Drawing Good Graphs for Chemistry Practicals

A good graph can reveal a relationship in seconds that would take pages to describe in words. A poor graph can hide it completely, or suggest one that isn’t there. In chemistry practicals and exams, graphs are also one of the easiest places to pick up (or lose) marks, because examiners use clear, predictable rules.

Step 1: choose the right type of graph

Type of data	Graph	Example
Independent variable is continuous (numbers on a scale)	line graph / scatter graph	rate vs temperature; volume of gas vs time
Independent variable is categoric (names or groups)	bar chart (bars separated by gaps)	energy released by different fuels; reactivity of different metals
Showing proportions of a whole	pie chart (rare in chemistry)	composition of air
Showing a spread of repeated measurements	histogram	distribution of titre values in a class

Most chemistry practical graphs are line graphs, because variables like concentration, temperature, time and volume are continuous. See independent, dependent and control variables.

Step 2: axes

• Independent variable on the x-axis (horizontal).
• Dependent variable on the y-axis (vertical).
• Label both axes with the quantity and its unit, using a slash or brackets:
  • “Temperature / °C” or “Temperature (°C)”
  • “Volume of hydrogen / cm³”
  • “Rate / s⁻¹”

A missing unit is one of the most common ways to lose a mark.

Step 3: scales

A good scale:

• Uses at least half the graph paper in both directions, ideally much more. Tiny graphs squeezed into a corner lose marks.
• Goes up in even, sensible steps: 1, 2, 5, 10, 20, 25, 50… Avoid awkward steps like 3 or 7 per square, which make plotting and reading difficult.
• Is linear: each square represents the same amount along the whole axis.
• Doesn’t have to start at zero, unless you need the origin (for example, to show that a line passes through it, or to read an intercept). If you start elsewhere, make it clear.

Example: data for temperature from 20 to 60 °C and rate from 0.008 to 0.071 s⁻¹. A good choice: x-axis from 20 to 60 °C at 5 °C per large square; y-axis from 0 to 0.080 s⁻¹ at 0.010 s⁻¹ per large square.

Step 4: plot the points

• Use a sharp pencil.
• Mark each point with a small cross (×) or a small dot in a circle (⊙). Large blobs make it impossible to tell where the point is.
• Plot accurately to within half a small square.
• Double-check each point against the table.

Step 5: the line or curve of best fit

Never join the dots. Experimental data contain random errors, so real points scatter around the true relationship. A line of best fit shows the underlying trend.

• If the points follow a straight-line trend, draw a single straight line with a ruler.
• If they follow a curve, draw a smooth curve freehand in one continuous stroke.
• Aim for roughly equal numbers of points on each side of the line, balanced along its length.
• The line doesn’t have to pass through any particular point, and it only passes through the origin if the science says it should (and the data support it).
• Don’t extend the line far beyond your data unless you’re deliberately extrapolating (and say so).

Step 6: anomalies

An anomalous point doesn’t fit the pattern of the others. It’s usually caused by a mistake or a random error.

• Circle it on the graph.
• Ignore it when drawing the line of best fit.
• In your evaluation, suggest a reason (for example, “the stopwatch was started late”) and, ideally, repeat that measurement.

Reading information from graphs

Describing the relationship

• Positive correlation: as x increases, y increases.
• Negative correlation: as x increases, y decreases.
• Directly proportional: a straight line through the origin. Doubling x doubles y.
• Linear but not proportional: a straight line that doesn’t pass through the origin.
• Levelling off: the curve flattens, meaning y approaches a maximum (common when a reactant is used up).

Always describe with data: “The volume of gas increased rapidly for the first 30 s, then more slowly, levelling off at 48 cm³ after about 90 s.”

Gradient

The gradient of a straight line is:

gradient = change in y ÷ change in x = Δy ÷ Δx

Use a large triangle (at least half the line’s length) to reduce reading errors, and include units (y units ÷ x units).

Example: on a graph of volume of gas (cm³) against time (s), a straight section rises from 10 cm³ at 5 s to 40 cm³ at 25 s.

gradient = (40 − 10) ÷ (25 − 5) = 30 ÷ 20 = 1.5 cm³/s

That gradient is the rate of reaction during that period.

Gradient of a curve: tangents

For a curve, the gradient changes. To find it at a particular point:

1. Draw a tangent: a straight line that just touches the curve at that point, with the same slope as the curve there.
2. Make the tangent long.
3. Calculate its gradient with a large triangle.

This is how you find the initial rate of a reaction (tangent at time = 0) or the rate at any moment on a concentration–time graph.

Intercepts

The y-intercept is where the line crosses the y-axis (x = 0). The x-intercept is where it crosses the x-axis (y = 0). Intercepts often have physical meaning, such as a background reading or a starting value.

Interpolation and extrapolation

• Interpolation: reading a value within the range of your data. Reliable.
• Extrapolation: extending the line beyond your data to predict values. Less reliable, because the relationship may change outside the tested range.

Common graph types in chemistry

Graph	What it shows
Volume of gas vs time	reaction rate; curve levels off as a reactant runs out
Mass loss vs time	rate of reactions releasing gas
Rate vs concentration	order of reaction
pH vs volume of titrant	titration curve; see titration curves
Temperature vs time (cooling curve)	melting/freezing point plateau
Absorbance vs concentration	calibration graph; see calibration curves
ln(rate constant) vs 1/T	Arrhenius plot for activation energy

Common mistakes

• Axes the wrong way round.
• Missing units or labels.
• Scales that use less than half the paper or go up in awkward steps.
• Joining the dots instead of drawing a line of best fit.
• Forcing a line through the origin when the data don’t support it.
• Using a small triangle for the gradient.
• Plotting a bar chart for continuous data or a line graph for categoric data.

A checklist examiners use

1. Axes the right way round, labelled with quantity and unit ✔
2. Sensible scale using more than half the grid ✔
3. Points plotted accurately ✔
4. Suitable line or curve of best fit ✔

Key takeaways

• Use line graphs for continuous data and bar charts for categoric data.
• Independent variable on x, dependent on y; label both with units.
• Choose scales that fill the paper and go up in easy steps.
• Draw a line or smooth curve of best fit, circling and ignoring anomalies.
• Gradient = Δy ÷ Δx using a large triangle; use tangents for curves.