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

This study guide covers section 1.1, Practical skills assessed in a written examination, of Module 1 in the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. It teaches every outcome in 1.1.1 Planning, 1.1.2 Implementing, 1.1.3 Analysis and 1.1.4 Evaluation. The specification does not split this section into tiers, so all of it applies to you.

Use it with the revision notes and the practice questions. See also the OCR A Level Chemistry hub, the printable checklist and the free 10-minute diagnostics.

What this unit covers

Statement	What you need to do	Linked skills
1.1.1(a)	Design experiments, including problems in a practical context; choose apparatus and techniques	HSW3
1.1.1(b)	Identify the variables that must be controlled	–
1.1.1(c)	Judge whether a method is appropriate to meet the expected outcomes	HSW6
1.1.2(a)	Use a wide range of practical apparatus and techniques correctly	HSW4
1.1.2(b)	Use appropriate units for measurements	M0.0
1.1.2(c)	Present observations and data in an appropriate format	HSW8
1.1.3(a)	Process and interpret qualitative and quantitative results; reach valid conclusions	HSW5
1.1.3(b)	Use appropriate mathematical skills to analyse quantitative data	HSW3
1.1.3(c)	Use significant figures appropriately	M1.1
1.1.3(d)	Plot graphs: (i) axes, scales, quantities and units; (ii) gradients and intercepts	M3.2 to M3.5
1.1.4(a)	Evaluate results and draw conclusions	HSW6
1.1.4(b)	Identify anomalies in measurements	–
1.1.4(c)	Identify limitations in experimental procedures	–
1.1.4(d)	Precision and accuracy: margins of error, percentage errors, apparatus uncertainties	M1.3
1.1.4(e)	Refine a design by suggesting improvements to procedures and apparatus	HSW3

How this section is assessed

The specification says the skills in 1.1 are assessed in the written papers. Module 1 is assessed in Components 01, 02 and 03, so a practical question can be set on any chemistry you study. The Practical Endorsement (section 1.2) is reported separately and is not covered here.

1.1.1 Planning

Experimental design and choice of apparatus

A plan states what you change (independent variable), what you measure (dependent variable), what stays the same (controlled variables) and how the results give the answer. Choose apparatus to match the size of the measurement and the precision you need:

• Exact volumes: pipette or burette; a measuring cylinder is fine for a reagent in excess.
• Standard solutions: volumetric flask, solid weighed by difference.
• Gases: gas syringe, or collection over water for a gas that is not very soluble.
• Temperature: a thermometer whose resolution suits the size of the change.

Say how much reagent to use and why, so the measurement fills a good part of the apparatus range.

Controlled variables

Name each one specifically; “keep conditions the same” is too vague. In a rate experiment they are typically temperature, the concentration and volume of the other reagent, the surface area of a solid and any catalyst. Say how you control each (for example, a thermostatic water bath).

For 1.1.1(c), judge a method by asking: does it measure the right quantity, are the quantities within the apparatus range, and are there losses or side reactions that make the result unreliable?

Worked example 1: choosing a method

You want the percentage of calcium carbonate in powdered eggshell. Method A: add excess hydrochloric acid with the flask standing on a balance, and record the mass lost as carbon dioxide. Method B: react it in a flask connected to a 100 cm³ gas syringe. Use Mr(CaCO₃) = 100.1, Mr(CO₂) = 44.0 and a molar gas volume of 24.0 dm³ mol⁻¹.

Step 1: estimate the gas produced. If the shell were pure calcium carbonate, 0.50 g gives 0.50 ÷ 100.1 = 0.00500 mol CO₂, or 0.00500 × 24 000 = 120 cm³. That is beyond the syringe’s range.

Step 2: adjust the quantity. 0.40 g gives 0.00400 mol, or 95.9 cm³, so at most 0.40 g should be used with a 100 cm³ syringe.

Step 3: judge Method A. 0.40 g releases only 0.176 g of CO₂. On a two-decimal-place balance the loss, from two readings of ±0.005 g, is ±0.01 g: a 5.7% uncertainty. Acid spray leaving the flask also adds to the apparent loss.

Conclusion: Method B is more appropriate, with no more than about 0.40 g of eggshell.

1.1.2 Implementing

Apparatus and techniques

The additional guidance links 1.1.2(a) to the techniques in the content and the Practical Endorsement: titration, standard solutions, heating, pH measurement, distillation and reflux, filtration, recrystallisation, separating funnels, melting points, chromatography, electrochemical cells and rate measurements. Questions often ask why a step is done: rinse a burette with the solution it will hold so water does not dilute it; read the bottom of the meniscus at eye level; heat under reflux so volatile reactants condense and return.

Units

Every measured quantity needs a unit (M0.0). The common conversions: cm³ to dm³ (divide by 1000) before using mol dm⁻³; q in J to kJ before quoting ΔH in kJ mol⁻¹; entropy in J K⁻¹ mol⁻¹ to kJ before combining it with ΔH. Units of Kc and of a rate constant depend on the expression, so work them out each time.

Presenting data

A results table has the independent variable in the first column, headings with quantity and unit (for example “volume of gas / cm³”), no units in the body, and each column recorded to the same number of decimal places, matching the apparatus resolution. Burette readings go to two decimal places, ending in 0 or 5. Write full observations: “white precipitate”, not “goes cloudy”.

1.1.3 Analysis

Qualitative results and valid conclusions

A conclusion is valid only if the observations support it and nothing else could explain them. For ions, the specification gives the order of tests: carbonate, then sulfate, then halide.

Worked example 2. A solution of solid Y fizzes with dilute nitric acid and the gas turns limewater milky. No precipitate forms with aqueous barium nitrate. Aqueous silver nitrate gives a cream precipitate that stays undissolved in dilute ammonia yet dissolves when concentrated ammonia is added.

