OCR A Level Chemistry A: Practical skills assessed in the practical endorsement (H432)

This guide teaches section 1.2, Practical skills assessed in the practical endorsement, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. It covers every statement in 1.2.1 and 1.2.2, plus what the specification says about the Practical Endorsement, the Practical Activity Groups (PAGs) and the Common Practical Assessment Criteria (CPAC). H432 is the full A level; none of this content is labelled AS or A2.

Two companion pages go with this guide: condensed revision notes for the final weeks, and a set of original practice questions. Every H432 page is listed on the OCR A Level Chemistry hub, and the printable checklist gives you every statement to tick. Weak areas? Try a free diagnostic.

How the Practical Endorsement works

Section 1.1 skills (planning, implementing, analysis, evaluation) are assessed in the written papers. Section 1.2 skills are assessed through the Practical Endorsement, Component 04. Techniques still matter in the written papers, because 1.1.2(a) links to the endorsement skills.

What the specification says:

• The endorsement is teacher assessed against the CPAC and reported separately from your A level grade, as Pass or Not Classified.
• You carry out a minimum of 12 assessed practical activities, covering all of PAG1 to PAG12, and show competence in every skill in 1.2.1 and every technique in 1.2.2 before the course ends.
• You may work in groups, but you need independent evidence of your own competence.
• You keep a record of your practical work. Properly recorded work from a standalone AS year can count.
• If you retake the A level, you can carry forward a previous endorsement result.
• Teachers follow JCQ guidance on AI: work must be your own, and AI use that stops you showing a competency independently earns no credit.

CPAC	What you must show
1 Follows written procedures	Follows instructions correctly
2 Applies investigative approaches	Uses apparatus and ICT with minimal help; works methodically; controls variables; picks equipment for suitably accurate results
3 Safely uses equipment and materials	Identifies hazards, assesses risks, adjusts for safety; uses safety equipment with minimal prompting
4 Makes and records observations	Accurate, precise and sufficient data, recorded with units
5 Researches, references and reports	Uses software to process and report; cites sources

The standard is “consistently and routinely”, not one good session.

Coverage table

Spec ref	What you must be able to do	Main PAG link
1.2.1(a)	Apply investigative approaches; solve problems in a practical context	PAG12
1.2.1(b)	Use equipment safely; identify hazards and minimise risks	All; risk assessment in PAG5, PAG6
1.2.1(c)–(f)	Follow instructions; record observations; keep records; present data	All
1.2.1(g)	Use software and tools to process data and report	PAG9 or PAG10
1.2.1(h)–(i)	Research online and offline; cite sources	PAG12
1.2.1(j)	Use a wide range of instruments and techniques	All
1.2.2(a)	Measure mass, time, volume of liquids and gases, temperature	PAG1, PAG2, PAG3, PAG9
1.2.2(b)	Heat with a water bath, electric heater or sand bath	PAG7
1.2.2(c)	Measure pH	PAG11
1.2.2(d)	Titration; distillation and reflux; qualitative tests; filtration	PAG2, PAG4–PAG7
1.2.2(e)–(f)	Make a standard solution; use acid–base indicators	PAG2
1.2.2(g)–(i)	Recrystallise; use a separating funnel; melting point; chromatography	PAG5, PAG6
1.2.2(j)	Set up electrochemical cells and measure voltages	PAG8
1.2.2(k)	Handle hazardous solids and liquids safely	Across the activities
1.2.2(l)	Measure rates by at least two methods	PAG9, PAG10

1.2.1 Practical skills

Investigative approaches, 1.2.1(a). Plan your own method: pick a measurable dependent variable, decide what to control, and trial it.

Safety, 1.2.1(b) and 1.2.2(k). A hazard is something that could cause harm. A risk is the chance of harm from what you actually do. Link each hazard to a matching control: “cyclohexane vapour could ignite, so heat it in a water bath with no flames nearby” beats “wear goggles”.

Instructions and records, 1.2.1(c)–(f). Follow methods in order. Record raw data straight into a dated lab book, with “quantity / unit” column headers and consistent decimal places.

Software, 1.2.1(g). Use spreadsheets to convert times to 1/t and plot graphs, and data loggers to record pH or temperature against time.

Research and citation, 1.2.1(h)–(i). Use books, journals and reputable websites, and give each source’s author, title, date, and URL with access date. The specification points to the Practical Skills Handbook for citation methods.

1.2.2 Measurement, heating and pH

Measurements, 1.2.2(a). Weigh by difference: weigh the container with solid, tip the solid out, reweigh the container. Use a burette or pipette for precise liquid volumes, and a gas syringe or an inverted burette over water for gases.

Worked example 1: gas volume (PAG1)

0.250 g of calcium carbonate (M = 100.1 g mol⁻¹) reacts with excess hydrochloric acid. Take the molar gas volume as 24.0 dm³ mol⁻¹ and choose a gas syringe.

CaCO3 + 2HCl -> CaCl2 + H2O + CO2
n(CO2) = n(CaCO3) = 0.250 / 100.1 = 2.498 x 10^-3 mol
V(CO2) = 2.498 x 10^-3 x 24 000 = 59.9 cm3

A 100 cm³ syringe is needed.

