NCERT Exemplar Solutions 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

Solution 1: (c) Evaporation, diffusion, expansion of gases. 

Gas evaporation, diffusion, and expansion increase as the temperature increases. The rate of evaporation increases since the kinetic energy of molecules increases with increasing temperature, so that the molecules present on the surface of the liquid leave the surface rapidly and join the vapor state. As the molecules move more quickly and try to fill more space, diffusion and expansion of gases also increase.

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.

Solution 2: (c) Low temperature, high pressure.

The liquefaction of gases into liquids requires a low temperature and high pressure. The particles of a gas have a lot of space between them. The particles of gas pass so closely when high pressure is applied that they begin to attract each other enough to form a liquid. When too much gas is compressed, heat is generated, so cooling is important. Cooling reduces the compressed gas temperature and assists in its liquefaction. Thus, by applying high pressure and reducing the temperature, a gas may be liquefied (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

Solution 3: (c) Gases and liquids behave like fluids. 

Liquids and gases behave like fluids. Owing to the lower attraction force between their particles, both gases and liquids appear to flow. They will need vessels to carry 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

Solution 4: (d) evaporation. 

Because of the evaporation phenomenon, water kept in an earthen pot becomes cool during summer. There are a large number of tiny pores in the earthen pot in its walls and some of the water molecules continually penetrate outside the pot through these pores.

This water evaporates continuously and extracts from the remaining water the latent heat needed for vaporization. The remaining water loses heat and gets cooled in this way.

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 

Solution 5: (c) Oxygen, water, sugar. 

Oxygen < Water < Sugar is the right order of the increasing 'attraction force' between their particles.

That is because the attraction force increases in the following 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 

Solution 6: (a) 298 K, 311 K and 339 K. 

We get the following temperatures when translating 25 ° C, 38 ° C and 66 ° C to the Kelvin scale:

25°C + 273 = 298 K => 38°C + 273 = 311 K => 66°C + 273 = 339 K.

The right temperature series would therefore be 298 K, 311 K, and 339 K respectively.

Question 7:  Choose the correct statement of the following.
(a) Conversion of solid into vapors without passing through the liquid state is called vaporization.
(b) Conversion of vapors into solid without passing through the liquid state is called sublimation.
(c) Conversion of vapors into solid without passing through the liquid state is called freezing.
(d) Conversion of solid into liquid is called sublimation.

Solution 7: (b) Conversion of vapors into solid without passing through the liquid state is called sublimation. 

Sublimation is the so-called conversion of solids into heating vapors or vapors into solids during cooling without liquidation.

Vaporization is called the conversion of liquid into gas (vapor).

Freezing is called the conversion of liquid to solid.

Melting is called the conversion of a solid into a liquid.

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

Solution 8: (c) 308 K, 329 K, 391 K. 

The right order of boiling points of diethyl ether, acetone and n-butyl alcohol on a kelvin scale is

308 K, 329 K, 391 K, which can be defined as (v T°C + 273 = 7K).

Diethyl ether boiling point = 35 ° C + 273 = 308 K.

Boiling point of acetone = 56 °C + 273 = 329 K.

N-butyl alcohol boiling point = 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

Solution 9:  (a) Increase in temperature of water. 

The rise in water temperature will increase water evaporation. This is because the kinetic energy of water molecules increases as the temperature increases, and more particles get enough kinetic energy to reach the vapor state. This boosts the evaporation rate. The surface phenomenon is evaporation, because it relies on the area of the exposed surface. If common salt is applied to water, the solvent as well as non-volatile solute particles inhabit the surface. Therefore, the escape tendency of solvent particles decreases and thereby decreases the evaporation of water. Other choices (b), (c) and (d) would, thus, limit water evaporation.

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)

Solution 10:   (c) (ii) and (iii).

In choice (ii), hydrogen gas escaping from the container leaves some empty space inside the container. So, inside the container, hydrogen gas molecules consume all the available space and the distance between the hydrogen gas molecules will be increased.

In choice (iii), to increase the volume of the hydrogen gas container, more space will be available within the container and all available space will be filled by hydrogen gas molecules and the distance between the molecules will also be increased.

If choice (i) raises the strain, the molecules of hydrogen will come closer and the distance between them will be reduced.

Choice (iv) provides that more hydrogen gas molecules are available 

in smaller amounts, so the distance between them will be reduced.

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.

Solution 11:  No, it is not pure water. This is because the pure water's boiling point is 100 ° C, but the sample boils at 102 ° C. It reveals there are some dissolved impurities in it. No, at 0°C the water will not freeze. Instead, it can freeze below 0 ° C because it has dissolved impurities 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.

Solution 12:  Figure (d) will reflect the outcome correctly. Since the ice absorbs this heat and transforms it into water at 0°C when heat is supplied to the mixture of water and ice at 0°C. During this time, the temperature does not increase. The temperature continues to increase as further heating takes place.

This is because the particles attract each other with strong forces in the ice (solid). During melting, the heat we supply to the ice is all used to resolve the attraction forces between ice particles, so that they become loose and form liquid water.

