PhysicsICSE Class 8 · Matter

Heating and Cooling Curves: Reading Temperature–Time Graphs (ICSE Class 8 Physics)

Updated 17 October 2026 · Fact-checked

A heating curve plots temperature against time as a substance is heated steadily. The sloping parts show temperature rising in one state; the flat parts show a change of state (melting or boiling) at a fixed temperature, with both states present. A substance that melts over a range of temperatures, without a flat part, is not pure.

Notice the temperature stays flat at 0 °C and 100 °C while the state changes, and rises only while one state is being heated.

Illustration drawn for QuestionBuddy.

Understand Heating and Cooling Curves

Heating ice steadily from −10 °C and noting the temperature every minute gives four parts:
1. a rising line: ice warms up to 0 °C;
2. a flat line at 0 °C: melting, ice and water together;
3. a rising line: water warms up to 100 °C;
4. a flat line at 100 °C: boiling, water and steam together.

Reading the graph.

The temperature of the flat part is the melting point (lower flat) or boiling point (upper flat).
The length of a flat part is the time taken for the change of state at the steady heating rate.
A steeper sloping line means the temperature rises faster. Ice (and steam) usually warm up faster than the same mass of water, because it needs less heat per degree.
The rate of rise in a sloping part is $\frac{change in temperature}{time taken}$ in °C per minute.

Why flat? The heat supplied during a change of state is used to separate the particles (this hidden heat is called latent heat), not to raise their average energy, so the temperature stays constant until the change is complete.

Cooling curve. Cool steam steadily: the graph is the mirror image. It falls, is flat at 100 °C (condensation), falls again, is flat at 0 °C (freezing), then falls further. The flat parts are the condensation and freezing points.

Purity. A pure substance melts and boils at fixed temperatures, so its graph has flat parts. A substance that melts over a range of temperatures, with no flat part, is not pure.

Mixture at a flat part. At the flat part of melting, both solid and liquid are present together; the boiling flat has liquid and vapour.

Heating and Cooling Curves: formulas and key facts

Parts of the heating curve of ice: Rise to 0 °C
Flat at 0 °C melting
Rise to 100 °C
Flat at 100 °C boiling; Four parts.
Rate of rise: $rate = \frac{Δ T}{Δ t}$; °C per minute on a sloping part.
Time of a change of state: $t = t_{end of flat} - t_{start of flat}$; From the length of the flat.

How to solve Heating and Cooling Curves questions

Read the graph or table in sections: sloping, flat, sloping, flat.

1Look for readings that stay constant: those are the flat parts.
2Name the change at each flat part and the temperature as the melting or boiling point.
3Subtract times to find how long each flat part lasts.
4For slopes, divide the change in temperature by the time to get °C per minute.
5If there is no flat part, the substance is not pure.

Heating and Cooling Curves in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

The pot of water on the stove

Suppose you heat 1 litre of water steadily from room temperature and record its temperature.

Describe the graph from about 30 °C until the water has all boiled away.

Try it first, then show the solutionSolution

The temperature rises steadily until about 100 °C.
Then the graph becomes flat while the water boils into steam, until all of it has gone.

Answer: A rising line, then a flat line at 100 °C.

Memory hook: Rise, then flat when boiling.

Ice cream melting

Imagine a pure ice block is taken out of a freezer at −10 °C and warmed steadily.

What does the thermometer show while it melts?

Try it first, then show the solutionSolution

The reading rises to 0 °C.
It stays at 0 °C until all the ice has melted.

Answer: 0 °C during melting.

Memory hook: Melting parks the thermometer.

Tricks to remember Heating and Cooling Curves

Short memory aids. Say each one aloud once, then use it on a question the same day.

Flat means changing state
A flat part = change of state at a fixed temperature.
Use it when: Reading graphs.
Two flats for water
Heating ice to steam: flats at 0 °C and 100 °C.
Use it when: Water curve.
No flat, not pure
A substance that melts over a range is impure.
Use it when: Purity.

Heating and Cooling Curves: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Constant temperature · 1 mark · MCQBoard level

While pure ice is melting under steady heating, its temperature:
(a) rises steadily
(b) falls slowly
(c) stays at 0 °C
(d) rises to 100 °C at once

Try it first, then show the solutionSolution

The heat supplied is used for melting; the temperature stays at the melting point until all the ice has melted.

Answer: (c) stays at 0 °C

Example 2: At the flat 100 °C · 2 marks · Short answerBoard level

On the heating curve of water, what does the flat part at 100 °C represent? What is present in the vessel at that time?

Try it first, then show the solutionSolution

It represents boiling: liquid changing to vapour at the boiling point. (1)
Water and steam are present together. (1)

Answer: Boiling; water and steam together.

Example 3: Heating ice · 4 marks · Long answerHarder board level

Ice is heated steadily and the temperature recorded:
Time (min): 0, 1, 3, 5, 7, 9, 11, 13
Temp (°C): −10, 0, 0, 0, 0, 10, 20, 30
(a) Find the melting point. (b) How long does melting take? (c) Find the rate of rise of temperature for the ice, and for the water after melting. (d) What is the state at 4 minutes?

