A Journey through States of Water · Lesson 6 of 8
Cooling Through Evaporation
“Explain why wet skin and earthen pots become cool, then design a simple pot-in-pot cooler.”
• Explain the cooling effect when liquid evaporates from a surface. • Connect cooling to sweat, moving air, sprinkling water, and porous earthen pots. • Describe how to build and observe a pot-in-pot cooler. • Identify conditions that could affect a cooler’s performance and food freshness.
What happens to the surface as water leaves?
Wet one hand and leave the other dry. Blow air over both. The wet hand often feels cooler because some of the water evaporates. Evaporation requires energy; it can take heat from the water and its nearby surroundings, including your skin. The surface loses heat and can feel cooler. This does not mean that every wind is cold or that evaporation creates coldness from nothing.
Cooling of a surface or its surroundings as a liquid evaporates and takes in heat.
Sweat cools us when it evaporates from the skin. A fan moves moist air away and can help sweat evaporate faster, so a person may feel cooler. Rubbing sanitiser on skin can give a similar cool feeling as some of its liquid ingredients evaporate; sanitiser is not simply water. Sprinkling water on a hot roof or floor can also cool its surface as the water evaporates.
Problem
One hand is wet, the other dry. You blow air across both. Explain the difference in feeling.
- 1.The wet hand has liquid on its surface that can evaporate.
- 2.Moving air can help that liquid evaporate more quickly.
- 3.Evaporation takes in heat from the nearby skin, so the wet hand may feel cooler.
Why an earthen pot cools its water
A porous earthen pot, or matka, lets a little water reach its outer surface. That water evaporates, taking in heat from the pot and its contents. Water stored inside can therefore become cooler. An ordinary stainless-steel pot does not let water seep to its outside in the same way. A narrow-necked surahi uses the same principle. Compare this with the earlier cold glass: droplets on that glass formed by condensation *from air*, whereas moisture on an earthen pot can come through the porous clay *from inside*.
Make and investigate a pot-in-pot cooler
Place a smaller earthen pot inside a larger one, with a layer of sand beneath and sand filling the space between them. Add water to make the sand moist, then cover the inner pot with a lid or wet jute sack. Allow several hours for cooling. A useful observation is to compare the inner temperature with an appropriate container kept under similar conditions. Add water regularly so the sand remains moist. Handle heavy or breakable pots with care.
The chapter suggests placing fruit or vegetables inside and observing their freshness daily for a week. Record what is stored, how often water is added, the weather, and the daily observations. Different air movement, humidity, starting temperature, coverings, and sand moisture can change the outcome. Suggest and test another safe material in place of sand if appropriate. The cooler can help, but do not assume that every food stays safe for a fixed number of days.
| Feature | Role in the model |
|---|---|
| Moist sand between pots | Provides water for evaporation near the inner pot |
| Outer earthen pot | Allows water to reach an exposed surface |
| Lid or wet jute covering | Shades and covers the inner opening; wet fabric can also evaporate |
| Regular observations | Show whether cooling and freshness persist over time |
If the sand dries out, evaporation and cooling decrease. Very humid air can also slow evaporation. A pot-in-pot cooler is not a substitute for temperature-controlled food storage when that is needed.
Quiz
Why can a wet hand feel cooler when air blows over it?
Where does the water evaporating from an earthen matka chiefly come from?
Why can a fan make sweating more effective for cooling?
What should be maintained in the pot-in-pot model?
Which comparison best distinguishes a cold glass from a matka?
Practice Problems
- Explain the wet-hand and dry-hand activity in terms of evaporation and heat.
- Why does water in an earthen matka often feel cooler than water in a steel pot?
- Draw and label the large pot, small pot, wet sand, and cover of a pot-in-pot cooler.
- Design a daily table for observing vegetables inside a pot-in-pot cooler for one week.
- Predict what happens if the sand is allowed to dry out; justify your answer.
- Compare the source of drops on a cold glass with the source of moisture on the outer surface of a matka.
Key Takeaways
• Evaporation takes in heat from nearby water and surfaces, producing a cooling effect. • Sweat, wet roofs, matkas, and pot-in-pot coolers use the same basic process. • Moisture, air movement, and humidity affect how well evaporative cooling works.