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# Changes in state

## Melting

The change of a solid into a liquid is called melting. This typically occurs when a solid absorbs heat.

The molecules of the solid start vibrating more vigorously as they absorb heat. The kinetic energy of the molecules increases when the temperature increases.

At the melting point, the molecules gain enough energy to overcome the intermolecular attractive forces. The molecules then begin to break away from their fixed positions and the solid begins to melt.

The temperature at which a solid turns into a liquid is known as the melting point. The melting point of a particular compound at a set pressure is always the same.

This is because the molecules need a fixed amount of energy to overcome the intermolecular forces that hold them together in a solid.

Ice changing into liquid water. This happens at a temperature of $0^{\circ}\text{C}$

## Melting latent heat

Heat absorbed during melting breaks intermolecular bonds in the solid, rather than raising the temperature.

Heat that does not cause a change in temperature in an object is known as latent heat. Latent heat increases the potential energy of molecules in a sample but not the kinetic energy.

The heat gives molecules enough energy to overcome the intermolecular forces, allowing them to be freed from their fixed positions.

The heat is not used to raise the temperature. Temperature therefore remains constant during melting.

When melting occurs, potential energy increases instead of kinetic energy (which is related to temperature).

When a solid with a temperature below the melting point is heated, the heat is used to increase the kinetic energy (and thus raise the temperature). This is not latent heat because a change in temperature is observed.

## Boiling

The change of a liquid into a gas is boiling. This occurs when a liquid absorbs heat.

The liquid molecules gain kinetic energy as they absorb heat, making them move around more vigorously.

At the boiling point, the molecules gain enough energy to overcome the intermolecular attractive forces and begin to move freely in a gaseous state.

The temperature at which a liquid turns into a gas is the boiling point.

The boiling point of a substance (at a particular pressure) is constant because the molecules have to possess a fixed amount of energy to overcome the attractive forces.

During boiling, the temperature remains constant because the heat absorbed at the boiling point is used to overcome the attractive forces between particles. The heat does not raise the temperature.

Heat added during boiling is latent heat.

Water turning into steam. This happens at a temperature of $100^{\circ}\text{C}$

## Sublimation

Sublimation is the change of a solid directly into a gas. The intermediate liquid phase is skipped.

Sublimation typically occurs when certain solids absorb heat. Some notable examples are solid $\ce{CO2}$ (known as dry ice) and iodine.

Iodine appears as a blue-black solid at room temperature. Upon heating, iodine crystals begin to sublime into violet fumes.

Low outside pressure (usually less than atmospheric pressure) often favours sublimation over melting.

Dry ice is one of the few solids that will sublimate at atmospheric pressure. Solid dry ice is spontaneously converted to carbon dioxide gas.

The solid molecules gain kinetic energy as they gain heat, allowing them to vibrate more vigorously.

Once the molecules are heated to a certain threshold, they obtain enough energy to overcome the intermolecular attractive forces. This enables them to move freely and randomly in the gaseous state.

As with melting, sublimation involves the transfer of latent heat. The temperature of the material therefore remains constant during sublimation.

## Evaporation

Evaporation is the change of a liquid into a gas without boiling. This process only occurs at the surface of the liquid.

When water is left in a glass at room temperature, it slowly evaporates and water is lost.

During evaporation, some liquid molecules on the surface of the liquid obtain enough energy to escape into the gas state.

Enough energy must be absorbed by these molecules to overcome the intermolecular attractive forces.

Evaporation can only occur for molecules on the surface of the liquid because they are not surrounded by other molecules.

The overall intermolecular forces holding these particles in the liquid are therefore weaker.

Evaporation is very slow because only a small proportion of liquid surface molecules gain enough energy to evaporate.

## Condensation

Condensation is the change of a gas into a liquid. This occurs when a gas loses heat to its surroundings.

The gas molecules lose kinetic energy as they lose heat, so they move less vigorously.

When they become unable to overcome the intermolecular attractive forces, the molecules move closer to each other and condense to form a liquid.

A gas starts condensing at its boiling point. The temperature remains constant during the process of condensation because only potential energy decreases.

The heat removed during condensation is therefore latent heat.

Water droplets found on plant surfaces during cold mornings are a result of condensation.

## Freezing

Freezing is the change of a liquid into a solid. This occurs when a liquid loses heat to its surroundings.

The liquid molecules lose kinetic energy as they lose heat, causing them to move around less vigorously.

At the melting point temperature, the molecules begin to lose potential energy. They become unable to overcome the intermolecular attractive forces.

The intermolecular forces cause molecules to become fixed in place. Movement becomes limited to vibrations and rotations, causing the substance to freeze.

The temperature remains constant during the freezing period. The heat removed is latent heat.

Lake Inari (in Finland) freezes up during the winter.