The melting point and the boiling point represent the temperature at which a substance changes state at a given pressure.
In the case of the melting point, the substance changes from a solid to a liquid state. The boiling point refers to the change from a liquid to a gaseous state.
For example, ice begins to turn into water in liquid form, when its temperature is equal to 0 ºC. Therefore, the melting point of the water is 0 ºC (under pressure of 1 atmosphere).
To change from liquid to steam, the water must reach a temperature of 100 ºC. Thus, the boiling point of water is 100 ºC (under pressure of 1 atmosphere).
Fusion point
When a substance in the solid state receives heat , there is an increase in the degree of agitation of its molecules. Consequently, its temperature also increases.
Upon reaching a certain temperature (melting point), the agitation of the molecules is such that it breaks the internal bonds between atoms and molecules.
At that point, the substance begins to change its state and will change to a liquid state if it continues to receive heat.
During the melting, its temperature remains constant, as the heat received is used only for the change of state.
The heat per unit mass necessary to change the phase is called latent heat of fusion (L f ) and is a characteristic of the substance.
Melting point table and latent heat
In the table below we indicate the melting point temperature and the latent heat of some substances at atmospheric pressure .
Boiling point
The boiling is characterized by the rapid passage of the liquid to the gaseous state, with the formation of vapor (bubbles) within the liquid.
As with fusion, there is a temperature (boiling point) at which a given substance changes from a liquid to a gaseous state.
For this to occur it is necessary that the substance receives heat. Throughout the phase change, the temperature remains constant.
The latent heat of vaporization (L v ) is the amount of heat per unit mass necessary for a substance to change phases.
Boiling point and latent heat table
In the table below, we indicate the boiling point temperature and the latent heat of vaporization of some substances at atmospheric pressure.
Pressure interference
The temperature of the melting point and the boiling point depends on the pressure exerted on the substance.
In general, substances increase in volume when they undergo fusion. This fact makes the higher the pressure, the higher the temperature must be for the substance to change phases.
The exception occurs with some substances, including water, which decreases its volume when undergoing fusion. In this case, greater pressure will lower the melting point.
A decrease in pressure causes the boiling point of a particular substance to be lower, that is, the substance will boil at a lower temperature.
For example, in places above sea level the water boils at temperatures below 100 ºC. As a result, in these places it takes much longer to cook than in places at sea level.