When heat is added to a liquid, the molecules within the liquid gain kinetic energy. This increased energy causes the molecules to move faster and further apart. Here's what happens:

1. Increased Molecular Motion: The heat energy causes the molecules to vibrate and move around more rapidly.

2. Weakening Intermolecular Forces: As the molecules move faster, the forces that hold them together (like hydrogen bonds or Van der Waals forces) weaken.

3. Evaporation: Some molecules gain enough energy to overcome the attractive forces and escape from the liquid's surface, becoming a gas. This is called evaporation.

4. Boiling Point: As the temperature continues to rise, the rate of evaporation increases. At a specific temperature, called the boiling point, the liquid's vapor pressure equals the atmospheric pressure, and the liquid rapidly changes into a gas. This is called boiling.

In summary, the addition of heat to a liquid causes the molecules to gain energy, move faster, break free from the liquid's surface, and become a gas.