Calorimetry — Part 2: Change of state and latent heat of fusion: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Heat that does not raise the temperature

Part 2 — when ice melts, it absorbs heat but its temperature stays at 0 °C until all of it has melted. Where does that heat go?

In this lesson you will

Interpret the heating curve of ice to steam
Define specific latent heat of fusion and use Q = mL
Solve mixture problems involving melting ice
Explain everyday effects of the latent heat of fusion

Table of four changes of state and whether heat is absorbed or given out
Change of state	Name	Heat
Solid → liquid	Melting (fusion)	Absorbed
Liquid → solid	Freezing	Given out
Liquid → gas	Boiling (vaporisation)	Absorbed
Gas → liquid	Condensation	Given out

Concept

Change of state and latent heat

A change of state (melting, boiling) happens at a fixed temperature for a pure substance at a given pressure. During melting, heat is absorbed to separate the molecules from their fixed positions, not to raise the temperature. Specific latent heat of fusion $L$ : heat needed to change 1 kg of a solid at its melting point into liquid at the same temperature; unit J kg⁻¹. For ice, $L = 336000\ \text{J kg}^{-1}$ (336 J g⁻¹). Heat to melt mass $m$ : $Q = mL$ . The same heat is given out when the liquid freezes.

Table of the two heat formulae and the latent heat of fusion of ice
Quantity	Formula	Unit
Heat for temperature change	Q = mcΔT	J
Heat for melting	Q = mL	J
Specific latent heat of fusion of ice	336 000	J kg⁻¹

Diagram

Heating curve of ice to steam

1 kg of ice at −10 °C heated steadily

Flat parts: change of state at constant temperature (latent heat). Sloping parts: temperature rise (Q = mcΔT).

Heating 1 kg of ice from −10 °C at a steady 1000 W: AB ice warms to 0 °C ( $Q = mc_{\text{ice}}\Delta T$ , 21 s); BC ice melts at 0 °C ( $Q = mL$ , 336 s) — flat; CD water warms to 100 °C ( $Q = mc_{\text{water}}\Delta T$ , 420 s); DE water boils at 100 °C — flat. The flat parts show latent heat: heat absorbed with no change in temperature. The melting part is much longer than the ice-warming part.

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