Melting/Solidification and Boiling/Condensation are processes that involve energy transfer, where a substance changes its phase without a change in temperature. Here's how these processes work:
Melting (Fusion): When a solid is heated, it absorbs energy. At a specific temperature, known as the melting point, the substance begins to change from a solid to a liquid. During this phase change, the absorbed energy is used to break the bonds between the molecules, allowing them to move more freely and transition into the liquid state. The temperature of the substance does not increase during this process, even though energy is being added, because all the energy goes into overcoming the intermolecular forces rather than increasing kinetic energy.
Freezing (Solidification): In the reverse process, when a liquid cools to its freezing point, it releases energy. As the substance loses energy, the molecules slow down and begin to form bonds, transitioning from a liquid to a solid. During this phase change, the temperature remains constant, even though energy is being released, because the energy is being used to form the bonds that hold the molecules together in the solid state.
Boiling (Vaporization): When a liquid is heated, it absorbs energy. At a specific temperature, known as the boiling point, the substance begins to change from a liquid to a gas. During boiling, the absorbed energy is used to break the intermolecular forces that hold the molecules together in the liquid state, allowing them to move freely as gas molecules. Despite the continuous addition of heat, the temperature of the substance does not increase during this phase change because the energy is fully utilized in overcoming the intermolecular forces.
The boiling point of a liquid is the temperature at which it starts to turn into gas. This point depends on the external pressure because higher pressure makes it harder for the liquid molecules to escape, requiring a higher temperature for boiling, while lower pressure makes it easier, requiring a lower temperature. The video below demonstrates how water can be made to boil at a lower boiling point by reducing the external pressure.
Condensation: When a gas cools, it releases energy. At the condensation point (which is the same as the boiling point for a given substance), the gas molecules lose energy and slow down, allowing intermolecular forces to pull them closer together to form a liquid. During this phase change, the temperature remains constant because the energy released during condensation is used to form intermolecular bonds, rather than changing the kinetic energy of the molecules.
Evaporation is the process by which a liquid turns into a gas at temperatures below its boiling point. It occurs when individual molecules at the surface of the liquid gain enough kinetic energy to overcome the intermolecular forces binding them to the liquid and escape into the air as vapor.
Not all molecules in a liquid have the same kinetic energy. Molecules with higher kinetic energy, typically near the surface, are the ones that can escape into the air. As these higher-energy molecules leave, the average kinetic energy of the remaining molecules decreases, which can cause the temperature of the liquid to drop. This is why evaporation has a cooling effect.
The factors affecting the rate of evaporation are:
Boiling and evaporation are similar in that both of them involve a change from liquid to gas. The energy required to boil an amount of a substance is the same as the amount required to evaporate the same amount of that substance.
Their differences are summarised in the table below.
| Aspect | Boiling | Evaporation |
|---|---|---|
| Temperature | Occurs at a specific temperature (boiling point). | Occurs at any temperature. |
| Location | Occurs throughout the entire liquid. | Occurs only at the surface of the liquid. |
| Energy Requirement | Requires continuous external energy (heat). | Does not require external energy, though it can be sped up by heat. |
| Process Speed | Relatively rapid process. | Relatively slow process. |
In the following simulation, you will observe that during the process of phase change (e.g. melting and boiling), the temperatures of the substance remain constant.
Media Credits: Created by Chris Bruce. More simulations and resources are available on the SimBucket website.
Use the time slider in the interactive graph below to go through the different phases of the substance. You can choose to observe the heating curves for either water or nitrogen.
The following interactive graph allows you to observe the cooling curves for either water or ethanol.