During a laboratory experiment, 0.5 kg of liquid water at 25 °C is heated until it boils at 100 °C. If the specific heat capacity of water is 4.18 kJ kg⁻¹ °C⁻¹ and the specific latent heat of vaporisation is 2260 kJ kg⁻¹, calculate the total energy required for the process and explain how conservation of energy applies to the heating and phase change stages.

The energy needed to raise the temperature from 25 °C to 100 °C is 0.5 kg × 4.18 kJ kg⁻¹ °C⁻¹ × 75 °C = 156.75 kJ. The energy required for the phase change is 0.5 kg × 2260 kJ kg⁻¹ = 1130 kJ. The total energy supplied is 156.75 kJ + 1130 kJ = 1286.75 kJ. Conservation of energy means the energy supplied by the heating source is entirely used to increase the internal energy of the water: first by increasing its kinetic energy (temperature rise) and then by increasing its potential energy (phase change). No energy is lost to the surroundings in an idealised closed system.

Comparison lens: State both sides of the comparison so the contrast is explicit rather than implied. This question asks: During a laboratory experiment, 0.5 kg of liquid water at 25 °C is heated until it boils at 100 °C. If the specific heat capacity of water is 4.18 kJ kg⁻¹ °C⁻¹ and the specific latent heat of vaporisation is 2260 kJ kg⁻¹, calculate the total energy required for the process and explain how conservation of energy applies to the heating and phase change stages. The correct response is The energy needed to raise the temperature from 25 °C to 100 °C is 0.5 kg × 4.18 kJ kg⁻¹ °C⁻¹ × 75 °C = 156.75 kJ. The energy required for the phase change is 0.5 kg × 2260 kJ kg⁻¹ = 1130 kJ. The total energy supplied is 156.75 kJ + 1130 kJ = 1286.75 kJ. Conservation of energy means the energy supplied by the heating source is entirely used to increase the internal energy of the water: first by increasing its kinetic energy (temperature rise) and then by increasing its potential energy (phase change). No energy is lost to the surroundings in an idealised closed system., because changes of state are explained by particle energy and arrangement. In Internal energy, the marking point should connect directly to apply the conservation of energy idea to heating and changes of state. 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 269 is distinct because it uses this exact question context and the comparison lens rather than a generic particle-model sentence.