This heat does not increase particle kinetic energy and thus no temperature rise occurs during ice melting. Further heating increases the kinetic energy of water until all the ice has melted to form water, because of which the water temperature continues to rise 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 ……………

Solution 13:

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 particles 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 16:  ‘Osmosis is a special kind of diffusion’ Comment.

Solution 16:    Diffusion is the mechanism in which a substance's molecules shift from their maximum concentration to their lowest concentration (no membrane is required). During osmosis, however, the solvent (water) molecules pass through a semi-permeable membrane from the position of their greater concentration to the place of their lower concentration. Osmosis is thus referred to as a special form 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 throughout the house.
(g) Aquatic animals using oxygen dissolved in water during respiration.

Solution 17:  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.

Solution 18:   As ice melts, it absorbs energy from the atmosphere equal to the latent heat of fusion, thereby creating a cooling effect. But steam releases the extra heat that it has absorbed as water is converted into steam (equal to the latent vaporization heat). So, steam causes a serious fire. 

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.

Solution 19:   The temperature of both boiling water and steam is 100°C, but in contrast to boiling water, steam gives out more heat (due to latent vaporization heat). This is because it absorbs latent vaporization heat as water transforms into steam, so when steam condenses to form water, without changing the temperature, an equivalent amount of latent heat is released.

Thus, from the puff of steam, she felt strong heat 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 

Solution 20: (a) Figure (a) accurately reflects the change in temperature over time. First, the water temperature decreases to zero degrees Celsius, then stays steady for some time (until the ice is formed), then begins to decrease again.

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.

Solution 21:   In vessel C, the evaporation rate would be highest as the surface area exposed for evaporation is greater than 6 (smaller size). The moving fan boosts the speed of the wind, which also raises the evaporation rate. While A and D are both similar in size to C, A is at a greater distance from the fan and a lid covers D.

Question 22:

a)      Name the term used to denote the conversion of vapor to solid.

b)      Conversion of solid state to liquid state is called fusion; what is meant by latent heat of fusion?

Solution 22: 

a)      Vapor to solid conversion is known as sublimation.

b)      The amount of heat energy needed to turn 1 kg of solid into liquid at its melting point at atmospheric pressure is known as latent fusion heat.

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 labeled diagram. 

Solution 23: Naphthalene is water-insoluble but benzene-soluble (organic solvent). Ammonium chloride is water-soluble but benzene-insoluble. At room temperature, naphthalene transforms into vapors, while ammonium chloride changes when heated into vapors.

Methodology:-

a)      To dissolve ammonium chloride, apply water to the mixture and vigorously shake it.

b)      Filter the blend. As a residual, naphthalene is obtained, while filtrates contain ammonium chloride. Crystallize the filtrate before the saturated ammonium chloride solution is obtained by heating.

c)       To get crystals of ammonium chloride, cool the hot saturated solution.

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?

Solution 24:   Priyanshi will be more convenient. The explanation is that on a hot summer day, we get a lot of sweat on our bodies.

Cotton, a strong water absorber, absorbs moisture from the body and provides a greater evaporation surface area that causes more cooling results. Sweat is not dissolved by nylon, so the sweat does not evaporate and Ali will feel uncomfortable.

Question 25:  You want to wear your favorite 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?

Solution 25: This can be achieved by taking the following steps (any one or more).

Dry it under a fan, as the wind speed rises under a fan, the water vapor particles from the shirt travel away with the wind, minimizing the amount of water vapor in the atmosphere. It raises the rate of water evaporation, and the wet washed shirt dries faster.

Use the hanger to expose the wider surface to air. It has a wide surface area when the shirt is spread. This makes water evaporation quicker. Because of this, the shirt dries off quicker.

Dry it in the Sun, the temperature of the atmosphere on a sunny day is higher, so the rate of evaporation increases. That is because more particles of water get enough kinetic energy to go into the vapor state as the temperature increases. This boosts the evaporation rate.

Use iron, by the use of iron, the temperature increases and water very easily transforms into a vapor state (steam). Therefore, the evaporation rate rises and the shirt dries very rapidly.

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.

Solution 26:

a)      Cooling is provided by evaporation. This is based on the fact that it takes (or draws) the latent heat of vaporization from 'another matter' which it contacts when a liquid evaporates. This 'other matter' is losing heat and cooling down. Evaporation thus induces cooling.

b)      The air is already filled with water vapors if the humidity is high, i.e. it has a lot of water vapors. Therefore, more water vapors cannot be quickly extracted. Therefore, the evaporation rate is declining.

c)       The sponge has minute holes that are stuck in the air. Often, the material is not so rigid. Air is released when squeezed, which is why it can be compressed, but it is strong as it has a definite form and volume and does not alter its shape until compressed.

Question 27:  Why does the temperature of a substance remain constant during its melting point or boiling point?

Solution 27:   During the melting and boiling points, the temperature of a substance remains constant until the end of melting and boiling. This is because the material uses latent fusion heat to overcome the attraction force between solid particles to turn into liquid during melting and latent vaporization heat to overcome the attraction force between liquid particles to change into vapors during boiling. The temperature, therefore, remains constant.