Interpreting Graphs in Chemistry Exams
How to read, interpret and draw graphs in IGCSE Chemistry 0620. Rate curves, heating curves, solubility curves and what the examiner expects.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Graphs appear on every 0620 paper. Paper 2 tests graph interpretation in MCQs. Paper 4 asks you to read graphs, describe trends and sketch additional curves. Paper 6 asks you to draw graphs from data and analyse them. The skills overlap, but the exam expects specific techniques for each type. More detail on graph skills for Paper 6 is in Paper 6 tables and graphs.
Reading a graph: the first 30 seconds
Before answering any question about a graph, spend 30 seconds reading:
- Title (if given): what the graph shows.
- Axes: what variable is on each axis, including units.
- Scale: what each square or division represents.
- Shape: is the line straight, curved, or has flat sections? What does the shape tell you?
These 30 seconds prevent the most common errors: reading from the wrong axis, using the wrong units, and misidentifying the trend.
Rate of reaction curves
The most common graph type on Papers 2 and 4. Typically, volume of gas (y-axis) is plotted against time (x-axis).
What each feature tells you
| Feature | Meaning |
|---|---|
| Steep initial section | Fast initial rate (many reactant particles, frequent collisions) |
| Curve flattening | Rate slowing down (reactant being used up, fewer collisions) |
| Curve levels off (horizontal) | Reaction complete (limiting reagent fully consumed) |
| Higher final level | More product formed (more reactant was present) |
| Same final level, steeper curve | Same amount of product, formed faster (higher temperature, catalyst, or smaller particles) |
Sketching a second curve
When asked to sketch a curve for a changed condition on the same axes:
Higher temperature (same amounts): Steeper curve, levels off at the same height. The reaction is faster but produces the same amount of product.
More concentrated acid (same volume and mass of solid): Steeper curve, levels off at the same height (if the solid is the limiting reagent) or at a higher level (if the acid is the limiting reagent).
Catalyst added (same amounts): Steeper curve, same final height.
Larger pieces of solid (same mass): Less steep curve, same final height. Fewer surface collisions, but the same total amount of reactant.
Always ensure your sketched curve does not cross above the original curve if the final amount should be the same.
Heating and cooling curves
These show temperature (y-axis) against time (x-axis) as a substance is heated or cooled.
Reading a heating curve
- Rising sections: the substance is in one state and its temperature increases.
- Flat sections: a change of state is occurring. Temperature stays constant because energy is used to overcome intermolecular forces, not to increase kinetic energy.
- First flat section: melting point (solid to liquid).
- Second flat section: boiling point (liquid to gas).
Read the temperature of each flat section directly from the y-axis to find the melting and boiling points.
Cooling curves
The same shape reversed. Flat sections show condensing and freezing. If the substance is impure, the flat section may slope slightly instead of being perfectly horizontal.
Solubility curves
These show the mass of solute that dissolves in a fixed volume of solvent (usually 100 g of water) at different temperatures.
Using solubility curves
To find how much dissolves: Go to the temperature on the x-axis, go up to the curve, then read across to the y-axis.
To predict crystallisation: If a saturated solution at high temperature is cooled, the amount that crystallises out is the difference between the solubility at the high temperature and the solubility at the low temperature.
Example: If 80 g of potassium nitrate dissolves in 100 g water at 60 degrees C, and only 30 g dissolves at 20 degrees C, then cooling a saturated solution from 60 to 20 degrees C crystallises 80 - 30 = 50 g of potassium nitrate.
Drawing graphs on Paper 6
When asked to draw a graph from tabulated data:
- Choose the axes correctly. The independent variable (the one you controlled) goes on the x-axis. The dependent variable (the one you measured) goes on the y-axis.
- Choose a scale that uses more than half the grid in each direction. Awkward scales (multiples of 3 or 7) should be avoided; use multiples of 1, 2, 5 or 10.
- Label both axes with the variable name and unit.
- Plot each point as a small, precise cross (x) or a dot in a circle. Large blobs are imprecise.
- Draw a line of best fit. This is a smooth curve or a straight line that passes through or near the plotted points. Do not join the dots with ruler-straight segments.
- Identify anomalous points. Circle any point that does not fit the pattern and exclude it from your line of best fit.
Common errors in graph drawing
- Choosing a scale that wastes space (all points in one corner).
- Not labelling axes or omitting units.
- Joining dots with straight lines instead of drawing a smooth curve.
- Plotting points from the wrong column of the table.
- Forgetting to identify anomalous results.
Calculating gradient
When asked to find the rate from a graph:
- Draw a tangent to the curve at the specified point.
- Choose two points on the tangent that are far apart (for accuracy).
- Gradient = change in y / change in x. Include units.
Example: If the tangent passes through (0, 0) and (20, 40), gradient = 40 cm3 / 20 s = 2.0 cm3/s.
Show the triangle on the graph and the calculation on the paper. The working is worth a method mark.
Worked exam question
Q (Paper 4): The graph shows the volume of hydrogen gas collected when zinc reacts with excess dilute hydrochloric acid at 25 degrees C. (a) State the total volume of hydrogen gas produced. (1) (b) Using the graph, determine the rate of reaction at 30 seconds. (2) (c) Sketch on the same axes the curve expected at 35 degrees C with the same amounts of reactants. (2)
Model answer: (a) Read from the graph where the curve levels off: e.g. 60 cm3 (1). (b) Draw a tangent at t = 30 s (1). Calculate gradient from the tangent: e.g., (50 - 20) / (40 - 20) = 1.5 cm3/s (1). (c) Draw a curve that rises more steeply than the original (1) and levels off at the same final volume (60 cm3) (1).
The marks in (c) are specifically for steeper + same height. A curve that is steeper but reaches a higher volume, or one that is the same steepness, earns 0 or 1.
If graph interpretation is costing marks, the issue is usually about reading technique rather than chemistry knowledge. A trial lesson can practise graph reading and sketching on real past-paper examples.