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class nine science chap 1 NCERT solutions part 2

Chapter 1: Matter in Our Surroundings

PAGE 9

1. Convert the following temperature to Celsius scale: (a) 300 K (b) 573 K.

Ans. Temperature in °C = Temperature in Kelvin – 273.

(a) Temperature in °C = (300 – 273) = 27°C

(b) Temperature in °C = (573 – 273) = 300°C

2. What is the physical state of water at: (a) 250°C (b) 100°C?

Ans. (a) At 250 °C water is in the form of a gas i.e., steam.

(b) At 100 °C, water is in the liquid state. However, if heat energy is supplied at this stage, the water will start changing into a gaseous state (steam), but its temperature will remain 100°C.

3. For any substance, why does the temperature remain constant during the change of state?

Ans. Temperature represents the average kinetic energy of the molecules of a substance. At the melting point or boiling point of a substance, all the heat energy supplied is utilised in decreasing the intermolecular forces and hence the change of state takes place from solid to liquid or liquid to gas. However the kinetic energy and the average kinetic energy of the molecules do not increase, therefore, the temperature remains constant during the change of state.

4. Suggest a method to liquefy atmospheric gases.

Ans. Gases can be liquefied by applying high pressure and reducing its temperature or by cooling and compression.

PAGE 10

1. Why does a desert cooler cool better on a hot dry day?

Ans. Higher is the temperature of the surroundings, more is the rate of evaporation and hence, more cooling is produced on a hot dry day. As the hot air passes over wet Khas, rapid evaporation takes place. This results in a large temperature fall and hence, a desert cooler cools better.

2. How does the water keep in an earthen pot (matka) become cool during summer?

Ans. As the water oozes out of the pores of the earthen pot, it evaporates, partly taking heat energy away from the water within the pot. This continues and the temperature of water within pot falls hence, water becomes cool.

3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Ans. Our palm feels cold when we put some acetone or petrol or perfume on it because the particles of acetone or petrol or perfume gain heat energy from our palm or surroundings and evaporate quickly causing a cooling effect.

4. Why are we able to sip hot tea or milk faster from a saucer than from a cup?

Ans. We are able to sip hot tea or milk faster from a saucer than from a cup because a saucer provides more surface area than a cup for evaporation of liquid into vapour and in turn causes faster cooling.

5. What type of clothes should we wear in summers?

Ans. During the summer, we should use cotton clothes as we sweat more. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation and in turn, making our body cool.

The solution to Textbook Exercises

1. Convert the following temperatures to Celsius scale.

(a) 293 K (b) 470 K

Ans. (a) 293 – 273 = 20°C (b) 470–273 = 197°C

2. Convert the following temperatures to Kelvin scale.

(a) 25°C (b) 373 °C.

Ans. Temperature in Kelvin = Temperature in °C + 273

(a) Temperature in Kelvin = (25 + 273) = 298 K.

(b) Temperature in Kelvin = (373 + 273) = 646 K.

3. Give a reason for the following observations:

(a) Naphthalene balls disappear without leaving any solid.

(b) We can get the smell of perfume sitting several metres away.

Ans. (a) Naphthalene slowly sublimes to its gaseous form by absorbing heat energy from the

surroundings. Thus, solid naphthalene just disappears without any trace.

(b) Perfume is a volatile liquid. As its molecules enter the air, they diffuse rapidly in the air in all directions. Thus, we can smell the perfume sitting several metres away.

4. Arrange the following substances in the increasing order of forces of attraction between the

particles — water, sugar and oxygen.

Ans. Oxygen, (gas) < water (liquid) < sugar (solid).

5. What is the physical state of water at :

(a) 25°C (b) 0°C (c) 100°C

Ans. (a) At 25°C, water is in the liquid state.

(b) At 0°C, water is in the solid and liquid state.

(c) At 100°C, water is in the gaseous and liquid state.

6. Give two reasons to justify.

(a) Water at room temperature is a liquid.

(b) An iron almirah is a solid at room temperature.

Ans. (a) (i) Intermolecular forces between the particles of water are less so the molecules of water are bound to each other because of which it is a liquid.

(ii) Intermolecular spaces and kinetic energy of the particles are more.

Thus, the molecules of water can interchange their spaces and hence, water is in the liquid state at room temperature.

(b) (i) Intermolecular forces of the particles of iron are very large.

(ii) Intermolecular spaces, as well as the kinetic energy of the particles of iron, are very small. Thus, the molecules are held very, very tightly with the result that an iron almirah has a definite shape and definite volume and hence it is solid.

7. Why is ice at 273 K more effective in cooling than water at the same temperature?

Ans. Ice absorbs more amount of heat at 273 K due to its latent heat of fusion. Thus, ice at 273 K causes more cooling effect than water at the same temperature.

8 What produces more severe burns, boiling water or steam?

Ans. Steam will produce more severe burns. It is because 1 g of steam at 373 K (100 °C) contains 2260 J of more heat energy in the form of latent heat of vaporisation as compared to water at 373 K (100 °C). Thus, steam produces more severe burns.

9. Name A, B, C, D, E and F in the following diagram showing a change in its state.

Ans. A: Solid changes into a liquid state by the process of melting or fusion.

B: Liquid changes into a gaseous state by the process of boiling or vaporisation.

C: A gas changes into a liquid state by the process of liquefaction.

D: A liquid changes into a solid-state by the process of freezing or solidification.

E: Solid changes into a gaseous state by the process of sublimation.

F: Gas change into a solid-state by the process of solidification.
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