of their lower concentration (no membrane is required). But during osmosis, the solvent (water) molecules move from the place of their higher concentration to the place of their lower concentration through a semi-permeable membrane. Thus, osmosis is termed as a special kind of diffusion.

Question 17:
Classify the following into osmosis/diffusion.
(a) Swelling up of a raisin on keeping in water.
(b) Spreading of virus on sneezing.
(c) Earthworm dying on coming in contact with common salt.
(d) Shrinking of grapes kept in thick sugar syrup.
(e) Preserving pickles in salt.
(f) Spreading of smell of cake being baked through out the house.
(g) Aquatic animals using oxygen dissolved in water during respiration.
Answer:
Osmosis occurs in
(a) Swelling up of a raisin on keeping in water.
(c) Earthworm dying on coming in contact with common salt.
(d) Shrinking of grapes kept in thick sugar syrup.
(e) Preserving pickles in salt.
Diffusion occurs in
(b) Spreading of virus on sneezing.
(f) Spreading of smell of cake being baked throughout the house.
(g) Aquatic animals using oxygen dissolved in water during respiration.

Question 18:
Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain these observations.
Answer:
When ice melts, it absorbs the energy equal to the latent heat of fusion from the surroundings,therefore causes cooling effect. But steam releases the extra heat (equal to the latent heat of vaporisation) which it has absorbed when water was converted into steam. So, steam produces severe burn.

Question 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.
Answer:
The temperature of both boiling water and steam is 100°C, but steam gives out more heat (due to latent heat of vaporisation) in comparison to boiling water.
It is because, when water changes into steam, it absorbs latent heat of vaporisation, but when steam condenses to form water, an equal amount of latent heat is given out without changing the temperature.
Hence, she felt intense heat from puff of steam gushing out of the spout of the kettle.

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

Answer:
(a) Figure (a) represents the change of temperature with time correctly. The temperature of water first decreases up to zero degree celcius, then remains constant for some time (till the ice is formed), then again starts decreasing.

Question 21:
Look at the figure and suggest in which of the vessels A, B, C or D, the rate of evaporation will be the highest? Explain.

Answer:
The rate of evaporation will be highest in vessel C as the surface area exposed for evaporation is larger than 6 (smaller size). The moving fan increases the wind speed which also increases the rate of evaporation. Although A and D are also equal in size to that of C but A is at a greater distance from the fan and D is covered with a lid.

Question 22:
(a) Conversion of solid to vapour is called sublimation. Name the te’-m used to denote the conversion of vapour to solid.
(b) Conversion of solid state to liquid state is called fusion; what is meant by latent heat of fusion?
Answer:
(a) It is also called sublimation.
(b) The amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.

Long Answer Type Questions

Question 23:
You are provided with a mixture of naphthalene and ammonium chloride by your teacher. Suggest an activity to separate them with well labelled diagram
Answer:
Naphthalene is insoluble in water but soluble in benzene (organic solvent). Ammonium chloride is soluble in water but insoluble in benzene. Naphthalene changes into vapours at room temperature whereas ammonium chloride changes into vapours on heating.
Method
(a) Add water to the mixture and shake it vigorously to dissolve ammonium chloride.
(b) Filter the mixture. Naphthalene is obtained as residue whereas, filtrate contains ammonium    chloride. Crystallise the filtrate by heating till saturated solution of ammonium chloride is obtained.

Question 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?
Answer:
Priyanshi would be more comfortable. The reason is that we get a lot of sweat on our body in a hot summer day.
Cotton being a good absorber of water, absorbs sweat from the body and provides larger surface area for evaporation which causes more cooling effect. Nylon does not absorb sweat, so the sweat does not evaporate and Ali would feel uncomfortable.

Question 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?
Answer:
This can be done by the following (any one or more) steps
(i) Dry it under the fan Under the fan when the speed of wind increases, the particles of water vapour from the shirt move away with the wind, decreasing the amount of water vapour in the surroundings. It increases the rate of evaporation of water and the washed wet shirt dries more quickly.
(ii) Use hanger for exposing its larger surface to air When shirt is spread, it has large surface area. This makes the evaporation of water faster. Due to this, shirt dries up more quickly.
(iii) Dry it in Sun On sunny day, temperature of atmosphere is more, therefore, the rate of evaporation increases. It is because, on increasing temperature, more particles of water get enough kinetic energy to go into the vapour state. This increases the rate of evaporation.
(iv) Use iron By the use of iron, temperature increases and water converts into vapour state (steam)  very quickly. Hence, rate of evaporation increases and shirt dries very quickly.

Question 26:
Comment on the following statements.
(a) Evaporation produces cooling.
(b) Rate of evaporation of an aqueous solution decreases with increase in humidity.
(c) Sponge though compressible is a solid.
Answer:
(a) Evaporation produces cooling. This is based on the fact that when a liquid evaporates, it takes (or draws) the latent heat of vaporisation from ‘another matter’ which it touches. This ‘another matter’ looses heat and gets cooled. Therefore, evaporation causes cooling.
(b) If humidity is high, then air is already saturated with water vapours, i.e., it has a lot of water vapours. Therefore, it will not take more water vapours easily. Hence, rate of evaporation decreases.
(c) Sponge has minute holes in which air is trapped. The material is also not so rigid. On pressing this,  air is expelled out, that is why it can be compressed but it is solid as it has a definite shape and volume and does not change its shape unless compressed

Question 27:
Why does the temperature of a substance remain constant during its melting point or boiling point?
Answer:
The temperature of a substance remains constant during melting and boiling points till the completion of melting and boiling. It is because substance makes use of latent heat of fusion to overcome force of attraction between particles of solid to change into liquid during melting and latent heat of vaporisation to overcome force of attraction between particles of liquid to change into vapours during boiling.
Therefore, temperature remains constant.

NCERT Exemplar Class 9 Science Solutions