What is the primary factor that causes boiling to occur?

The correct answer is Increase in temperature.

Application lens: Apply the rule to the specific sample or situation instead of reciting a broad fact. This question asks: What is the primary factor that causes boiling to occur. The correct response is Increase in temperature, because changes of state are explained by particle energy and arrangement. In Changes of state, the marking point should connect directly to explain boiling using particles gaining enough energy to separate throughout a liquid. 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 139 is distinct because it uses this exact question context and the application lens rather than a generic particle-model sentence.