Try it first, then show the solutionSolution

(a) The flat part is at 0 °C. (1)
(b) Flat from 1 min to 7 min: 6 min. (1)
(c) Ice: $\frac{0 - ( - 10 )}{1} = 10$ °C/min. Water: from 7 min (0 °C) to 13 min (30 °C): $\frac{30}{6} = 5$ °C/min. (1)
(d) A mixture of ice and water. (1)

Answer: (a) 0 °C (b) 6 min (c) 10 and 5 °C/min (d) ice and water together

Example 4: Cooling steam · 3 marks · Short answerHarder board level

Steam at 110 °C is cooled steadily to −10 °C. Describe the shape of the cooling curve and name the processes at each stage.

Try it first, then show the solutionSolution

Falls from 110 °C to 100 °C (steam cooling). (½)
Flat at 100 °C: condensation. (1)
Falls from 100 °C to 0 °C (water cooling), flat at 0 °C: freezing, then falls to −10 °C (ice cooling). (1½)

Answer: Fall, flat at 100 °C, fall, flat at 0 °C, fall.

Example 5: Which is pure? · 4 marks · Case-basedHOTS / competency

Solid X: temperature readings every minute while heating: 60, 69, 78, 78, 78, 78, 85, 92. Solid Y: 60, 64, 68, 71, 74, 77, 80, 83. (a) Which one is pure and what is its melting point? (b) How long does X take to melt? (c) Explain why Y has no flat part. (d) Find the rate of rise of X before melting and after.

Try it first, then show the solutionSolution

(a) X is pure; melting point 78 °C. (1)
(b) The flat covers 78 °C at minutes 2 to 5: 3 min. (1)
(c) Y melts over a range of temperatures, which shows that it is not pure. (1)
(d) Before: $\frac{78 - 60}{2} = 9$ °C/min; after: $\frac{92 - 78}{2} = 7$ °C/min. (1)

Answer: (a) X, 78 °C (b) 3 min (c) melts over a range (d) 9 and 7 °C/min

Where students lose marks in Heating and Cooling Curves

The slips examiners see most often, and what to write instead.

Mistake: Calling a flat part "heating stopped"
The heat is still supplied; it changes the state.
Wrong working
"The flat part means no heating."
Right working
"The flat part is a change of state at a fixed temperature."
Do this: Say what the particles do.
Mistake: Reading the wrong flat
The lower flat (0 °C for water) is melting; the upper is boiling.
Wrong working
"Flat at 0 °C is boiling."
Right working
"Flat at 0 °C is melting; flat at 100 °C is boiling."
Do this: Match each temperature to its change.
Mistake: Rate with the wrong time
Use the time between the two readings.
Wrong working
$\frac{30}{13} = 2.3$ °C/min.
Right working
$\frac{30}{13 - 7} = 5$ °C/min.
Do this: Subtract start from end time.

Exam tips

Write what is rising or flat and what it means: state, temperature.
For a table, mark the repeated readings first.
Give units with every rate (°C per minute).
Link "not pure" to a missing flat part.

Practice questions on Heating and Cooling Curves

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
What is the melting point of ice?
Hint
Flat part of the curve.
Answer
0 °C.
Q2 · 2 marks · Short answerHarder board level
A pure solid Z gives these readings at minutes 0, 1, 2, ...: 54, 66, 78, 78, 78, 78, 85, 92. Find its melting point and how long it takes to melt.
Hint
Find the flat part.
Answer
Melting point 78 °C. The flat part runs from minute 2 to minute 5, so melting takes 3 minutes.
Q3 · 3 marks · Short answerHarder board level
Why does the temperature of boiling water not rise although the burner keeps supplying heat? What happens to the water if the burner is switched off?
Hint
Think of latent heat.
Answer
The heat is used to change water into steam, not to raise the temperature, so it stays at 100 °C. If the burner is switched off, the heat supply stops, boiling stops and the water cools.
Q4 · 3 marks · Short answerHarder board level
Using the readings of Z in the previous question, find the rate of temperature rise (a) before melting, (b) after melting. Which state of Z warms faster?
Hint
Change in temperature divided by time.
Answer
(a) 54 to 78 °C in 2 min: $\frac{24}{2} = 12$ °C/min. (b) Minute 5 (78 °C) to minute 7 (92 °C): $\frac{14}{2} = 7$ °C/min. The solid warms faster.
Q5 · 4 marks · Long answerHOTS / competency
A student heats an unknown solid and gets the readings: 60, 64, 68, 71, 74, 77, 80. He says it is pure. Do you agree? What does the graph show, and what would you expect for a pure solid?
Hint
Look for a flat part.
Answer
No. The temperature keeps rising with no flat part, so the solid melts over a range of temperatures, which shows it is impure. A pure solid would show a constant temperature (a flat part) while it melts.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What does a flat part show?Show answer
A change of state at a fixed temperature.
2.Flat temperatures of water?Show answer
0 °C and 100 °C.
3.Pure or not if no flat part?Show answer
Not pure.
4.What is present at the melting flat?Show answer
Solid and liquid together.
5.Cooling curve is?Show answer
The mirror image of the heating curve.

Heating and Cooling Curves: frequently asked questions

What is a heating curve?

A heating curve is a graph of temperature against time as a substance is heated steadily. Sloping parts show warming; flat parts show a change of state.

Why is the graph flat during melting?

The heat supplied is used to loosen the particles, not to raise the temperature, so the temperature stays constant until the whole solid has melted.

How can a graph show if a solid is pure?

A pure solid melts at one fixed temperature, so the graph has a flat part. An impure one melts over a range with no flat part.