Changes in water
In general, water takes various forms like solid, liquid and gas in different conditions. The physical state of water changes in different processes.
The process in which a liquid changes into a solid on cooling is known as freezing.
Water turns to ice on cooling.
The process in which a solid changes into a liquid on heating is known as melting.
Ice turns to water on heating.
For example, the length of a candle is measured. It is then lighted and allowed to burn for some time. Now, the length of a candle is measured again. But the measurement will not be the same again. It would be different.
When the water is boiled for some time, its quantity will decrease by changing into water vapour.
This process where the water changes into water vapour on heating is known as evaporation.
Water boiling in a kettle.
The water vapour can be changed again into liquid by cooling.
This process where the water vapour changes into water on cooling is known as condensation.
The steam released from the kettle condenses into water droplets on the lid.
These processes can be picturized as below.
The process in which a solid directly changes into a vapour on heating is known as sublimation.
If an incense stick is lighted and allowed to burn for some time, it produces ash along with some gases. A pleasant smell is sensed even though we cannot see the gases. This type of change cannot be reversed. The same thing applies for a matchstick used for lighting.
When some amount of salt is dissolved in water, a salt solution is formed. This change occurred in the salt can be reversed by heating the solution in a beaker. On heating, the water evaporates, and the salt remains at the bottom of the beaker. So, dissolving salt in water is a reversible change.
Some amount of curd is added to warm milk and stirred for some time. The milk then changed to curd when it is kept at a warm place for a few hours. This change is irreversible.
Thus, a substance is changed either by heating, cooling or by mixing it with some other substance. Some of the changes are reversible, whereas the others are not.