Plotting and reading graphs

Plotting

• The independent variable (the one you choose) goes on the x-axis; the dependent variable (the one you measure) on the y-axis.
• Choose scales so the points cover at least half the grid in each direction, with easy steps (1, 2, 5 or 10 per large square, not 3).
• The origin need not be included: an axis from 20 °C to 40 °C is fine for readings of 21 °C to 39 °C.
• Label each axis with quantity / unit, and plot points as small crosses.
• Draw one straight line with a ruler or one smooth curve, with points spread evenly either side. Never join dot to dot.

Gradients

• Use a large triangle covering more than half the line, and show it on the graph.
• Gradient = change in y / change in x, with units (y unit / x unit).
• For a curve, draw a tangent at the point and find its gradient: this gives a rate at that instant.

Extrapolation in enthalpy experiments

In a temperature-time graph, the reactants are mixed at a known time (say 3 minutes). After mixing, the temperature peaks and then falls as heat is lost. Draw the best line through the cooling points and extrapolate it back to the time of mixing. The temperature change is read at the time of mixing, from the steady starting temperature to the extrapolated line. This corrects for heat lost while the reaction was happening. Do not just use the maximum reading.

Worked example

A straight line passes through (0.10 g, 24 cm³) and (0.40 g, 96 cm³) on a graph of gas volume against mass of solid. Find the gradient.

1. Change in y = 96 - 24 = 72 cm³
2. Change in x = 0.40 - 0.10 = 0.30 g
3. Gradient = 72 / 0.30

Answer: 240 cm³ g^-1

Check yourself

1. A best-fit line passes through (2.0, 0.40) and (12.0, 1.90). Calculate its gradient.
   Show answer

   0.15. (1.90 - 0.40) / (12.0 - 2.0) = 0.15.

2. In a temperature-time graph the steady starting temperature is 21.4 °C. The cooling line, extrapolated back to the time of mixing, reaches 31.6 °C. What temperature change should be used?
   Show answer

   10.2 °C. 31.6 - 21.4 = 10.2 °C.

3. In an experiment to see how temperature affects the time for a precipitate to form, which variable goes on the x-axis?
   • Temperature
   • Time for the precipitate to form
   • Volume of solution
   • Concentration of product
   Show answer

   Temperature. Temperature is chosen by the experimenter: it is the independent variable.

4. A student's points occupy only the bottom-left quarter of the grid. What is wrong?
   • The axes should be swapped
   • The points should be dots, not crosses
   • The line should be dot to dot
   • The scales are too large, so the points do not cover at least half the grid
   Show answer

   The scales are too large, so the points do not cover at least half the grid. Choose scales so the plotted data fill most of the grid.

5. Why is the cooling line extrapolated back to the time of mixing in an enthalpy experiment?
   • To make the graph look smoother
   • To allow for heat lost to the surroundings while the reaction was taking place
   • Because the thermometer was not working at the time of mixing
   • To find the time the reaction finished
   Show answer

   To allow for heat lost to the surroundings while the reaction was taking place. The maximum reading is too low because heat is lost during the reaction.

6. A tangent to a curve of gas volume against time passes through (0 s, 48.0 cm³) and (60.0 s, 0 cm³). What is the magnitude of the gradient of the tangent?
   Show answer

   0.80 cm³ s^-1. |0 - 48.0| / 60.0 = 0.80 cm³ s^-1.

7. How should the axis showing the volume of gas in cubic centimetres be labelled?
   Show answer

   volume of gas / cm³. Quantity, slash, unit.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH13 June 2024
• Pearson examiners' report, WCH13 October 2021

Last updated 8 November 2026. Spotted a mistake? Tell us.