Explain why a solid has a fixed shape while a liquid does not, using the particle model.

In a solid, particles are closely packed in a fixed arrangement and can only vibrate in place, which gives the solid a definite shape. In contrast, the particles in a liquid are less tightly packed and can move around each other, allowing the liquid to flow and take the shape of its container.

Definition lens: Give the precise definition, then add a context sentence that shows how it is used. This question asks: Explain why a solid has a fixed shape while a liquid does not, using the particle model. The correct response is In a solid, particles are closely packed in a fixed arrangement and can only vibrate in place, which gives the solid a definite shape. In contrast, the particles in a liquid are less tightly packed and can move around each other, allowing the liquid to flow and take the shape of its container., because changes of state are explained by particle energy and arrangement. In Changes of state, the marking point should connect directly to explain that the material recovers its original properties if a change of state is reversed. If the question includes values, the working must keep the appropriate unit and operation; if it is an explanation, it must name the relevant particle behaviour or energy change. This item belongs to Changes of state and the particle model, so avoid answers that switch to a different quantity, confuse heat with temperature, or describe gas pressure without collisions when collisions are the reason. Checkpoint 186 is distinct because it uses this exact question context and the definition lens rather than a generic particle-model sentence.