The practical papers (5 and 6)
🎯What you need to be able to do
- Read burettes, thermometers, measuring cylinders, balances and stop-watches to the right precision.
- Record results in a table with headings and units, and plot a graph with a best-fit line or curve.
- Identify anomalous results, draw conclusions, and read values from a graph.
- Evaluate a method, suggest sources of error and improvements, and plan an investigation, including safety.
- Use the qualitative analysis notes (topic 12b) to identify ions and gases from observations.
📚The papers
Every candidate, Core or Extended, takes one of these, worth 20% of the grade:
Paper 5, Practical Test: 1 hour 15 minutes, 40 marks, done in a laboratory
Paper 6, Alternative to Practical: 1 hour, 40 marks, written — you read results from diagrams instead of taking them
Both test the same skills. The difference is only where the data come from, so revise the skills below whichever paper you sit.
Reading apparatus
- Burette: to the nearest 0.05 cm³, two decimal places (23.45, 0.00 — not 0). Read the bottom of the meniscus at eye level (topic 12a).
- Thermometer: usually to the nearest 0.5 °C, one decimal place.
- Measuring cylinder: to the nearest graduation, usually 0.5 or 1 cm³.
- Stop-watch: to the nearest second (reaction time makes hundredths meaningless).
Tables and graphs
- Axes: independent variable on x, dependent on y; label both with quantity and unit.
- Scale: linear and easy to read (2, 5 or 10 per large square), with the points using more than half the grid. The axes need not start at zero.
- Points: small crosses, plotted to within half a small square.
- Line: a single best-fit straight line or smooth curve — never dot to dot. Ignore an anomalous point when drawing it.
Evaluating and planning
- Anomalous result: a point well away from the pattern. Suggest a cause specific to the method (a reading taken before the mixture was stirred, a misread scale, gas escaping before the bung was fitted).
- Improvements must be specific: “insulate the cup with a lid”, “use a burette instead of a measuring cylinder”, “take readings at smaller intervals near the end-point”, “repeat and take a mean”. “Be more careful” scores nothing.
- Variables: the independent variable is changed, the dependent variable is measured, controlled variables are kept the same.
- A plan names apparatus and quantities, the measurements to take, the variables to control, how to process the results, and one safety precaution linked to a real hazard.
✏️Worked example
25.0 cm³ of aqueous sodium hydroxide is put in a polystyrene cup. Dilute hydrochloric acid is added 5.0 cm³ at a time, and the highest temperature
is recorded after each addition (results table above).
(a) Plot the graph and draw two straight lines of best fit. [4]
(b) Identify the anomalous result and suggest a cause. [2]
(c) Use the graph to find the volume of acid that exactly neutralised the alkali. [1]
(d) Suggest one improvement to the method. [1]
(a) See the graph: one line through the rising points, a second through the falling points, extended until they cross.
(b) 15.0 cm³, 25.2 °C: it is 1.2 °C below the line. The reading was probably taken before the temperature had reached its maximum (or before the mixture was stirred).
(c) 20.0 cm³, where the lines cross (the maximum temperature, 28.2 °C).
(d) Add a lid to reduce heat loss, or add the acid in smaller portions (1.0 cm³) near 20 cm³.
Check it. The temperature rises while neutralisation (exothermic) is still happening, then falls because cold acid is added after all the
alkali has been used up. The turning point is the end-point ✓.
Joining the points. Dot-to-dot lines through the anomaly give a peak at the wrong volume. Two ruled straight lines, ignoring the anomaly,
give 20.0 cm³.
📝Practise
In the style of Papers 5 and 6.
1. A burette reads 1.20 cm³ at the start and 24.65 cm³ at the end. Calculate the volume added. [1]
\( 24.65 - 1.20 = 23.45 \) cm³.
2. Titres of 24.10, 23.35, 23.50 and 23.55 cm³ were recorded; the first was a rough titration. Which titres are concordant, and what is their mean? [2]
23.50 and 23.55 cm³ (within 0.10 cm³). Mean \( = (23.50 + 23.55) \div 2 = 23.525 \), recorded as 23.53 cm³.
3. A student records a temperature as “24” from a thermometer marked every 0.5 °C. What is wrong, and how should it be written? [1]
It is not recorded to the precision of the thermometer: write 24.0 °C.
4. In an investigation of how the concentration of hydrochloric acid affects the rate of its reaction with magnesium ribbon, name the independent variable, the dependent variable and two controlled variables. [4]
Independent: concentration of the acid. Dependent: rate (e.g. time for the magnesium to disappear, or gas volume per unit time). Controlled (any two): volume of acid; length/mass of magnesium ribbon; temperature.
5. Plan an experiment to find which of two solid fuels releases more heat per gram when burned. [6]
Measure 100 cm³ of water into a copper can, record its temperature. Weigh the first fuel in its burner. Burn it under the can until the temperature rises by about 20 °C, stir, record the highest temperature, and reweigh the burner. Calculate temperature rise per gram burned. Repeat with the second fuel with the same volume of water, the same can and the same distance from the flame. Repeat each and take a mean. The fuel with the larger temperature rise per gram releases more heat. Safety: tie back hair / keep flammables away.
6. When measuring the gas given off by a reaction, a student adds the solid to the acid and then fits the bung. Explain why the volume recorded is too low, and suggest an improvement. [2]
Some gas escapes before the bung is fitted. Put the solid in a small tube inside the flask (or in a separate compartment), fit the bung, then tip it over to start the reaction.
7. Why is a lid used on the polystyrene cup in a temperature-change experiment, and why polystyrene rather than glass? [2]
Both reduce heat loss to the surroundings: the lid stops loss from the surface, and polystyrene is a poor thermal conductor (glass conducts heat away).
🔗Go deeper — other people’s work
These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.
- Cambridge International — the 0620 syllabus section on practical assessment, and the Paper 5 confidential instructions (ask your teacher)
- Royal Society of Chemistry — Practical Chemistry, method sheets for the standard experiments