Why does temperature remain constant during a change of state?

The correct answer is Energy is used to change the potential energy of particles..

Graph lens: Read the trend, flat section, gradient, or axis labels before explaining the physical meaning. This question asks: Why does temperature remain constant during a change of state. The correct response is Energy is used to change the potential energy of particles., because internal energy combines particle kinetic and potential energy. In Internal energy, the marking point should connect directly to distinguish temperature from internal energy. 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 Internal energy and energy transfers, 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 243 is distinct because it uses this exact question context and the graph lens rather than a generic particle-model sentence.