Liquid state, Typical properties, evaporation vapour pressure and boiling points

Typical properties of Liquids

Evaporation

Conversion of a liquid to a gas are vapour at all temperature is called vaporization or evaporation.

Example

When we place some liquids such as ether or acetone in an open container and observe the we notice that volume of liquid gradually decreases and finally no more of the liquid is left. This is because liquid constantly change into gas are paper even temperature is less than boiling point of liquid.

Question is evaporation a cooling process?

During evaporation process, only those molecule which have greater kinetic energy than average can break intermolecular forces first and enter into gas or vapour state. The molecules which have lower kinetic energy than average are left behind. Due to these lower energy molecules, the temperature of liquid decreases. These low energy molecules absorb heat from the surrounding and decrease its temperature. Thus evaporation causes cooling.

Vapour pressure

The pressure exerted by the weapons of liquid in equilibrium with its liquid state is called vapour pressure.

Explanation

Consider a close container containing some liquid in it. Some of liquid molecules enter gas or vapour state (evaporation). As time passes, number of molecules changing into vapour increases. Some of these vapours collide with one an other as well as with liquid surface and convert into liquid again. This process is called condensation. After some time, number of molecules entring vapour state become equal to the number of molecules entering liquid state. At this stage, an equilibrium is established between liquid and vapour state. At this equilibrium state, the pressure exerted by vapours is called vapour pressure

Effect of temperature on vapour pressure

When we increase the temperature vapour pressure of liquid in also increases. This is because an increase in temperature of liquid increases the kinetic energy of molecule. As a result, more of the molecule will have minimum kinetic energy needed to escape the surface of liquid. Thus, vapour pressure of liquid also increases.

Boiling points

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

Explanation

Let us consider an open container containing a liquid and atmospher exerts pressure of surface of liquid. When liquid is heated, average kinetic energy of molecules increases. As a result number of molecules that overcome intermolecular forces also increas. Therefore, vapour pressure of liquid also increases. If liquid is heated continuously, at certain temperature vapour pressure of the liquid becomes equal to the external atmospheric pressure. At this stage, first buble coe out of the surface of liquid and liquid starts boiling.

Effect of external pressure on boiling points

Liquid boil when they are vapour pressure is equal to the pressure exerted on the liquid by the surroundings the normal boiling point of water is 100 degree centigrade in the Mountain that must rec pressure is less than 1 ATM so water ball below 100 degree certain in a pressure cooker at 2:18 water boil at 120

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