Change of State of Matter - science lessons for life

## Thursday, January 5, 2017

Change of State of Matter
You have learnt before that there are three states of matter known as solid, liquid and gas. As an example, when it is being heated ice melts into water and water converts into vapor. By absorbing or releasing heat, water changes from one state to another state.

The conversion of the state of a substance from solid, liquid or gas into another state is known as a change of state. Condensation of a gas, melting of a solid, solidification of a liquid, boiling of a liquid are examples of changes of state.

Melting Point
The temperature at which a solid substance that is being heated changes state from the solid state to the liquid state is known as its melting point.

Freezing Point
The temperature at which a liquid substance that is being cooled changes state from the liquid state to the solid state is known as its freezing point.

The melting point and the freezing point of a given substance have the same value.

Melting points of some solids are given in table
Melting points of some solids
Boiling Point
The temperature at which a liquid starts to boil (i.e. the temperature at which the liquid turns to vapour by forming bubbles inside the liquid) is known as its boiling point.

The temperature at which changes of states of matter occur depend on the pressure. Normally, the boiling points and melting points of materials are specified as the temperatures at which boiling or melting occur under the pressure of 1 atmosphere.

Boiling points of some solids

Latent Heat
The change of state of a substance takes place as a result of supplying heat to the substance or removing heat from it. Atoms of substances that exist as solids at room temperature possess some amount of kinetic energy. When heat is supplied, this kinetic energy increases gradually and along with it the temperature of the substance increases. When heat is continuously supplied, at a certain point the kinetic energy of the atoms becomes large enough to break the bonds between the atoms and allow the atoms to move freely. This is the point that the substance changes state from the solid state to the liquid state.

At the point that the change of state takes place, the heat supplied is spent on breaking the bonds between molecules and therefore, the temperature of the substance does not increase. When the change of state of all atoms is complete, the heat supplied is spent again on increasing the temperature of the system.

The heat absorbed by the system without changing its temperature while the change of state is taking place is known as the latent heat.

Consider an instance when heat is being supplied to a block of ice at a temperature slightly below 0 oC.

At first, its temperature would increase gradually up to 0 oC. Since 0 oC is the melting point of ice, the heat supplied thereafter is spent on doing work against the intermolecular attractive forces between the water molecules and the ice at 0 oC becomes water at 0 oC. If heat is supplied further after the block of ice has completely melted into water, then the heat supplied will be spent on increasing the temperature of the water again.

Conversion of a solid into a liquid is known as fusion and the heat absorbed in the conversion of ice at 0 oC into water at 0 oC is known as the latent heat of fusion.

Any solid substance that undergoes fusion absorbs latent heat, not only ice. If the fused substance is cooled, it solidifies again, releasing the same amount of heat that it absorbed during fusion. Therefore, when the water mass at 0 oC is cooled, the same quantity of latent heat is released and the water becomes ice.

Now let us consider an instance where heat is supplied to water at 100 oC.

Since water is at its boiling point, here too a change of state takes place. Here again, work has to be performed against the intermolecular attractive forces. Therefore, the heat supplied is first spent on doing work against the intermolecular attractive forces and the temperature does not change until all the water at 100 oC becomes steam. The latent heat absorbed in this instance is known as the latent heat of vaporization.

Any liquid that vaporizes absorbs latent heat while this vapor releases the same amount of latent heat upon condensation back into the liquid.

Specific Latent Heat of Fusion
The amount of latent heat that has to be supplied in order to convert 1 kg of ice at 0 oC into liquid water at the same temperature is 3.36 × 10^5 J. This quantity of heat is known as the specific latent heat of fusion of ice.

The amount of heat required to change the state of a unit mass of a solid substance at its melting point into the liquid state is known as the specific latent heat of fusion of the substance.

Specific Latent Heat of Vaporization

The amount of latent heat required in order to convert 1 kg of water at 100 oC into steam at the same temperature is 2.26 × 10^6 J. This quantity of heat is known as the specific latent heat of vaporization of water.

The amount of heat required to change the state of a unit mass of a solid substance at its boiling point into the gas state is known as the specific latent heat of vaporization of the substance.

Evaporation and Vaporization
The conversion of a liquid into a gaseous state is called vaporization. Liquid into a gaseous can happen in one of two ways. One is the boiling that takes place at the boiling point of a liquid when further heat is supplied. The other is the conversion of the liquid into a gas gradually at temperatures below the boiling point. The conversion of a liquid at a temperature below the boiling point is known
as evaporation.

In vaporization due to either of the processes boiling and evaporation, latent heat is absorbed. Generally, evaporation takes place only at the surface of a liquid exposed to air. However in boiling vaporization takes place even below the liquid surface. This is why bubbles are formed in a boiling liquid.

In drying clothes and in perspiring to regulate our body temperature, evaporation is the process that plays an important role. Since the specific latent heat of vaporization has a fairly large value, in the evaporation of water through the process of perspiration taking place from our skin, a large amount of heat is removed from our body.

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