Question

Does the nature of intermolecular forces change when a substance goes from a solid to a liquid, or from a liquid to a gas? What causes a substance to undergo a phase change?

Short answer

In short, the nature of intermolecular forces does not change during a phase change, only the degree of interaction between the molecules changes due to their relative positions and kinetic energy. Phase changes occur when a substance gains or loses energy, causing the strength of the intermolecular forces to either decrease or increase, leading to a change in the substance's phase.

Step-by-step solution

Intermolecular forces

Intermolecular forces are the forces that hold molecules together in a substance. They can be classified into three types: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. These forces determine many properties of substances, such as their boiling and melting points.

Phase changes

A phase change, or a physical change, occurs when a substance changes from one phase of matter (solid, liquid, or gas) to another while remaining chemically the same. The energy required to initiate a phase change is often related to the strength of the intermolecular forces in that substance. The most common phase changes are melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), and condensation (gas to liquid).

Forces during phase changes

The nature of intermolecular forces does not change during phase changes; only the degree of interaction between the molecules changes. For example, the molecules in a liquid are held closely together by intermolecular forces, but when a liquid becomes a gas, the molecules become more separated and move more freely, leading to a decrease in the strength of the intermolecular forces.

Causes of phase changes

Phase changes occur when energy (usually in the form of heat) is added or removed from a substance. When enough energy is added to a substance, the molecules gain enough kinetic energy to overcome the intermolecular forces holding them together, leading to a change in phase. Conversely, when energy is removed from a substance, the molecules lose kinetic energy, and the intermolecular forces become stronger, causing a phase change in the opposite direction (for example, condensation). To conclude, the nature of intermolecular forces does not change during a phase transition. However, the strength of the intermolecular forces changes as the substance gains or loses energy, leading to a change in its phase.