question_answer

  Comment on the following statements: (a) Evaporation causes cooling. (b) Rate of evaporation of an aqueous solution decreases with increase in humidity. (c) Sponge though compressible is a solid,                           (d) Ice is solid at 0°C, while water is liquid at room temperature.       (e) Sugar crystals dissolve faster in hot water than cold water.                        [NCERT Exemplar]

Answer:

                                                      (a) Evaporation produces cooling as the particles at the surface of the liquid gain energy from the surroundings and change into vapour, thereby producing a cooling effect. (b) Air around us cannot hold more than a definite amount of water vapour al a given temperature which is known as humidity. So, if the air is already rich in water vapour, it will not take up more water; therefore, rate of evaporation of water will decrease. (c) A sponge has minute holes in which air is trapped. Also the material is not rigid. When we press it, the air is expelled out and we are able to compress it. (d) Ice is solid at 0°C because it has a definite volume and definite shape due to strong intermolecular forces. Water is liquid at room temperature because it has definite volume and no definite shape due to weak intermolecular forces of attraction. (e) Sugar crystals dissolve faster in hot water than cold water because hot water molecules contain more kinetic energy. Due to this, they strike faster on the particles of sugar than cold water molecules. As a result, hot water will