When does water start boiling process?

The boiling point is the temperature at which the vapour pressure of the liquid becomes equal to the provided atmospheric pressure and the substance starts changing from liquid to gas phase.

It is easy to identify when water is boiling because a large bubble starts rising from the bottom, as soon as the vapour pressure starts exceeding atmospheric pressure. The thermometer temperature becomes constant.

Heat energy when absorbed by the liquid particles starts increasing their kinetic energies. When the temperature (kinetic energy) becomes high enough to overcome the intermolecular interactions, evaporation starts increasing. Now the heat energy given to the system is utilized to break those intermolecular bonds and the average kinetic energy (the temperature) remains constant as shown in the temperature vs time graph below.

Heating curve for water

Effect of atmospheric pressure on boiling point

At sea level, the atmospheric pressure is 760 mmHg (1 torr). The boiling point of pure water is 100 °C or 212 °F or 273.16 °K.

At higher altitudes e.g. at Mount Everest, where atmospheric pressure is relatively low, the low temperature corresponds to more vapour pressure or internal pressure. A point comes when the vapour pressure of water becomes equal to external or atmospheric pressure. this is the point when water starts boiling.

The boiling point of water and impurities

The boiling point is elevated whenever an impurity (solute) is added to it. When a compound is dissolved, it starts interacting with water molecules and lowers the vapour pressure of that very liquid. This phenomenon is governed by Raoult’s law which states the colligative properties of boiling sciences e.g. freezing point depression, boiling point elevation, etc.

Enthalpy of vaporization of water

It is the amount of energy required to convert one mole of water from a liquid phase to the gaseous at a  pressure of 1atm.

ΔH = Hvapours – Hliquid

Application of high pressure in cooking

High pressure is exerted to enhance the boiling point of water. It gives deep cooking because now water boils at higher temperatures i.e. The boiling point in a household pressure cooker is 120° C.

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