Topic outline

• Matter in Our Surroundings

Q61. State any two characteristic properties of a gas.OrWhat are the main characteristics of a gas?OrWhat are the characteristics of gases?

Ans. Characteristics of gases

a. Gases are highly compressible as compared to solids and liquids.

b. Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.

Q62. Convert the following temperatures to the Celsius scale.(a) 300 K      (b) 573 K

Ans. To convert a temperature on Kelvin scale to the Celsius scale, we have to subtract 273 from the Kelvin temperature.

(a) 300 K = (300-273)°C = 27°C

(b) 573 K = (573-273)°C = 300°C

Q63. Convert the following temperatures to the Kelvin scale.(a) 25°C        (b) 373°C.

Ans. To convert a temperature on Celsius scale to the Kelvin scale, we have to add 273 to the Celsius temperature.

(a) 25°C = (25+273) K = 298 K

(b) 373°C = (373+273) K = 646 K

Q64. What produces more severe burns, boiling water or steam?

Ans. The burns caused by steam are more severe than those caused by boiling water though both of them are at the same temperature of 100°C. This happens because steam contains more heat, in the form of latent heat, than boiling water.

Q65. We come to know of what is being cooked in the kitchen without even entering there, by the smell that reaches our nostrils. How does this smell reach us?

Ans. The particles of the aroma of food mix with the particles of air spread from the kitchen, reach us and even farther away. Due to high speed of particles and large space between them, gases show the property of diffusing very fast into other gases.

Q66. Give reasons for the following observation:The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.

Ans. The rate of diffusion of hot gases (released by the hot sizzling food) is much faster than the rate of diffusion of cold gases released by the cold food. Thus, the smell of hot sizzling food reaches us several metres away, but to get the smell from cold food we have to go close to it.

Q67. Why is ice at 273 K more effective in cooling than water at the same temperature?

Ans. 0°C = 273 K

Ice at 273 K more effective in cooling than water at the same temperature because ice at 273 K takes its latent heat from the substance and hence cool the substance more effectively. On other hand, water at 273 K cannot take any such latent heat from the substance.

Q68. Why does a desert cooler cool better on a hot dry day?

Ans. The outer walls of the cooler get sprinkled by water constantly. The higher temperature and low humidity of air on a hot day increases the rate of evaporation of water. And due to increased rate of evaporation of water, a desert room cooler cools better on a hot and dry day.

Q69. How does the water kept in an earthen pot (matka) become cool during summer?

Ans. The earthen pot has lot of pores on it; the water oozes out through these pores. This water evaporates continuously and takes the latent heat required for vaporization from the earthen pot and the remaining water. In this way remaining water loses heat and gets cooled.

Q70. Why is heat energy needed to melt a solid? What is this heat energy called?

Ans. The heat energy supplied gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by solid without showing any rise in temperature, it is considered as hidden is known as the latent heat.

Q71. 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. This is due to the fact that to change from the liquid to the vapour state, it requires latent heat of vaporization. It takes this heat of vaporization from our palm. The palm loses heat and gets cooled.

Q72. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Ans. Saucer has a larger surface area than a cup. Due to the large surface area of saucer, the evaporation of hot tea (or milk) from the saucer is faster. The faster evaporation cools the hot tea (or milk) more quickly and thus, we are able to sip hot tea or milk faster from a saucer rather than a cup.