Question 8: Give reasons
(a) A gas fills completely the vessel in which it is kept.
Answer: The force of attraction between particles of gas is negligible. Because of this, particles of gas move in all directions. Thus, a gas fills the vessel completely in which it is kept.
(b) A gas exerts pressure on the walls of the container.
Answer: Because of negligible force of attraction between particles of gas, the particles of gas have the highest kinetic energy. These properties enable the particles of gas to move in all directions and hit the walls of container from all sides. Because of this a gas exerts pressure on the walls of the container in which it is kept.
(c) A wooden table should be called a solid.
Answer: A wooden table has fixed shape and fixed volume, which are the main characteristics of solid. Thus a wooden table should be called a solid.
(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Answer: Since, air is gas, so its particles are loosely packed and there is negligible force of attraction between its particles. Because of that we can easily move our hand in air.
But wood is a solid, so the force of attraction between its particles is greatest. The particles of wooden block are closely packed. That’s why we cannot move our hand through a solid block of wood. However a karate expert can exert required pressure to break the great force of attraction of the particles of a solid wooden block.
Question 9: Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer: During freezing of water, some space between the particles of water is left vacant with some air trapped between them. These empty spaces having air in them makes the density of ice; lower than that of water. That’s why ice floats on water.
Question 10: Convert the following temperature to Celsius scale.
(a)300 K (b) 573 K
Answer: (a) Temperature on Kelvin scale = Temperature on Celsius scale + 273
Temperature on Celsius scale = Temperature of Kelvin scale – 273
Hence, Temperature on Celsius scale = 300K – 273 = 27⁰C
Answer: (b) Temperature on Kelvin scale = Temperature on Celsius scale + 273
Therefore, Temperature on Celsius scale = Temperature of Kelvin scale – 273
Hence, Temperature on Celsius scale = 573K – 273 = 300⁰C
Question 11: What is the physical state of water at
(a) 250⁰C (b) 100⁰C
Answer: (a) Since, water boils at 100⁰C, thus it is in gaseous state at 250⁰C.
Answer: (b) Since, water boils at 100⁰C, thus at 100⁰C water is in transition phase, i.e. in both liquid and gaseous states.
Question 12: For any substance why does the temperature remain constant during the change of state.
Answer: During the change of state of any substance, the heat supplied or released is utilised in phase change. Such heat is called latent heat. So, the temperature of any substance remains constant during the change of state.
Question 13: Suggest a method to liquefy atmospheric gas.
Answer: Atmospheric gas is liquefied by increasing pressure and decreasing temperature.
Question 14: Why does a desert cooler cool better on a hot dry day?
Answer: Desert cooler works on the basis of evaporation. In hot and dry days the moisture level is very low in atmosphere which increases the rate of evaporation. Because of faster evaporation, cooler works well. That’s why desert cooler cool better on a hot dry day.
Question 15: How does the water kept in an earthen pot (matka) become cool during summer?
Answer: Water from porous wall of earthen pot evaporates continuously, which lowers the temperature of water kept in the earthen pot.
In summer moisture level is very low in the atmosphere, which increases the rate of evaporation as evaporation is inversely proportional to the moisture level in atmosphere. That is why in summer water kept in earthen pot becomes cool.
Question 16: Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer: When acetone or petrol or perfume is poured over palm, it evaporates quickly as these are volatile liquids. The evaporation lowers the temperature of palm and our palm feels cold.
Question 17: Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer: When hot tea or milk is kept in a saucer, the liquid is exposed over a larger surface area as compared to in case of the liquid being kept in a cup. The larger surface area enables the faster cooling. That’s why we are able to sip hot tea or milk faster from a saucer rather than from a cup.
Question 18: What type of clothes should we wear in summer?
Answer: We should wear cotton cloths in summer. Because cotton cloths are good absorbent and they absorb sweat excreted in summer. Because of evaporation of sweat from the surface of cotton cloth, we feel colder and comfortable.
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