IGCSE Chemistry (0620)

Sources of error and improving accuracy

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

When approaching the titration end-point, acid must be added dropwise with swirling because adding too quickly can overshoot the exact volume needed for the indicator to change, leading to an inaccurate result.
Repeating experiments and averaging results reduces the impact of random errors and helps identify anomalies, making the data more reliable than relying on a single measurement that could be affected by chance mistakes.
To ensure all experiments start at the same temperature, the solution must be allowed to reach thermal equilibrium in the controlled environment and checked with a thermometer before proceeding.
Common mistake: Students often skip waiting or verifying with a thermometer.
A metal oxide layer can prevent the metal from reacting with acid.
Common mistake: Students must recognize that removing this layer (e.g., by abrasion) is necessary for the reaction to proceed; forgetting this leads to no or slow reaction.
The precision of a volume measurement depends on the smallest scale division of the apparatus used.
The precision of the measuring instrument affects the reliability of results: a burette's finer graduations allow for more accurate and consistent measurement of liquid volumes than a measuring cylinder, reducing variation between trials.

Worked example

State how the student should run the dilute nitric acid from the burette when the end-point is close, so that the end-point is not overshot. Do not give any observations in your answer.

Try it first, then show the working

Close to the end-point a single drop changes the colour permanently. Adding dropwise with swirling means the exact volume needed is found rather than being passed.
Answer: Add the acid one drop at a time (dropwise) while swirling the flask.
Examiner's mark scheme: add the nitric acid dropwise / one drop at a time near the end-point; rinse the flask sides with distilled water so all acid added is mixed in

Answer: Add the acid one drop at a time (dropwise) while swirling the flask.

Try one yourself

Explain why carrying out each experiment three times and calculating a mean time would improve the results.Answer it and get it marked → Describe how the temperature of the acid can be held at the same value in every experiment.Answer it and get it marked → Suggest how the aluminium strip should be treated before it is added to the acid.Answer it and get it marked →

27 exam-style questions on Sources of error and improving accuracy, each marked to the scheme.

Sources of error and improving accuracy

Key ideas and common mistakes

Worked example

Try one yourself

More IGCSE Chemistry topics