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wise otter.Chemistry
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K324 / 2027
Data and Experimental Skills

Lesson 2 of 8

Graphs and calculations

Turn numbers into a trend, gradient or justified result.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can present data in an appropriate, useful form.Syllabus

    Syllabus K324, PDO. Present data in an appropriate, useful form

  • I can analyse, conclude and evaluate.Syllabus

    Syllabus K324, ACE. Analyse, conclude and evaluate

  • I can measure reaction rates.Syllabus

    Syllabus K324, T2. Measure reaction rates

Turn measurements into a graph or calculation

Keep the meaning of each axis, ratio and unit visible.

Put the independent variable on the horizontal axis and the measured or derived dependent variable on the vertical axis. Label each axis with a quantity and unit, use a clear uniform scale, and plot the actual points. A best-fit line or smooth curve represents a trend; do not force it through every point or through the origin without a reason.

Gas collected in an illustrative reaction
Time / sGas volume / cm3
00
2024
4039
6045
8048

An interval rate is a gradient between two points

Between 20 and 40 seconds, gas volume increases from 24 to 39 cubic centimetres. The horizontal change is 20 seconds and the vertical change is 15 cubic centimetres. The curve becomes less steep later.

Illustrative measurements at constant collection temperature and pressure. The curve shows the trend; use the table for exact values.

Worked exampleCalculate and interpret a gradient

Find the average gas-production rate between 20 and 40 s.

Try it first. Then show the worked stepsHide the worked steps
  1. Change in gas volume = 39 - 24 = 15 cm3.
  2. Time interval = 40 - 20 = 20 s. Average rate = 15 / 20 = 0.75 cm3 s-1.
  3. This is an interval average. A rate at one instant is the gradient of a tangent to the curve at that instant, where required.
Answer

0.75 cm3 s-1. The graph later becomes flatter, so the gas-production rate decreases.

Interpolation estimates a value within the measured range. Extrapolation extends a trend outside it and needs an extra assumption that the relationship continues. If a graph is linear, choose two well-separated points on the best-fit line for a gradient, not two close points chosen because they happen to be measurements.

Keep guard digits during working. Round the final answer to a precision justified by the inputs and any question instruction. A unit conversion changes the numerical value, not the physical quantity: 25.0 cm3 = 0.0250 dm3. Check dimensions before inserting numbers into a formula.

Check your understanding

A gas-volume curve is high but nearly horizontal. Does its height show a high rate at that moment?

  1. No. Height shows the rate, but this curve is too high to read accurately.
  2. Yes. More gas has been collected, so the reaction is going faster now.
  3. No. Height shows gas made so far; the flat gradient shows a low rate now.
Show the answer

No. Height shows gas made so far; the flat gradient shows a low rate now.

The rate at a moment is the gradient of the curve. A high, flat curve means a lot of gas has been made but very little is being made now.