5 high-yield, syllabus-aligned questions on Heat & thermal physics, covering 13 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define2 marks
Define specific heat capacity and state its SI unit.
TopMarks model answer
The specific heat capacity of a substance is the quantity of heat required to raise the temperature of one kilogram of the substance by one kelvin. Its SI unit is the joule per kilogram per kelvin, J kg⁻¹ K⁻¹.
Suggested marking guide
1Heat to raise unit mass through unit temperature rise
2Correct SI unit
Why it scores
The word SPECIFIC means “per unit mass”, and that is precisely what distinguishes specific heat capacity from heat capacity. A definition that omits “one kilogram” has defined heat capacity instead, whose unit is J K⁻¹, and neither mark can be given.
A common incomplete answer
“It is the amount of heat required to raise the temperature of a substance.”
What it costs: Both marks. Neither the unit mass nor the unit temperature rise appears, so this could describe any quantity of heat at all, and no unit is given.
Key ideas to include: quantity of heat, one kilogram, one kelvin, J kg⁻¹ K⁻¹.
Calculate3 marks
Calculate the quantity of heat required to raise the temperature of 2 kg of water from 20 °C to 70 °C. [specific heat capacity of water = 4200 J kg⁻¹ K⁻¹]
TopMarks model answer
Temperature rise θ = 70 - 20 = 50 K. Quantity of heat Q = mcθ = 2 x 4200 x 50 = 420000 J.
Suggested marking guide
1Correct formula stated
2Correct temperature rise used
3Correct answer with the unit
The working, line by line
1θ = 70 - 20 = 50 K — The formula needs the temperature RISE, not the final temperature. Work it out on a line of its own so you cannot forget.
2Q = mcθ — Write the formula. It is a mark whether or not the arithmetic afterwards is right.
3Q = 2 x 4200 x 50 = 420000 J — A temperature DIFFERENCE is the same number in °C and in K, so no conversion is needed here — but the answer must carry the joule.
Why it scores
Using 70 instead of 50 is the classic error and it is worth understanding why it is wrong: heat is needed to CHANGE the temperature, so the change is what enters the formula. Note too that a temperature difference has the same numerical value in kelvin as in degrees Celsius, which is why no conversion is required.
A common incomplete answer
“Q = mcθ = 2 x 4200 x 70 = 588000 J”
What it costs: Two marks of three. The formula is credited, but 70 °C is the final temperature, not the rise. The rise is 50 K, so both the substitution and the answer are wrong.
Key ideas to include: temperature rise, specific heat capacity, Q = mcθ, joule.
Explain3 marks
Explain why evaporation causes cooling.
TopMarks model answer
The molecules in a liquid have a range of kinetic energies. During evaporation it is the most energetic molecules that escape from the surface of the liquid. The molecules left behind therefore have a lower average kinetic energy, and since the temperature of a body is a measure of the average kinetic energy of its molecules, the temperature of the remaining liquid falls. The escaping molecules also carry away latent heat of vaporisation, which is drawn from the liquid and its surroundings.
Suggested marking guide
1The most energetic molecules escape
2The average kinetic energy of the rest falls
3Latent heat is taken from the liquid and its surroundings
Why it scores
The chain of reasoning has three links and each is a mark: the fastest molecules leave → the average energy of those remaining falls → temperature IS average kinetic energy, so the temperature falls. Skipping the middle link makes the argument a claim rather than an explanation.
A common incomplete answer
“The fastest molecules escape from the surface of the liquid.”
What it costs: Two marks of three. The first link is credited, but nothing says the average kinetic energy of the remaining molecules falls, and latent heat is not mentioned.
Key ideas to include: kinetic energy, most energetic molecules, average kinetic energy, latent heat of vaporisation.
State3 marks
State three differences between boiling and evaporation.
TopMarks model answer
Boiling occurs at one fixed temperature, the boiling point, whereas evaporation occurs at all temperatures. Boiling takes place throughout the whole body of the liquid, with bubbles forming within it, while evaporation takes place only at the surface. Boiling requires an external supply of heat and the temperature remains constant during it, whereas evaporation draws heat from the liquid itself and so produces cooling.
Suggested marking guide
1Fixed temperature versus all temperatures
2Throughout the liquid versus at the surface only
3Heat supplied versus cooling of the liquid
Why it scores
Boiling and evaporation are both changes from liquid to vapour, which is why the examiner asks how they differ. Each mark is a paired contrast; give both halves in the same sentence and the pairing is unmistakable.
A common incomplete answer
“Boiling takes place at the boiling point while evaporation takes place at any temperature.”
What it costs: Two marks of three. A correctly paired difference, but where each happens in the liquid, and the cooling effect, are both missing.
Key ideas to include: boiling point, all temperatures, throughout the liquid, surface only, cooling.
Explain2 marks
Explain, in terms of the kinetic theory, why a solid expands when it is heated.
TopMarks model answer
When a solid is heated its molecules absorb energy, so their vibration about their mean positions becomes more vigorous and the amplitude of vibration increases. The increased vibration pushes neighbouring molecules slightly further apart, so the average separation between the molecules increases and the solid as a whole expands.
Suggested marking guide
1Vibration of the molecules increases
2The average separation between molecules increases
Why it scores
The second mark is the one that answers the question. Increased vibration on its own would not change the size of the solid — the molecules have to end up FURTHER APART on average. Say that explicitly; it is the step from “the molecules move more” to “the solid gets bigger”.
A common incomplete answer
“The molecules vibrate more when the solid is heated.”
What it costs: One mark of two. The vibration is credited, but nothing connects it to expansion — the increase in the average separation of the molecules is never stated.
Key ideas to include: kinetic theory, vibration, amplitude, mean position, average separation.
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