Write notes on the following properties of liquids
a) Vapour pressure (b) Surface tension ( c) Viscosity.

Explanation

(a) Vapour. Pressure : The pressure exerted by vapour of a liquid when it is in equilibrium with liquid is known as vapour pressure. Effect of temperature: When the temperature of a liquid increased the average kinetic energy of molecules increases. This increase in kinetic energy overcomes the attractive forces between the liquid molecules so that liquid molecules rapidly escape into air. Thus rise in temperature raises the escaping tendency of molecules. Hence the vapour pressure of a liquid increases with increase in temperature. The vapour pressure of a liquid increases with increase of temperature. This goes on until the critical temperature of the liquid is reached.. Above the critical temperature liquid state does not exist when the vapour pressure of the liquid becomes equal to the external (atmospheric pressure the liquid is said to be boiling and the temperature at which this happens is known as boiling point. For water the boiling point is `100^@ `C at 1. aim pressure. If the external pressure is reduced, the liquid boils at lower temperature: The boiling point of a liquid can be increased by increasing the external pressure. b) Surface tension property (`gamma` ): "It is defined as the force acting along the surface of a liquid. at . right angles to any line of 1 unit length." It is numerically and dimensionally equal to surface energy. It has dimensions kg. `s^(-2)` and in SI unit `Nm^(-1)`

If we consider a molecule in the bulk of the liquid it experiences equal intermolecular forces in all directions. Hence there is no net force acting on it. But a molecule at the surface has intermolecular forces from inside only. Therefore there is a net attractive force on it towards the interior of the liquid. Due to this the surface area of the liquid tends to minimise. That is the molecules experience a downward force and have more energy than the molecules in the bulk.
Surface tension decreases with increase of temperature because of increase in K.E. of molecules and decrease in intermolecular forces.
e.g .

c) Viscosity : Viscosity is a measure of resistance to flow of liquids. This arises due to internal friction between layers of fluids as they slip past one another while liquid flows.
Coefficient of Viscosity:
The force of friction. required to maintain velocity difference of 1 cm. `sec^(-1)` between two parallel layers of a liquid 1 `cm^2` . apart and each layer having an area 1cm is called coefficient of viscosity.
` to ` It is denoted by ` eta`
` to F= eta A (du)/(dx)`
` to ` Units :.Poise : In CGS system 1 poise = 1g. `cm^(-1) sec^(-1)`
`to` Viscosity of liquids decrease with increase of temperature due to high kinetic energy of molecules that over come the inter molecular forces.