IEMDaily - Video Lecture Notes

Definition

Difference between Boiling and Evaporation

1. Boiling occurs at the entire length of liquid whereas evaporation occurs on the surface of the liquid.
2. Boiling occurs rapidly whereas evaporation occurs slowly.
3. Boiling occurs at a specific temperature whereas evaporation occurs at any temperature.
4. The motion of particles is fast in boiling whereas in evaporation few particles move slowly and few at a faster rate.
5. There is formation of bubbles in boiling, but bubbles are not seen in evaporation.

Result

Factors affecting rate of vaporisation

Rate of vaporisation is affected in the following ways:

It increases with increase in the temperature of the fluid.
It depends on the material of the fluid.
It increases with the increase in surface area of liquid.
It increases with increase in the speed of air flowing above the water.
It increases with the increase in the temperature of the surroundings.
It increases with the dryness of air above the water.

Example

Applications of evaporation

Some applications of evaporation are as follows:

Wet handkerchief on the forehead of a person having high fever relieves the patient.
Pouring tea in the saucer helps it cool faster.
Earthen pots help in the cooling of drinking water in summers.

Definition

Melting point and normal melting point

The temperature at which the solid and liquid states of a substance is in thermal equilibrium with each other is called melting point. Normal melting point is the temperature at which a solid melts at 1 atmosphere of pressure.

Definition

Define freezing point

The freezing point is the temperature at which a liquid changes to a solid. The freezing point of a substance is not necessarily the same as its melting point. Example: Freezing point of water is

0^{o} C

Definition

Melting

The change in solid to liquid phase at a constant temperature is called melting (or fusion). The temperature at which melting occurs is called melting point of the solid. Heat is released from the solid in this process.

Definition

Vaporisation and condensation

Change of state of a material from liquid to gas on heating at a constant temperature is called boiling (or vaporisation). The temperature of this phase change is called boiling point of liquid. For water, boiling point is

100^{o} C

at STP.
Condensation is the change of a substance from its gaseous form (water vapor) into liquid form.Having a cold soda on a hot day, the can "sweats." Water molecules in the air as a vapor hit the colder surface of the can and turn into liquid water.

Definition

Evaporation v/s Vaporisation

S.No.	Evaporation	Vaporisation
1	Occurs at all temperatures	Occurs only at boiling point
2	Slow process	Rapid and violent process
3	Occurs only at surface of liquid	Occurs over the entire volume of the liquid
4	Causes cooling in atmosphere.	Does not cause cooling in atmosphere.

Definition

Sublimation

The change from solid state to vapour state without passing through the liquid state is called sublimation and the substance is said to sublime. Dry ice (solid

C O_{2}

) and iodine sublimes.

Example

Describe everyday phenomena based on high latent heat of fusion of ice

Everyday phenomena based on latent heat of fusion of ice are as follows:
1. When we add ice to an already cold drink, only some of the ice melts, not all of it. This is because the soda does not contain enough energy as heat to overcome the latent heat of fusion of the ice.
2. When a pot of water is kept boiling, the temperature remains at

100^{o} C (212^{o} F)

until the last drop evaporates, because all the heat being added to the liquid is absorbed as latent heat of vaporization and carried away by the escaping vapour molecules.
3. While ice melts, it remains at