Intermolecular forces are stronger in liquid state than in gaseous state.

Molecules in liquids are so close that there is very little empty space between them and under normal conditions

liquids are denser than gases.

Molecules of liquids are held together by attractive intermolecular forces.

Liquids have definite volume because molecules do not separate from each other. Molecules of liquids can move past one another freely, therefore, liquids can flow, can be poured and can assume the shape of the container in which these are stored.

When a container is partially filled with a liquid, a portion of the liquid evaporates to fill the remaining volume with vapor and this process, known as vapourisation,

Vapour pressure is the pressure exerted by vapors on the container walls, which increases as the liquid evaporates.

This equilibrium is known as equilibrium vapour pressure or saturated vapour pressure. The process of vapourisation is temperature-dependent,

When heated in an open vessel, the liquid vaporises from the surface, and at a temperature equal to external pressure, vapourisation can occur throughout the bulk of the liquid,

allowing vapors to expand freely into the surroundings and  this condition is called boiling.

The temperature at which vapour pressure of liquid is equal to the external pressure is called boiling temperature at that pressure.

At 1 atm pressure boiling temperature is called normal boiling point.

If pressure is 1 bar then the boiling point is called standard boiling point of the liquid.

The boiling point of a liquid is slightly lower than the normal boiling point at 1 atm pressure.

1 bar pressure is slightly less than 1 atm pressure.

The normal boiling point of water is 100 °C (373 K), its standard boiling point is

99.6 °C (372.6 K).

At high altitudes atmospheric pressure is low.

liquids at high altitudes boil at lower temperatures in comparison to that at sea level. Since water boils at low temperature on hills, the pressure cooker is used for cooking food.

In hospitals surgical instruments are sterilized in autoclaves in which boiling point of water is increased by increasing the pressure above the atmospheric pressure by using a weight covering the vent.

Boiling does not occur when liquid is heated in a closed vessel.

On heating continuously vapour pressure increases.

At first a clear boundary is visible between liquid and vapour phase because liquid is more dense than vapour.

As the temperature increases more and more molecules go to vapour phase and density of vapours rises.

At the same time liquid becomes less dense.

It expands because molecules move apart.

When density of liquid and vapours becomes the same; the clear boundary between liquid and vapours disappears.