Heating and cooling curves

One graph, read both ways

Figure 14.2 plots temperature against the internal energy of a substance. Read from A to F it is a heating curve. Read from F back to A it is a cooling curve.

Sloping parts are one state warming up. Flat parts are a change of state, where the temperature stays constant.

For water, the heating curve is flat at 0 °C while ice melts and at 100 °C while water turns to steam. The cooling curve is flat at the same two temperatures.

Key terms

Heating curve

Temperature against internal energy as a substance is heated.

Cooling curve

The same graph read backwards as a substance is cooled.

Liquefaction point

Where a cooling gas starts to condense; same temperature as the boiling point.

Freezing point

Where a cooling liquid starts to freeze; same temperature as the melting point.

Short questions with model answers

1. Q1. Ice at −10 °C is heated until it is steam above 100 °C. Describe the heating curve.

   • A to B, then B to C.
   • C to D, then D to E.
   • E to F.

   Rise, flat at 0 °C, rise, flat at 100 °C, rise.

2. Q2. Read the curve backwards from F. Name the points where the steam condenses and where the water freezes.

   • Point E.
   • Point C.

   E at 100 °C (condensation) and C at 0 °C (freezing).

Common mistakes

• ✗ On a flat part, no energy is going in.

  ✓ Energy still goes in; it changes the state, not the temperature.

• ✗ Water freezes at a different temperature from where ice melts.

  ✓ Freezing point and melting point are the same temperature (point C).

• ✗ The horizontal axis of Figure 14.2 is time.

  ✓ In your book it is internal energy.

MCQs

1. 1. In Figure 14.2, the line from B to C shows:

   1. (a) The liquid warming up
   2. (b) The liquid boiling
   3. (c) The solid melting at constant temperature
   4. (d) The gas warming up
   Show answer

   (c) p. 4: heat between B and C melts all the solid, so the temperature does not rise.

2. 2. The condensation (liquefaction) point E is at the same temperature as the:

   1. (a) Boiling point
   2. (b) Melting point
   3. (c) Freezing point
   4. (d) Room temperature
   Show answer

   (a) p. 5: the condensation point represents the same temperature as the boiling point.

3. 3. Along D to E, the heat supplied is used to:

   1. (a) Raise the temperature of the liquid
   2. (b) Melt the solid
   3. (c) Raise the temperature of the gas
   4. (d) Change the liquid into gas
   Show answer

   (d) p. 6: lines B to C and D to E change the phase, from solid to liquid and from liquid to gas.

4. 4. On the cooling curve, the freezing point C is the same temperature as the:

   1. (a) Boiling point
   2. (b) Melting point
   3. (c) Liquefaction point
   4. (d) Point F
   Show answer

   (b) p. 5: the freezing point represents the same temperature as the melting point.

5. 5. The heating curve of water is flat at:

   1. (a) 0 °C only
   2. (b) 100 °C only
   3. (c) 0 °C and 100 °C
   4. (d) −10 °C and 0 °C
   Show answer

   (c) pp. 5–6: constant at 0 °C while ice melts and at 100 °C while water turns to steam.

Quick revision

• A heating curve (A to F) slopes while one state warms and is flat while the state changes.
• Read backwards (F to A) it is a cooling curve: E is the liquefaction point, C the freezing point.
• For water the flat parts are at 0 °C and 100 °C, heating or cooling.