NCERT Exemplar for Class 9 Science Chapter 1 - Matter in Our Surroundings (Book Solutions)

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: 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. 

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). 

Increase in temperature increases the kinetic energy of gas particles which causes the gases to expand when temperature is increased. 

Solubility of solute particles is also increased by increasing the temperature.

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 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 intermolecular forces of attraction which reduces the intermolecular 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. 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 - 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. 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. 

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. On converting \[{25^ \circ }C\], \[{38^ \circ }C\] and \[{66^ \circ }C\] to kelvin scale, the correct sequence of temperature will be

(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, \[{25^ \circ }C + 273 = 298K\], \[{38^ \circ }C + 273 = 311K\],  and \[{66^ \circ }C + 273 = 339K\].

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. The boiling points of diethyl ether, acetone and n-butyl alcohol are \[{35^ \circ }C\], \[{56^ \circ }C\] and \[{118^ \circ }C\] respectively. Which of the following sets 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, \[{35^ \circ }C + 273 = 308K\], \[{56^ \circ }C + 273 = 329K\], and \[{118^ \circ }C + 273 = 391K\].

9. Which condition out of the following 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 an 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. In which of the following conditions, the distance between the molecules of hydrogen gas would increase?

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

(b) Leaking some hydrogen out of the container. 

(c) Increasing the volume of the container. 

(d) 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 \[{102^ \circ }C\] at normal pressure. Is the water pure? Will this water freeze at \[{0^ \circ }C\]? Comment. 

Ans: The boiling point of pure water at $1$ atm is \[{100^ \circ }C\]. The sample of water provided to us boils at \[{102^ \circ }C\]. Therefore, the sample provided to us is not pure water. The freezing point of pure water is \[{0^ \circ }C\]. But the sample is not pure. So, it will not freeze at \[{0^ \circ }C\]. It will freeze at a temperature below its freezing point, \[{0^ \circ }C\].

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 the slope of the graph (increasing temperature).

13. Fill in the blanks:- 

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

Ans: cooling

(b) At room temperature, the forces of attraction between the particles of solid substances are _______ than those which exist in the gaseous state. 

Ans: stronger 

(c) The arrangement of particles is less ordered in the _______ state. However, there is no order in the _______ state. 

Ans: liquid, gaseous

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

Ans: sublimation, liquid 

(e) The phenomenon of change of a liquid into the gaseous state at any temperature below its boiling point is called _______. 

Ans: evaporation 

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

Ans: 

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: 

Ans: 

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

Ans: Both the processes, osmosis and diffusion have a lot in common. In both phenomena, there is movement of particles to regions 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 semipermeable 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 semipermeable 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, ${100^ \circ }C$. 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. ${0^ \circ }C$. At the freezing point, the ice and water coexist at equilibrium. The temperature will remain constant till all the water freezes to ice. After this point, the temperature would decrease again. 

21. Look at Fig. 1.3 and suggest in which of the vessels A,B, C or D the rate of evaporation will be the highest? 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. 

Long Answer Questions

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. 

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:- 

1. 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 hang line, the rate of evaporation will increase. 

2. 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. 

3. 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 the 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.