OCR A Level Chemistry A: Practical skills assessed in a written examination (planning, implementing, analysis and evaluation) (H432) – Revision Notes

These notes condense section 1.1, Practical skills assessed in a written examination, from the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover 1.1.1 Planning, 1.1.2 Implementing, 1.1.3 Analysis and 1.1.4 Evaluation. The section has no tiers; all of it applies to the full A level. Explanations and longer worked examples are in the practical skills study guide.

Then test yourself with the practice questions. Course links: OCR A Level Chemistry hub, printable checklist, free diagnostics.

Definitions to know word for word

Term	Meaning
Independent variable	The one you deliberately change
Dependent variable	The one you measure
Controlled variable	Any other variable that could affect the result, kept constant
Accurate	Close to the true or accepted value
Precise	Repeated values are close to one another
Repeatable	Same person, same method and equipment, similar results
Reproducible	Different person, method or equipment, similar results
Resolution	Smallest change the instrument can show
Uncertainty	The range, ± a value, within which the true value lies
Random error	Unpredictable scatter either side; reduced by repeats
Systematic error	Same direction every time; repeats do not remove it
Anomaly	A value that does not fit the pattern of the others
Valid conclusion	Supported by the data and answers the question asked

1.1.1 Planning

Method in steps: writing a plan

1. State the independent and dependent variables, with units.
2. Choose apparatus for each measurement and say why (pipette for an exact volume, gas syringe for gas volume).
3. Choose quantities: estimate the expected reading so it fits the apparatus range.
4. List controlled variables by name and say how each is controlled.
5. Describe the measurements in order, including repeats.
6. Say how the results are processed to give the answer (a calculation, or a graph and what you read from it).

1.1.1(c) appropriate method? Ask: does it measure the right quantity; are the readings within range and large enough for a small percentage uncertainty; is anything lost, gained or side-reacting?

Small reminder: a student plans to collect oxygen from hydrogen peroxide in a 50 cm³ syringe but expects about 80 cm³ of gas. The method is not appropriate as planned; use a 100 cm³ syringe or less peroxide.

1.1.2 Implementing

Techniques (1.1.2(a)). Expect “why” questions on titration, standard solutions, reflux, distillation, filtration, recrystallisation, separating funnels, melting points, chromatography, electrochemical cells, pH measurement and rate methods. One-line reasons:

• Rinse the burette with the titrant, the pipette with the solution it delivers.
• Fill the volumetric flask to the mark with the bottom of the meniscus on the line at eye level, then invert to mix.
• Use a white tile under the flask to see the end point.
• Anti-bumping granules give smooth boiling.
• Reflux stops volatile substances escaping; distillation separates by boiling point.

Units (1.1.2(b), M0.0). Use cm³ or dm³, mol dm⁻³, kJ mol⁻¹, J K⁻¹ mol⁻¹, s, K or °C. Work out units of Kc and k each time.

Worked reminder: Kc = [C]² ÷ ([A][B]³). Units = (mol dm⁻³)² ÷ (mol dm⁻³)⁴ = (mol dm⁻³)⁻² = dm⁶ mol⁻². For rate = k[A][B], k has units dm³ mol⁻¹ s⁻¹.

Presenting data (1.1.2(c)).

• Column headings: quantity / unit. No units in the body of the table.
• Independent variable in the first column.
• Same number of decimal places down a column, matching resolution (burette: 2 d.p., ending 0 or 5).
• Observations in full: colour before and after, “effervescence”, “white precipitate”.

1.1.3 Analysis

Significant figures (M1.1). Give the answer to the least number of significant figures in the data used. Round once, at the end. Keep trailing zeros when converting between ordinary and standard form.

Worked reminder: 1.37 g of NaCl (M = 58.44) made up to 250.0 cm³. c = 1.37 ÷ 58.44 ÷ 0.2500 = 0.09377… The mass has 3 s.f., so c = 0.0938 mol dm⁻³.

Graph rules (1.1.3(d)(i)).

• x-axis: independent variable. Both axes: quantity / unit.
• Scale: points fill more than half the grid; steps of 1, 2 or 5; no awkward scales such as 3 per square.
• Plot points as small crosses; draw a single thin best-fit line or smooth curve.
• The origin is included only if it helps.

Gradients and intercepts (1.1.3(d)(ii)).

Task	How
Gradient of a line	Δy ÷ Δx from two far-apart points on the line
Gradient of a curve	Draw a tangent at the point; Δy ÷ Δx of the tangent
Intercept, axis from zero	Read where the line crosses the y-axis
Intercept, axis not from zero	c = y − mx using a point on the line
Units of gradient	y-unit ÷ x-unit

Worked reminder: a calibration line of absorbance against concentration passes through (0.0040 mol dm⁻³, 0.25) and (0.0240 mol dm⁻³, 1.45). Gradient = 1.20 ÷ 0.0200 = 60 dm³ mol⁻¹. Intercept = 0.25 − 60 × 0.0040 = 0.01, close to zero, as expected when the blank reads zero.

Qualitative conclusions (1.1.3(a)). Follow the specification’s order for anions: carbonate, then sulfate, then halide. Conclude only what the tests show; a negative test rules an ion out, it does not identify another.

