Boiling of water

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We are all familiar with the phenomenon of boiling. Boiling of water is the best way to make the water fit for consumption as it kills most of the harmful microbes. Boiling is essential when cooking food items which are suitable to be cooked in water or a water-based medium.

Usually, we are told that boiling point of water is 100°C or 212F. But it is possible for water to boil even early at 80°C or boil later at 120°C. This begs the question: what does boiling of water actually depend on?

Boiling experiment

Take a pot with some water and put it on a Bunsen burner stand. Using a Bunsen burner, heat the pot. Using a thermometer you can observe that the amount of heat that is supplied is being utilized in raising the temperature of water in the pot. The temperature of the water keeps on rising till it reaches 100°C. If you still keep on heating the water, you will observe that the thermometer does not report a further change in temperature. Instead, the heat that is supplied is now used to convert water into steam. The change of state from liquid to vapour is referred to as vaporization. As this change is taking place, the temperature of the system remains constant until the entire amount of the water gets converted into steam.

Boiling point of water

The change in state of water from liquid to gaseous takes place with the two states being in thermal equilibrium. Boiling point of any substance is simply the temperature at which the liquid and gaseous state coexist. Now, cover the pot and shut it strongly so that no vapour can escape into the atmosphere. As a result of this, the steam that is formed will accumulate in a volume directly above the surface of water. This steam will apply pressure over water and the boiling process will stop. The heat that is constantly being supplied will now be used to increase the temperature of the water once more, which you can observe from the temperature reading in the thermometer. After the temperature of the water rises well beyond 100°C, you will notice that boiling starts again.

Role of pressure in boiling

When the pot was open, only atmospheric pressure was acting over the surface of the water. The boiling point in such a case is called normal boiling point and for water the normal boiling point is 100°C. But when the pot was closed, the steam present in the volume directly above water was exerting pressure over the surface of water.  This pressure builds up as the amount of steam that is collected directly above the water increases. Eventually, it surpasses the atmospheric pressure and so the boiling stops. The increased pressure prevents the water molecules from escaping into the atmosphere. Alternatively, when boiling at places way above the sea level, boiling takes place at a lower temperature because of lower atmospheric pressure at such places than at the sea level. That is why it is difficult to cook near hill stations because water boils faster and converts *into steam.

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