During the melting of ice at 0 °C, the temperature of the water remains constant even though energy is being supplied. Explain why this happens, using the concepts of internal energy, particle arrangement and temperature.

The supplied energy is used to break the hydrogen bonds that hold the ice lattice together, increasing the potential energy of the water molecules. Because the kinetic energy (and therefore the temperature) of the molecules does not change, the temperature stays at 0 °C while the state changes from solid to liquid.

Boundary lens: Keep this separate from nearby specification points that use similar words but test a different idea. This question asks: During the melting of ice at 0 °C, the temperature of the water remains constant even though energy is being supplied. Explain why this happens, using the concepts of internal energy, particle arrangement and temperature. The correct response is The supplied energy is used to break the hydrogen bonds that hold the ice lattice together, increasing the potential energy of the water molecules. Because the kinetic energy (and therefore the temperature) of the molecules does not change, the temperature stays at 0 °C while the state changes from solid to liquid., because changes of state are explained by particle energy and arrangement. In Changes of state and specific latent heat, the marking point should connect directly to explain why temperature remains constant during a change of state even though energy is transferred. 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 418 is distinct because it uses this exact question context and the boundary lens rather than a generic particle-model sentence.