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LearnYCH11WCH16 · Unit 6: Practical Skills in Chemistry IIPractical skillsPlanning and evaluating experiments
Practical skills · Subtopic 1 of 1

Planning and Evaluating Experiments

Planning: state the independent (changed), dependent (measured) and controlled variables; write a hypothesis; choose apparatus and quantities (with reasons); give a step-by-step method; identify hazards and precautions. Analysis: tabulate with units; plot graphs with the independent variable on xx; draw a line of best fit; calculate gradients using a large triangle; use derived quantities (1/t, ln k). Evaluation: identify anomalies; distinguish random errors (scatter; reduce by repeats) from systematic errors (shift all results; reduce by changing method); calculate % error against a reference value and compare with total % uncertainty; suggest specific improvements.

Cambridge 9701 Paper 5Edexcel YCH11 WCH16AQA 7405 Paper 3≈ 45 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Identify variables and write a hypothesis.
  • Plan a safe, detailed method.
  • Process data and calculate gradients.
  • Evaluate errors and suggest improvements.
Pause and think

Your measured enthalpy of neutralisation is 10% less exothermic than the data book value, but your total uncertainty is only 3%. What does that tell you?

Section 2 of 14

Core idea

Start hereThinking like an examiner

A good plan names its variables, uses real quantities, and explains how it will reduce error. "Be careful" earns nothing.

The big idea

Control every variable except one, process the data with graphs, and compare your error with your uncertainty.

From plan to evaluation

Each stage feeds the next.

Worked examplePercentage error

Measured −52.0, accepted −57.1 kJ mol⁻¹. % error?

Goal
%.
Strategy
|E − A| / A.
  1. ∣−52.0+57.1∣=5.1|{-52.0}+57.1|=5.1
    Difference.
  2. 5.157.1×100=8.9%\frac{5.1}{57.1}\times100=8.9\%
    Relative to the accepted value.
Answer8.9%.
✓ CheckBigger than a 2% uncertainty, so systematic error ✓.

Summary

  • Independent, dependent, controlled.
  • Large triangle for gradients.
  • Random vs systematic error.
Key words
Systematic error
Error that shifts every result the same way.
Random error
Unpredictable scatter; reduced by repeats.
HistoryLavoisier's balance. Antoine Lavoisier's careful planning and precise weighing in the 1770s proved conservation of mass and overturned phlogiston theory.
Fun factAnomalies can be discoveries. Argon was discovered in 1894 because nitrogen from air was 0.5% denser than nitrogen from chemicals: an "anomaly" that turned out to be a new element.
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