Practical skills — Cambridge IGCSE Combined Science
Planning, tables, graphs, biological drawings, magnification, evaluating experiments, errors and uncertainty, and indicators for Papers 5 and 6.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Planning an investigation means stating exactly what you change, what you measure, what you keep the same and how. The independent variable is changed, usually over at least five evenly spaced values; the dependent variable is measured with named apparatus. Keep controlled variables constant, include a control experiment, repeat each test at least three times and calculate a mean.
A Cambridge results table is ruled, with the independent variable in the first column and each heading giving the quantity and unit separated by a solidus, such as time / s. Units go only in the heading, never in the body. Record data to the precision of the instrument, to the same decimal places in each column.
A line graph is used when the independent variable is continuous, such as temperature, while a bar chart, with bars that do not touch, is used for categories. Put the independent variable on the x-axis, label axes with quantity and unit, use more than half the grid, plot small crosses and draw a single thin best-fit line or curve.
A biological drawing must be large, with clear, unbroken pencil lines, no shading or colour, and ruled label lines that touch the feature without arrowheads. Measure sizes in millimetres with a ruler. Magnification equals image size divided by actual size, both in the same unit, and has no unit; it is written with a × sign, such as ×3.
Evaluating an experiment means judging how good the method and data are, then suggesting specific improvements. Identify any anomalous result, a result that does not fit the pattern, and leave it out of the mean. Name sources of error and exact fixes, such as using a syringe instead of a beaker. Human error or being more accurate scores nothing.
Read scales to the nearest half division, correct zero errors, tell random from systematic error, and treat results within 10% as equal.
Indicators are substances that change colour to show a chemical change, and you must give the colour before and after. Limewater turns from colourless to milky with carbon dioxide. Hydrogencarbonate indicator is red at the level in air, yellow when carbon dioxide increases and purple when it decreases. Universal indicator gives the pH number; litmus shows only acid or alkali.
Exam questions
6 exam-style questions (29 marks), each with its mark scheme.
Answer the questionsFlashcards
27 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of experimental skills on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
How should a column heading for time in seconds be written?
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time / s
Which variable goes on the x-axis?
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The independent variable, unless told otherwise.
State the formula for magnification.
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magnification = image size ÷ actual size
What colour does hydrogencarbonate indicator turn when carbon dioxide increases?
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Yellow (from red).
A balance reads 0.3 g with nothing on it. What is this, and how do you correct for it?
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A zero error (a systematic error). Subtract 0.3 g from every reading.
Two results are 40 s and 43 s. Are they equal within the limits of experimental accuracy?
Show answer
Yes. 10% of 40 s is 4 s, so any result from 36 s to 44 s counts as equal.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026