Chemistry Required Practicals and Data Analysis
The GCSE Chemistry required practicals are the eight investigations every student must carry out: making a soluble salt, titration, electrolysis, temperature changes in reactions, rates of reaction, chromatography, water purification, and identifying ions. Around 15 percent of the marks on the papers are for practical skills such as naming variables, justifying repeats and concordant results, and calculating means and rates.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Name the variables before describing anything: the independent variable you change, the dependent variable you measure, and the control variables that keep the comparison fair, such as the volume and concentration of acid, the mass and surface area of the solid, and the temperature.
- Write methods as numbered steps with quantities: state the volumes in cubic centimetres, the concentrations in moles per cubic decimetre, the mass in grams, and the time interval between readings. A method without numbers cannot be repeated and does not earn full marks.
- In titration, learn the sequence and the vocabulary: rinse the burette with the solution it will hold, fill and record the initial reading to the nearest 0.05 cubic centimetres, add a measured volume with a pipette to the conical flask with a few drops of indicator, add from the burette swirling constantly, slow to drops near the end point, then repeat until two titres are concordant, meaning within 0.10 cubic centimetres of each other, and mean only those.
- For rates, know both ways of measuring: the volume of gas produced against time (gas syringe or inverted measuring cylinder), or the loss of mass against time, or the time for a cross to disappear through a cloudy precipitate. Rate = change in quantity / time, and the gradient of the tangent gives the rate at an instant.
- For temperature-change experiments, use a polystyrene cup with a lid to reduce heat loss to the surroundings, and record the temperature before mixing and at the maximum or minimum reached, not at a fixed time.
- Evaluate with a specific improvement, not a general one: name the apparatus with better resolution (a burette rather than a measuring cylinder, a balance reading to 0.01 g), the control variable that was not held constant, or the source of loss (gas escaping before the bung was replaced, heat lost to the air).
Worked example
A student carries out a titration and records titres of 24.60, 23.85, 23.90 and 23.80 cubic centimetres. Calculate the mean titre that should be used.
- Identify the rough titre: 24.60 is the first result and is well outside the others, so it is discarded as the rough run.
- Check the remaining results for concordance: 23.85, 23.90 and 23.80 all lie within 0.10 cubic centimetres of each other, so all three are concordant.
- Add the concordant titres: 23.85 + 23.90 + 23.80 = 71.55.
- Divide by three: 71.55 / 3 = 23.85.
- Final answer: mean titre = 23.85 cubic centimetres, using only the concordant results and stating that the rough titre was excluded.
Practice questions
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Q1What does 'concordant' mean when applied to titres?Show answer
Answer: Titres that agree within 0.10 cubic centimetres of each other. Only concordant titres are used to calculate the mean.
Q2Why is a polystyrene cup with a lid used in energy-change experiments?Show answer
Answer: It insulates the reaction mixture, reducing heat transfer to the surroundings so the temperature change measured is closer to the true value.
Q3Give the resolution of a burette that can be read to the nearest half of a small division.Show answer
Answer: 0.05 cubic centimetres.
Q4A gas syringe collects 48 cubic centimetres of gas in 40 seconds. Calculate the mean rate.Show answer
Answer: 48 / 40 = 1.2 cubic centimetres per second.
Q5Why is the burette rinsed with the solution it is going to contain rather than with water?Show answer
Answer: Water left in the burette would dilute the solution, so the concentration would be lower than stated and the titre would be too large.
Q6Name one way to make a rate of reaction experiment more valid rather than simply more precise.Show answer
Answer: Control the temperature with a water bath, since temperature strongly affects the rate and an uncontrolled room temperature would confound the results.
Q7In the rate experiment where a cross disappears, why is the result subjective?Show answer
Answer: The point at which the cross is judged to have disappeared depends on the observer, so different people record different times. Using a light sensor and data logger removes the judgement.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Describe how a student could measure the rate of reaction between marble chips and hydrochloric acid using a gas syringe. Include two control variables.
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A student obtains a mean titre that is consistently 0.50 cubic centimetres higher than the class average. Suggest one cause and explain whether repeating the titration more times would fix it.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the chemistry required practicals and data analysis worksheet pack - 19 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 11 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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