1.1.4 Evaluation

Formulae

Quantity	Formula
Uncertainty of one reading	Usually ± half the smallest division, unless stated
Quantity from two readings	Add the two uncertainties
Percentage uncertainty	(uncertainty ÷ value) × 100
Result from × or ÷	Add the percentage uncertainties
Percentage error	(│experimental − accepted│ ÷ accepted) × 100

Worked reminders:

• 10.0 cm³ pipette, ±0.04 cm³: 0.04 ÷ 10.0 × 100 = 0.40%.
• Temperature change of 6.0 °C on a thermometer read to ±0.5 °C: two readings give ±1.0 °C, so 1.0 ÷ 6.0 × 100 = 17%. Fix: a thermometer with finer resolution, or a larger temperature change.
• Gas syringe ±0.5 cm³, 40.0 cm³ collected: 1.3%.

Method in steps: an evaluation answer

1. Identify anomalies and say what you did with them.
2. Calculate the percentage error, if an accepted value is given.
3. Calculate the total percentage uncertainty.
4. Compare: error larger than uncertainty means a systematic limitation.
5. Name the specific limitation and its direction of effect.
6. Give an improvement that removes that limitation.

Limitation → improvement pairs

Limitation	Improvement
Heat lost to surroundings	Lid and insulation; extrapolate the temperature–time graph
Gas escapes before the bung is fitted	Keep reactants apart inside a sealed flask, then mix
Small titre, large percentage uncertainty	Larger sample or more concentrated solution
End point hard to see	White tile; a more suitable indicator
Timing a colour change by eye	Colorimeter or data logger
Solid not fully dehydrated	Heat to constant mass

Must-know distinctions

• Accuracy vs precision: results can be tightly grouped yet all wrong by the same amount.
• Random vs systematic error: only random error falls when you repeat and average.
• Percentage uncertainty vs percentage error: the first comes from apparatus; the second compares your answer with an accepted value.
• Limitation vs improvement: the improvement must address the limitation you named.
• Repeatable vs reproducible: same experimenter and method, or different ones.

Quick self-test

1. Define precision.
2. A student repeats a titration three times with very similar titres, but the mean is 5% above the accepted value. Are the results accurate, precise, both or neither?
3. Burette readings are 0.40 cm³ and 21.65 cm³, each ±0.05 cm³. Give the titre and its percentage uncertainty.
4. Give the units of Kc = [C]² ÷ ([A][B]³).
5. Write 0.000470 mol in standard form without losing significant figures.
6. Calculate the amount in 22.5 cm³ of 0.0815 mol dm⁻³ solution, to an appropriate number of significant figures.
7. Why can an intercept not be read from a graph whose x-axis starts at 2.9 × 10⁻³ K⁻¹?
8. Titres (cm³): rough 26.85, then 26.30, 26.55, 26.40, 26.35. Which do you use, and what is the mean?
9. Name one systematic error in a calorimetry experiment, and its effect on an exothermic ΔH.
10. An experimental value is 52.3 kJ mol⁻¹ and the accepted value is 57.1 kJ mol⁻¹. Calculate the percentage error.

Answers

1. Repeated measurements are close to each other.
2. Precise but not accurate.
3. Titre = 21.25 cm³; uncertainty ±0.10 cm³; 0.10 ÷ 21.25 × 100 = 0.47%.
4. dm⁶ mol⁻².
5. 4.70 × 10⁻⁴ mol.
6. 0.0815 × 0.0225 = 0.00183375, so 1.83 × 10⁻³ mol (3 s.f.).
7. The y-axis is not at x = 0, so the line’s crossing point there is not the intercept; calculate it with c = y − mx.
8. Use 26.30, 26.40 and 26.35 (within 0.10 cm³); mean = 26.35 cm³.
9. Heat loss to the surroundings; the measured temperature rise is too small, so ΔH is less negative than it should be.
10. (57.1 − 52.3) ÷ 57.1 × 100 = 8.4%.

Where marks are usually lost

• Listing “amount” or “conditions” as controlled variables without naming the quantity.
• Proposing a quantity that would overflow the apparatus, with no estimate of the expected reading.
• Units missing from table headings, or written in every cell.
• Burette readings recorded to 1 d.p. instead of 2 d.p.
• Using ±0.05 cm³ for a titre instead of ±0.10 cm³.
• Giving a final answer with more significant figures than the weakest measurement supports.
• Taking a gradient from two data points that are not on the best-fit line.
• Reading a “y-intercept” off an axis that does not start at zero.
• Saying “human error” or “use better equipment” instead of naming a limitation and a matching improvement.
• Comparing percentage error with percentage uncertainty but not saying what the comparison shows.

Next steps

• Full explanations: study guide
• Exam-style questions: practice set
• Titration calculations: amount of substance revision notes
• Free 10-minute diagnostics

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, published by OCR – Module 1, section 1.1 Practical skills assessed in a written examination (1.1.1 Planning, 1.1.2 Implementing, 1.1.3 Analysis, 1.1.4 Evaluation).