Required practicals and exam skills
Chemistry · GCSE · The School
What is examined
The practicals themselves are examined on paper, not in the lab, so the method has to be known in words. The standard set covers making a soluble salt, titration, electrolysis, temperature change, rates of reaction, chromatography and water purification. For each, know the equipment, the steps in order, and the one control that makes it fair.
Measurement and error
Name the instrument and the precision: a measuring cylinder to the nearest cm³, a burette to 0.05 cm³, a balance to 0.01 g. RANDOM error is reduced by repeating and taking a mean; SYSTEMATIC error (an uncalibrated balance, a misread meniscus every time) is not — repeating just repeats it. Saying which kind you have is what earns the mark.
Writing the long answer
For six marks, write a method someone else could follow: equipment, quantities, the variable changed, what is kept the same, what is measured and how often, repeats, and the safety precaution WITH its reason — 'goggles because acid can splash', not just 'wear goggles'. Bullet points in a sensible order beat a paragraph.
"Describe how to investigate the effect of concentration on rate." (1) Measure 50 cm³ of sodium thiosulfate into a conical flask on a paper cross. (2) Add 5 cm³ of hydrochloric acid and start a stopwatch. (3) Stop timing when the cross disappears from above. (4) Repeat with concentrations from 10 to 50 g/dm³, keeping volume, temperature and the same cross throughout. (5) Repeat each three times and take a mean. (6) Plot rate (1/time) against concentration. Six steps, six marks.
A second worked method: titration
"Describe how to find the concentration of a hydrochloric acid solution using a standard solution of sodium hydroxide." (1) Rinse a burette with the acid, then fill it and note the starting volume. (2) Pipette 25.0 cm³ of the sodium hydroxide into a conical flask and add a few drops of indicator. (3) Add acid from the burette, swirling constantly, until the indicator just changes colour — this is the END POINT. (4) Record the volume added. (5) Repeat until two readings agree within 0.10 cm³, and use those to calculate a mean. (6) Use the mean volume, the known concentration, and the equation to find the unknown concentration. A different practical, the same six-mark shape: numbered steps, a stated precision, and repeats used properly.
Anomalies are identified, not deleted
A result that does not fit is reported, excluded from the mean with a reason, and if possible explained. Quietly dropping it is not science, and examiners look for the reasoning. Repeats matter because they make anomalies visible; a single run cannot tell you whether a value is odd.