Phase Change and Latent Heat

Flat plateaus on a heating curve occur at the melting point and boiling point of a substance. During these sections, temperature does not change even though energy is being added.

During a phase change, particles need energy to overcome the attractions holding them together — these are called intermolecular bonds. All the added energy goes into breaking these bonds.

Because the energy goes into breaking bonds rather than speeding up the particles, the kinetic energy of the particles does not increase. Since temperature is a measure of average kinetic energy, the temperature remains constant.

The correct answer is that energy is breaking intermolecular bonds between particles. This is the defining feature of a phase change at constant temperature.

We need to find what happens to energy during the flat (plateau) regions of a heating curve, where temperature is not increasing.

'The energy is increasing the kinetic energy of the particles' is incorrect. If kinetic energy increased, the temperature would rise — but the plateau shows temperature is constant.

'The energy is being lost to the surroundings' is incorrect. Energy is being deliberately added at a constant rate, and the question is about where it goes — not that it disappears.

'Stored as gravitational potential energy' is not relevant here. Phase changes involve molecular-level changes, not changes in height or gravitational position.

The remaining option — 'the energy is breaking intermolecular bonds between particles' — is correct. This is exactly what happens during a phase change, which is why temperature stays constant.

We have: mass $m=2.0$ kg and specific latent heat of fusion $L_f=334,000$ J/kg. This is a phase change (melting), so we use $Q=mL$.

$Q=mL=2.0\times 334,000$

$Q=668,000\text{ J}$ This is the energy needed to break all the intermolecular bonds in 2.0 kg of ice, converting it entirely to liquid water at 0°C.

The answer is $668,000$ J. Note that the temperature does not change during this process — it stays at 0°C throughout.

We need $Q=mL$ with $m=2.0$ kg and $L_f=334,000$ J/kg. We can check each option by working backwards.

$334,000$ J would be correct only if $m=1.0$ kg (since $1.0\times 334,000=334,000$). Our mass is 2.0 kg, so this is wrong.

$167,000$ J corresponds to $0.5\times 334,000$ — that would be for only 0.5 kg of ice, not 2.0 kg.

$836,000$ J would require $m=836,000÷334,000\approx 2.5$ kg, which does not match our given mass of 2.0 kg.

$668,000$ J is correct: $Q=2.0\times 334,000=668,000$ J.