AQA A-Level Chemistry Paper 3, 2023: Question 15

What happens when water is vaporised?

• A: Covalent bonds break within molecules.
• B: Intermolecular forces are overcome.
• C: The enthalpy of the molecules decreases.
• D: The disorder of the molecules decreases.

The correct explanation is that intermolecular forces are overcome when water is vaporised. These forces, primarily hydrogen bonds in water, hold the molecules together in the liquid state. When water is vaporised, these forces are broken, allowing the molecules to separate and transition into the gaseous state.

• A: Covalent bonds within the water molecules do not break. This would require a chemical reaction, not a physical change like vaporisation.
• C: The enthalpy of the molecules increases during vaporisation as energy is absorbed to overcome intermolecular forces.
• D: The disorder (or entropy) of the molecules increases as they move from a more ordered liquid state to a less ordered gaseous state.

Vaporisation is a physical change where a substance transitions from the liquid to the gas phase. Energy is absorbed to break intermolecular forces, increasing the potential energy of the molecules.

The correct answer is:

B: Intermolecular forces are overcome.