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NCERT Solutions Curiosity Chapter 8 .4 How can we change the States of Water? — In-text Questions

Book page 1528 Updated on2026-09-05

Q1.
How can you change the state of water? How can you quickly change ice to its liquid state, water?
Answer

The state of water is changed by supplying heat or by taking heat away — nothing else is needed.

Ice  + heat  → Water  + heat  → Water vapour
Water vapour  − heat  → Water  − heat  → Ice

To change ice quickly into water, supply heat fast. Practical ways:

  • Put the ice in a warm room or in sunlight instead of leaving it in the freezer.
  • Hold it in your palm, or pour warm water over it.
  • Put it in a metal (steel) bowl rather than a plastic one — metal carries heat to the ice faster.
  • Break the ice into small pieces or crush it. Small pieces have a much larger surface touching the warm air, so they melt much faster than one big block.
  • Stir the ice in water — moving water keeps bringing fresh heat to the surface of the ice.
Why heat is what matters: ice melts at 0 °C. To melt it, we must bring it to that temperature and then keep feeding it heat while it melts. Everything on the list above is simply a way of delivering heat to the ice faster.
Did you know? While ice is melting, its temperature stays at 0 °C even though heat is going in. All that heat is used up in breaking the particles out of their fixed places. Only after the last bit of ice has melted does the water start getting warmer.
Q2.
If we want to change water into ice, what should be done?
Answer

We must take heat away from the water — that is, cool it down to 0 °C and keep cooling it.

In the words of the book: It can be done by placing water in a cold environment, such as a freezer. Water freezes and is converted into ice. If we take the ice out of the freezer, it melts and is converted into water.

ChangeName of the processWhat must be doneTemperature
Solid → LiquidMeltingSupply heatIce melts at 0 °C
Liquid → SolidFreezingRemove heatWater freezes at 0 °C
Liquid → GasEvaporation (and boiling)Supply heatEvaporation at any temperature; boiling at 100 °C
Gas → LiquidCondensationRemove heatWhenever vapour meets a colder surface
Why a freezer works: a freezer does not "make cold". It pumps heat out of the space inside and throws it away at the back of the refrigerator — which is why the back of a fridge feels warm. The water inside loses heat, its particles slow down and lock into fixed places, and it becomes ice.
More to know! An Atmospheric Water Generator (AWG) uses the reverse idea. It cools humid air so that the water vapour in it condenses, and collects that water for drinking. It is the same process as the drops forming outside a tumbler of ice-cold water — done on a large scale.
Q3.
Can you think of any other example, besides water, that can change from solid to liquid? How can we turn candle wax into liquid state? How can we change the liquid wax back into solid state? What are the other liquids you have seen which get converted into a solid?
Answer

Yes — a candle, which is made of wax, is the example the book gives. Many everyday substances behave in the same way.

Turning candle wax into a liquid: light the candle, or hold a piece of wax in a spoon over a flame. The wax takes in heat, melts, and a small pool of clear liquid wax collects around the wick.

Turning liquid wax back into a solid: cool it. Blow out the flame and the molten wax hardens in a minute; pour it into a cold spoon and it sets even faster. In the book's words — we should cool the liquid wax to change it into a solid.

Other liquids you have seen turning into solids:

  • Coconut oil — it becomes white and solid in the winter months and clears again in summer. The book asks about exactly this.
  • Ghee and butter — solid in winter, runny in May.
  • Vanaspati and dalda in a cold kitchen.
  • Molten jaggery or sugar syrup, which sets hard into chikki or a lollipop.
  • Melted chocolate setting again in the fridge.
  • Molten metal poured into a mould by a bell-maker, setting into a solid bell.
  • Water itself, of course, freezing into ice.
The general rule: the process of conversion of a solid into liquid state is called melting; the process of conversion of liquid into solid state is called freezing. Each substance has its own melting temperature — coconut oil sets at about 24 °C, so a cool night is enough, while water needs 0 °C and iron needs well over 1500 °C.
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