Melting and Boiling points

The process due to which a solid changes into liquid state by absorbing heat energy is called melting or fusion and the constant temperature at which a solid changes into liquid state by absorbing heat energy is called melting point.

Again,the process due to which a liquid changes into gaseous state by absorbing heat energy is called boiling or vaporization and the constant temperature at which a liquid rapidly changes into gaseous state by absorbing heat energy is called boiling point.

There are two types of energy in a substance, which we often call to be the internal energy. The two types of energy are potential and kinetic energy.

The kinetic energy is what defines the temperature of the substance. When the temperature of a substance reaches a state where it is ready to go through boiling/melting, energy is still being transferred but the temperature is still constant.

In this process, bonds are being broken/formed, so energy transfer is definitely required for this change in state. However, if the kinetic energy does not change here, what changes? The potential energy increases.

One difference between boiling and evaporation is that evaporation happens at the surface of a liquid where boiling occurs inside the body of the liquid.

Additionally, evaporation can literally happen at any temperature, as long there is enough Kinetic Energy to break bonds, but boiling can only happen at certain temperatures, e.g. 100°C for water.

Condensation: This is the change of the physical state of matter from a gas into a liquid.

Basically, condensation starts by the formation of atomic clusters of the substance in its gaseous (gas) state, like the formation of a rain drop in the clouds.

Solidification: This is the change of the physical state of matter from a liquid to a solid.

A good example is changing water into ice cubes. You put the water into the fridge, and it freezes to become ice.

Latent Heat of Vaporisation: The change of phase from liquid to gas

Latent heat of fusion: The change of phase from solid to liquid.

Latent Heat: Amount of energy associated with phase change.

The molecules in a liquid are nicely packed together, not as packed as the solids, but packed enough so they don’t have complete freedom as in where to move around. However, if you apply some “energy” into the liquid, the molecules inside (internal energy) starts to become more energetic and move around more often. As you heat it for a longer period of time, some of its molecules will have enough energy to overcome the forces of attraction holding the liquid together, hence evaporating and changing phase to a gas.

The concept is similar in the change of phase from solid to liquid. Let’s say you have an ice cube. As we all know, the particles in the solid have little freedom to move around and can only vibrate around a fixed position. If you heat the ice cube, the particles will vibrate more rapidly, and as you heat it for a longer period of time, naturally, some of the molecules in the ice cube will have enough energy to overcome the forces of attraction holding the solid together, hence melting into a liquid. Obviously, this does not  happen all at once, its a gradual process where more and more molecules have enough energy to overcome the forces of attraction between molecules.

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