Cooling Effect
Class 6 Science · Chapter 8: A Journey through States of Water · NCERT Curiosity section 8.6
Learn
Put a drop of water on the back of your hand and blow on it. Your hand feels cool. Evaporation needs heat, and it takes that heat from your skin.
That is why sweating cools us on a hot day, and why sitting under a fan feels good when we are sweaty. An earthen matka or surahi keeps water cool: water seeps through its tiny pores and evaporates from the outside, cooling the pot.
Trees give off water vapour from their leaves, which cools the air around them. That's one reason a tree-lined street feels cooler than a bare one on a summer afternoon.
Explore: does it cool by evaporation?
- Sort each one.
Remember
- Evaporation causes cooling: it takes heat from the surface.
- Sweating, matkas and trees all cool by evaporation.
Quick check
Why does water in a clay pot stay cool in summer?
- Water seeps out and evaporates, taking heat away
- Clay makes ice
- The pot is kept in the sun
- Clay stops all heat
Answer
Water seeps out and evaporates, taking heat away. Evaporation causes cooling.
Think about it
Why do we feel more uncomfortable on a hot, humid day than on a hot, dry day?
One answer
When the air is humid, sweat evaporates slowly, so it does not cool us well. On a dry day sweat evaporates quickly and cools the skin.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.