• Fizzing and milky limewater: carbonate is present.
• No precipitate with Ba²⁺(aq): sulfate is absent.
• Cream precipitate, soluble only in concentrated ammonia: bromide is present.

Valid conclusion: Y contains CO₃²⁻ and Br⁻; these tests cannot identify the cation. Testing for carbonate first with acid removes it, so it cannot give a false positive with Ba²⁺ or Ag⁺.

Mathematical skills and significant figures

Analysis uses the mathematical skills listed in Section 5 of the specification. For significant figures (M1.1), match the significant figures of your result to the least precise measurement used, and round only at the end. Keep trailing zeros: 0.0050 mol dm⁻³ is 5.0 × 10⁻³ mol dm⁻³.

Graphs

For 1.1.3(d)(i): independent variable on the x-axis, both axes labelled with quantity and unit, and a scale using more than half the grid with easy steps. Axes need not start at zero.

For 1.1.3(d)(ii): draw a best-fit line or curve, ignoring clear anomalies. Take a gradient from two far-apart points on the line, not from data points off it. For a curve, draw a tangent at the point required (M3.5). If the x-axis does not start at zero, calculate the intercept from c = y − mx.

Worked example 3: gradient and intercept

A student measures the rate constant k for a reaction at several temperatures and plots ln k against 1/T. The best-fit line passes through (3.000 × 10⁻³ K⁻¹, −4.20) and (3.300 × 10⁻³ K⁻¹, −6.45). Use ln k = −Ea/(RT) + ln A, with R = 8.314 J mol⁻¹ K⁻¹. k is in s⁻¹.

Gradient: (−6.45 − (−4.20)) ÷ (3.300 × 10⁻³ − 3.000 × 10⁻³) = −2.25 ÷ (3.00 × 10⁻⁴) = −7500 K.

Activation energy: gradient = −Ea/R, so Ea = 7500 × 8.314 = 62 355 J mol⁻¹ = 62.4 kJ mol⁻¹.

Intercept: the axis starts near 2.9 × 10⁻³ K⁻¹, so calculate: ln A = −4.20 − (−7500 × 3.000 × 10⁻³) = −4.20 + 22.5 = 18.3. A = e¹⁸·³ = 8.86 × 10⁷ s⁻¹.

1.1.4 Evaluation

Key terms

• Accurate: close to the true value.
• Precise: repeated measurements are close to each other.
• Uncertainty: the range either side of a reading within which the true value lies.
• Random error: scatter either way; reduced by repeating and averaging.
• Systematic error: a shift in one direction every time (such as heat loss); repeating does not remove it.
• Anomaly: a result that does not fit the pattern. Leave it out of means and best-fit lines, and repeat it if possible.

Uncertainties and percentage error

The uncertainty of a reading is usually taken as ± half the smallest division, unless the apparatus or question states a value. When a quantity comes from two readings (a titre, a temperature change, a mass by difference), add the two uncertainties (M1.3).

Percentage uncertainty = (uncertainty ÷ measured value) × 100. For a result calculated by multiplying or dividing measurements, add the percentage uncertainties. Percentage error = (difference from accepted value ÷ accepted value) × 100.

If the percentage error is larger than the total percentage uncertainty, apparatus uncertainty cannot explain the difference; look for a limitation in the procedure.

Worked example 4: evaluating an enthalpy experiment

0.120 g of magnesium ribbon is added to 50.0 cm³ of excess dilute hydrochloric acid in a polystyrene cup. The temperature rises from 21.3 °C to 31.1 °C. Use Ar(Mg) = 24.3, c = 4.18 J g⁻¹ K⁻¹, a solution density of 1.00 g cm⁻³ and a value for the reaction of −467 kJ mol⁻¹. Uncertainties: balance ±0.001 g for the mass, measuring cylinder ±0.5 cm³, thermometer ±0.1 °C per reading.

Result: q = 50.0 × 4.18 × 9.8 = 2048 J. n(Mg) = 0.120 ÷ 24.3 = 4.938 × 10⁻³ mol. ΔH = −2048 ÷ (4.938 × 10⁻³) = −414 760 J mol⁻¹ = −415 kJ mol⁻¹, given as −4.1 × 10² kJ mol⁻¹ because ΔT has only two significant figures.

Percentage error: (467 − 415) ÷ 467 × 100 = 11%.

Percentage uncertainties: mass 0.001 ÷ 0.120 × 100 = 0.83%; volume 0.5 ÷ 50.0 × 100 = 1.0%; ΔT 0.2 ÷ 9.8 × 100 = 2.0%. Total = 3.9%.

Evaluation: 11% is much larger than 3.9%, so the difference is mainly systematic. The value is less exothermic than expected, which fits heat loss during the temperature rise.

Improvements (1.1.4(e)): fit a lid; record the temperature every 30 s before and after adding the magnesium, and extrapolate the cooling section of the graph back to the moment the magnesium went in.

A limitation (1.1.4(c)) is a specific weakness in the procedure, and each improvement must fix one. “Use more accurate equipment” is too vague.

Common errors

• Naming controlled variables vaguely (“same amount”) instead of quantity and value.
• Using one reading’s uncertainty for a titre or temperature change instead of two.
• Rounding mid-calculation, then quoting too many significant figures at the end.
• Reading an intercept off an axis that does not start at zero.
• Calling a result “inaccurate” without saying whether error exceeds uncertainty.

Next steps

• Condensed recall: practical skills revision notes
• Test yourself: practical skills practice questions
• Titrations: amount of substance study guide
• Papers: OCR A Level Chemistry exam preparation
• Free 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 to 1.1.4).