While making dosa, we sprinkle some water on the hot pan and it disappears. Where does it go?
Answer
The water changes into steam and goes into the air. As the book puts it — the water which is sprinkled on the hot pan gets converted into steam. Steam is actually water vapour, some part of which converts into water droplets.
Step by step:
The tawa is very hot — far above 100 °C.
The moment the drops touch it they take in a great deal of heat and change into vapour almost at once, with a hissing sound.
Just above the tawa the vapour meets cooler air, and part of it turns back into tiny droplets. This is the white cloud you see rising.
Higher up these droplets evaporate again and the vapour mixes with the air in the kitchen.
Do you know?Water vapour is actually invisible but the presence of tiny droplets of water in the steam makes it visible. So the white plume you see is not vapour at all — it is a mist of very small water droplets, which is already condensation happening in front of your eyes.
Why the cook does it: sprinkling water tells the cook whether the tawa is hot enough — if the drops skitter and vanish instantly, the tawa is ready — and the vapour also carries away some heat, cooling the tawa a little so the dosa batter spreads evenly instead of sticking.
Q2.
The process of evaporation takes place continuously, even at room temperature. Can you think of other examples of evaporation?
Answer
The book gives three — drying of wet clothes, a mopped floor drying, and sweat on our body drying. Here are many more from an Indian home:
Where you see it
What is evaporating
Clothes on the line, a wet towel on a hook
Water from the cloth
Wet floor after mopping, a washed verandah
Water from the floor
Sweat drying under a fan
Water from the skin
Washed steel plates left on the rack
Water on the metal
Papads, badis, mango slices and red chillies drying on the terrace
Water inside the food
Water level in an open bucket or a birdbath falling
Water from the open surface
Wet hair drying after a bath
Water from the hair
Salt pans of Gujarat and Tamil Nadu
Sea water — the salt is left behind
Water in a matka going down over a week
Water seeping through the pores and evaporating
The common thread: in every case a liquid surface is open to the air and the temperature is ordinary. No flame, no 100 °C — just the constant, quiet escape of the fastest particles from the surface.
Q3.
Now what do you think is the reason for the disappearance of water from the puddles? Is it due to: (i) seeping of water into the ground or (ii) evaporation of water or (iii) both of these?
Answer
(iii) Both of these.
The playground has a soil surface, so both routes are open to the water:
Seeping (percolation): water sinks into the spaces between the soil grains and moves down to become groundwater.
Evaporation: water at the open surface of the puddle changes into vapour and mixes with the air. The Sun heating the ground makes this fast.
Why we cannot pick only one: Activity 8.2 showed that water evaporates even from a surface it cannot seep through, so evaporation is certainly happening in the playground too. At the same time, everyone has seen water sink into dry soil, so seeping is happening as well. When two causes can both operate, the honest answer is that both are at work — and their share differs from place to place. On a cemented ground only evaporation is possible; on loose sandy ground seeping does most of the work.
Check it yourself: After rain, mark the edge of one puddle on soil and one on a cemented floor with chalk. Note how long each takes to dry. The soil puddle usually goes first, because it has two exits instead of one.
Q4.
Hand sanitiser disappears as you rub it on your hands. What happens to it?
Answer
The sanitiser evaporates — it changes from the liquid state into vapour and mixes with the air. It is not absorbed by the skin and it is not "used up".
Two things make it disappear so quickly:
Sanitiser is mostly alcohol (spirit), and alcohol evaporates much faster than water at the same temperature.
Rubbing spreads it into a very thin film over both palms and fingers, so the exposed area becomes large — and a larger exposed area always means faster evaporation (Activity 8.7).
Why your hands feel cold: to change into vapour, the liquid needs heat, and it takes that heat from your skin. Your palm loses heat and feels cool. This is the same cooling effect that makes a matka keep water cold and makes sweating cool our body — you will meet it again in Section 8.6.
Try This: Put one drop of sanitiser on the back of one hand and one drop of water on the other. Blow gently on both. The sanitiser goes first and feels colder. Different liquids evaporate at different rates.