Not given in the exam, recall it.
What it is for
Specific latent heat is used in Heat (SPM Physics Form 4). Keep the SI units in every line of working. Heat
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| Q | heat energy | J |
| m | mass | kg |
| L | specific latent heat | J kg⁻¹ |
Rearrangements
L = Q / mm = Q / L
Worked example
Common trap
Understanding specific latent heat
The equation Q = mL gives the heat needed to change the state of a substance without changing its temperature. Q is heat energy in joules (J), m is mass in kilograms (kg), and L is the specific latent heat in J kg⁻¹.
Specific latent heat is the energy needed to change the state of 1 kg of a substance at constant temperature. This energy goes into breaking or forming the bonds between particles, which is why the temperature stays fixed and there is no θ term.
To find other quantities, rearrange to L = Q / m or m = Q / L. Use latent heat of fusion for melting and freezing, and latent heat of vaporisation for boiling and condensing.
A worked example, step by step
How much heat turns m = 0.2 kg of water at 100 °C completely into steam? The specific latent heat of vaporisation of water is L = 2.26 × 10⁶ J kg⁻¹.
Write the formula: Q = mL. Substitute with units: Q = 0.2 kg × 2.26 × 10⁶ J kg⁻¹.
Multiply: 0.2 × 2.26 × 10⁶ = 0.452 × 10⁶.
So Q = 4.52 × 10⁵ J. The units kg × J kg⁻¹ cancel to J, as required. Vaporising the water needs 4.52 × 10⁵ J, quoted to three significant figures.
Notice this is far more than heating the same water by a few degrees.
Common mistakes and how this is tested
The core trap is that temperature stays constant during a state change, so never include a θ term. Also choose the correct L: fusion for solid-liquid, vaporisation for liquid-gas.
More errors: using grams instead of kilograms for m; adding mcθ energy during the flat part of a heating curve, where only mL applies; and forgetting that the same L applies to the reverse change (freezing releases the fusion energy).
Under calculate, show the substitution with units. Define may ask for the meaning of specific latent heat of vaporisation, and explain may ask why steam causes worse burns than boiling water at the same temperature.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)