Science PDF - Rationalised 2023-24

What happens when you pour some acetone (nail polish remover) on your palm? The particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool. After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of w...

What happens when you pour some acetone (nail polish remover) on your palm? The particles gain energy from your palm or surroundings and evaporate causing the palm to feel cool. After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the hot surface. Can you cite some more examples from daily life where we can feel the effect of cooling due to evaporation? Why should we wear cotton clothes in summer? During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation. Why do we see water droplets on the outer surface of a glass containing ice-cold water? Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets. Q uestions 1. Why does a desert cooler cool better on a hot dry day? 2. How does the water kept in an earthen pot (matka) become cool during summer? 3. Why does our palm feel cold when we put some acetone or petrol or perfume on it? 4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup? 5. What type of clothes should we wear in summer? What you have learnt Matter is made up of small particles. The matter around us exists in three states—solid, liquid and gas. The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases. The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases. 10 SCIENCE Rationalised 2023-24