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NCERT Solutions Curiosity Chapter 8 and 157A pot-in-pot cooler (Fig. 8.7) and the surahi (Fig. 8.8) — Activity 8.9: Let us make a model

Book page 156 Updated on2026-09-05

Q1.
Observe and discuss how it creates a cooling effect inside the pots.
Answer

The pot-in-pot cooler is nothing but a large matka working on a bigger scale. Recall how it is built: a large earthen pot, a layer of sand at its bottom, a smaller earthen pot placed in the centre, more sand filling the gap, water poured into the sand, and a lid or a wet jute sack on top.

How the cooling happens, step by step:

  1. The wet sand keeps the outer pot damp all the time.
  2. Water seeps through the pores of the outer pot and reaches its outer surface.
  3. There it evaporates into the surrounding air.
  4. Evaporation needs heat, and that heat is drawn from the outer pot, the sand, the inner pot and the things kept inside.
  5. So the inside of the smaller pot becomes several degrees cooler than the room, and the wet jute sack on top does the same job for the opening.
  6. Adding water regularly keeps the sand moist, so the process never stops. No electricity is used at any stage.
Wet sand → water seeps to the outer surface → evaporates → takes heat from inside → vegetables and fruits stay cool and fresh
Why the sand is there at all: the sand acts as a reservoir. It holds a large quantity of water and feeds it steadily to the outer pot, so evaporation continues for hours instead of stopping in a few minutes. It also keeps the two pots apart, so the cool inner pot is not directly touching the warm air.
Did you know? This design is known worldwide as the zeer pot. It can keep tomatoes and leafy vegetables fresh for many days in a village that has no electricity, and it has been used to store medicines in hot, remote areas.
Q2.
Keep some vegetables and fruits in it and observe for a week on a daily basis. For how many days can the vegetables and fruits be kept fresh in it? What are the conditions which can affect the number of these days?
Answer

Roughly what you will find: in an ordinary Indian summer a well-made pot-in-pot cooler keeps most vegetables fresh far longer than the open kitchen shelf.

Item keptOn an open shelfIn the pot-in-pot cooler
Tomato, brinjal, capsicum2–3 daysAbout 8–12 days
Spinach, coriander and other leafy greensLess than 1 dayAbout 3–5 days
Carrot, beans, lady finger2–4 daysAbout 7–10 days
Guava, banana, grapes2–3 daysAbout 5–8 days

Conditions that change the number of days:

  • How dry the air is. Hot, dry air gives fast evaporation and the best cooling. In humid monsoon weather the cooler works much less well.
  • Movement of air. Standing the cooler in a breezy, shaded spot cools it far better than a closed corner.
  • Sunlight. Direct sun heats the pot faster than evaporation can cool it — always keep the cooler in the shade.
  • How regularly you add water. If the sand dries out, the cooling stops at once.
  • Quality of the pots. Unglazed, well-fired clay is porous and works well; a glazed or painted pot has sealed pores and hardly cools at all.
  • The covering. A wet jute sack works better than a dry lid, as it evaporates too.
  • What you put in, and how much. Only fresh, dry, undamaged produce should go in, and the pot should not be packed so tight that air cannot move.
Tip: Keep an identical set of vegetables outside the cooler as a control and compare them every day. Without the comparison you cannot claim that the cooler made any difference.
Q3.
What else can be used in place of sand for better cooling?
Answer

The material in the gap has just one job — to hold a lot of water and keep the outer pot wet. So anything that soaks up water well and stays in contact with the pot will do, and some materials do it better than sand.

MaterialHow well it works
Khus (vetiver) rootsExcellent — holds a great deal of water, lets air pass through, and adds a pleasant scent. Used in traditional coolers
Wet jute or gunny cloth, packed in layersVery good — soaks up much more water than sand for the same weight
Coir (coconut husk fibre) or coco peatVery good — holds many times its own weight of water
Sawdust or dry strawGood, and easily available in villages
Charcoal piecesGood — porous, holds water and also keeps smells away
Sand (the standard choice)Works, is cheap and easy, but holds less water than the materials above
Clay soil or cotton woolPoor — clay packs solid and blocks air; cotton stays soggy and rots
What makes a material good here: two things together — it must hold plenty of water (so the supply lasts) and it must be loose enough for air to move through (so the water can actually evaporate). Sand is only average on the first count, which is why khus roots and coir work better.
Try This: Make two small coolers, one with sand and one with wet jute, and measure the temperature inside each with a laboratory thermometer after 5 hours. Report both readings in °C — that turns your model into a real experiment.
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