# Latent heat of vaporization and fusion

Let’s start the definition of Latent heat of vaporization and Latent heat of fusion

Contents

## Latent heat

The heat energy which has to be supplied to change the state of a substance is known as its latent heat.

### Types of latent heat

1. #### Latent heat of fusion

The heat energy required to convert 1 kilogram of a solid into liquid at atmospheric pressure, at its melting point is called the latent heat of fusion.

• The latent heat of fusion of ice = 3.34 x 105 J/kg.
• The latent heat of fusion of water = 80 cal/gram.
• Ice at 0 degrees Celsius is more effective in cooling a substance than water at 0 degrees Celsius.
• If a liquid freezes to form a solid, an equal amount of heat is given out.
• A piece of ice held in our hands feels very cold because it takes latent heat from our hands. Our hand loses heat to ice making us feel cold.

#### 2. Latent heat of vaporization

The heat energy required to convert 1 kilogram of liquid into gas, at atmospheric pressure, as its boiling point, is known as the latent heat of vaporization.

• The latent heat of vaporization = 22.5 x 105 J/kg.
• The latent heat of vaporization of water = 540 cal/gram.
• Once the water has begun to boil, the temperature remains constant at 100 degrees Celsius until all the water has changed into steam.
• Water vapor at 100 degrees Celsius has more energy than water at the same temperature because particles in steam have absorbed extra energy in the form of latent heat of vaporization.

## Change of state of matter

Physical states of matter can be inter-converted into each other in the following ways:

1. By changing the temperature

2. By changing the pressure

### Effect of change of temperature

• Solid to liquid change
• Liquid to gas change
• Gas to liquid change
• Liquid to solid change

#### Solid to liquid change

Melting: The process in which a solid substance changes into a liquid on heating, is known as melting. It is also called fusion.

Melting point: The temperature at which solid melts to become a liquid at the atmospheric pressure is known as its melting point.

The melting point of ice = 0°C

The melting point of iron = 1535°C

#### Liquid to gas change

Boiling: The change of a liquid substance into gas on heating is known as boiling.

Boiling point: The temperature at which a liquid boils and changes rapidly into a gas at atmospheric pressure is called its boiling point.

The boiling point of water = 100°C

The boiling point of alcohol = 78°C

#### Gas to liquid change

On cooling a gas like steam (or water vapor), the kinetic energy of its particles is lowered down, causing them to move slowly and bringing them closer, forming a liquid.

Condensation: The process of changing a gas (or vapor) to a liquid by cooling is known as condensation.

Liquid to solid change: When a liquid is cooled down by lowering its temperature, its particles lose the kinetic energy and come to a stationary position, causing the liquid to turn to solid.

Freezing: The process of changing a liquid into a solid by cooling is known as freezing. It means solidification.

Freezing point: The temperature at which the state of substance changes from a liquid to a solid is called the freezing point of that substance.

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