Heating and Cooling Graphs

Section 1 of 6Reading a heating curve

Section of graph	What is happening	Energy supplied is used to...
Rising line (solid)	Solid gets warmer	Increase kinetic energy of the particles
First flat line	Melting; solid and liquid exist together	Break bonds, increasing potential energy
Rising line (liquid)	Liquid gets warmer	Increase kinetic energy of the particles
Second flat line	Boiling; liquid and gas exist together	Separate particles completely
Rising line (gas)	Gas gets hotter	Increase kinetic energy of the particles

Section 2 of 6Reading the graph correctly

1

Check the axes: temperature is normally on the y-axis and time on the x-axis.

2

Read the y-value of the lower flat section to find the melting point, and of the upper flat section to find the boiling point.

3

Find the state of the substance by looking at where the point lies relative to these two temperatures.

4

Compare lengths of flat sections: a longer flat section means more energy was needed for that change of state.

Section 3 of 6Cooling curves

1Gas cools

• Gas cools Particles slow down and the temperature falls steadily.

2Condensing

• Condensing Energy is released as bonds form; temperature stays at the boiling point.

3Liquid cools

• Liquid cools Temperature falls again.

4Freezing

• Freezing Energy is released while the substance solidifies; temperature stays at the melting point.

Section 4 of 6Why graph slopes differ

Gradient

• Gradient Since the energy is supplied at a constant rate, a steeper slope means a smaller specific heat capacity for that state.
• For water, the liquid slope is shallower than the ice and steam slopes.

Pure substances

• Pure substances A pure substance has sharp, fixed melting and boiling points;
  • an impure one melts over a range of temperatures.

Cooling graphs

• Cooling graphs The rate of cooling is greatest when the substance is much hotter than the surroundings, so the curve is steepest at the start.

Section 5 of 6Key terms

Term	Meaning
Heating curve	A graph of temperature against time as a substance is warmed.
Melting point	The temperature at which a solid becomes a liquid.
Boiling point	The temperature at which a liquid becomes a gas.
Plateau	A flat section of a temperature–time graph.
Pure substance	A single element or compound, with a sharp melting and boiling point.

Section 6 of 6Exam technique

1

Saying that nothing is happening during a flat section; energy is still being transferred and the state is changing.

2

Reading the melting point from the highest point of the graph rather than the first plateau.

3

Confusing a heating curve with a cooling curve and giving the wrong direction of the energy transfer.

4

Explaining the plateau as 'the particles stop moving'; they keep moving but the energy goes into potential stores.