Fig. 36.1 shows the arrangement of particles in two different substances, X and Y, at room temperature. (a) Identify the state of matter of substance X and ...

(a) Identifying the states of matter [2]

Substance X is a solid. [1] The particles are closely packed in a regular, ordered pattern with no gaps between them.

Substance Y is a liquid. [1] The particles are close together but arranged irregularly, with small gaps and no long-range order.

(b) Comparing forces between particles [1]

The forces between particles in X are stronger than in Y (particles in X are held more firmly in fixed positions). [1]

In a solid, the strong intermolecular forces lock particles into a rigid structure. In a liquid, the forces are still significant but not strong enough to hold particles in fixed positions, which is why liquids can flow.

(c) Motion of particles in substance X [1]

The particles vibrate about fixed positions. [1]

They do not move from place to place; they oscillate back and forth around their equilibrium positions in the regular lattice. The higher the temperature, the greater the amplitude of vibration, but the particles remain anchored in place.

(d) Changes to particle arrangement when Y becomes a gas [2]

1. The particles become much further apart / widely separated (with large spaces between them). [1]

2. The particles move randomly and rapidly in all directions, no longer sliding past each other as in a liquid. [1]

In a gas, the intermolecular forces are negligible, so particles travel in straight lines at high speed until they collide with each other or the container walls.