Heating, 1.2.2(b). A water bath heats evenly up to about 100 °C and suits flammable organic liquids. An electric heater, such as a heating mantle, reaches higher temperatures without a flame. A sand bath gives even heat above 100 °C.

pH, 1.2.2(c). A pH chart with universal indicator gives a rough value. A pH meter or a data-logger probe is more precise, but calibrate it with buffers of known pH and rinse it with distilled water between samples. In a PAG11-style task, benzoic acid at 0.100 mol dm⁻³ (use Ka = 6.3 × 10⁻⁵ mol dm⁻³) has [H⁺] = √(6.3 × 10⁻⁶) and pH 2.60, easy to tell from a strong acid of the same concentration at pH 1.00.

1.2.2 Titration, standard solutions and indicators

Standard solution, 1.2.2(e). Dissolve the weighed solid in distilled water in a beaker. Transfer it through a funnel into the volumetric flask, rinsing beaker, rod and funnel into the flask. Fill until the bottom of the meniscus sits on the line at eye level, then stopper and invert.

Worked example 2: standard solution and titration

Make 250.0 cm³ of 0.0800 mol dm⁻³ potassium hydrogen phthalate, KHC₈H₄O₄ (M = 204.1 g mol⁻¹). Then titrate 25.0 cm³ portions with sodium hydroxide (1 : 1). Titres: rough 21.70; then 21.35, 21.40, 21.30 cm³.

mass = 0.0800 x 0.2500 x 204.1 = 4.082 g
  (bottle + solid 14.562 g, bottle after 10.480 g)
mean concordant titre = (21.35 + 21.40 + 21.30) / 3 = 21.35 cm3
n(NaOH) = n(acid) = 0.0800 x 0.0250 = 2.00 x 10^-3 mol
c(NaOH) = 2.00 x 10^-3 / 0.02135 = 0.0937 mol dm-3
burette % uncertainty = (2 x 0.05) / 21.35 x 100 = 0.47%

Leave the rough titre out of the mean.

Titration, 1.2.2(d)(i). Rinse the pipette and burette with the solutions they will hold. Fill the pipette to the line with a filler. Add titrant dropwise near the end point, swirling, and repeat until titres are concordant, usually within 0.10 cm³.

Indicators, 1.2.2(f). The colour change must fall within the steep part of the pH curve.

Acid with alkali	Indicator
Strong with strong	Either methyl orange or phenolphthalein
Strong with weak	Methyl orange (end point below pH 7)
Weak with strong	Phenolphthalein (end point above pH 7)
Weak with weak	No sharp change; use a pH meter

The phthalate is a weak acid: use phenolphthalein.

1.2.2 Preparation and purification

Reflux and distillation, 1.2.2(d)(ii). Clamp flask and condenser to a retort stand. For reflux, the condenser is vertical and open at the top, so volatile reactants condense and return during long heating. For distillation, the thermometer bulb sits level with the side arm and you collect the fraction boiling at the expected temperature. Water enters each condenser at the bottom.

Separating funnel, 1.2.2(g)(ii). Shake with aqueous sodium hydrogencarbonate or water, opening the tap often to release pressure. Run off the lower layer and keep the correct one (test with a drop of water). Dry the organic layer with anhydrous magnesium sulfate, then redistil.

Filtration, 1.2.2(d)(iv). Fluted filter paper gives fast gravity filtration, useful for removing insoluble impurities from a hot solution before crystals form. Filtration under reduced pressure, with a Büchner funnel, collects crystals quickly and pulls more solvent off them.

Recrystallisation, 1.2.2(g)(i).

1. Add just enough hot solvent to dissolve the crude solid (the minimum).
2. Filter hot if insoluble impurities are present.
3. Cool slowly, then in ice; soluble impurities stay dissolved.
4. Filter under reduced pressure; wash with a little ice-cold solvent.
5. Dry the crystals.

Excess solvent lowers the recovery.

Melting point, 1.2.2(h). Heat dry solid in a capillary tube, slowly near the expected value. A pure compound melts sharply at its known melting point. Impurities lower the melting point and widen the range.

Chromatography, 1.2.2(i). Draw the baseline in pencil, above the solvent level. Cover the tank. Mark the solvent front as soon as you remove the plate.

Worked example 3: Rf values

The solvent front moves 7.5 cm; spot X moves 3.9 cm and spot Y 5.7 cm.

Rf(X) = 3.9 / 7.5 = 0.52      Rf(Y) = 5.7 / 7.5 = 0.76

1.2.2 Tests, cells and rates

Qualitative tests, 1.2.2(d)(iii). For ions (PAG4), test for carbonate, then sulfate, then halide, so earlier ions cannot give false positives: dilute nitric acid gives effervescence with carbonate; barium ions give a white precipitate with sulfate; silver nitrate gives white, cream or yellow precipitates with chloride, bromide or iodide, separated by their solubility in aqueous ammonia. Warm ammonium salts with sodium hydroxide; the ammonia turns damp red litmus blue. For organic groups (PAG7): bromine water decolourises with alkenes; acidified dichromate, warmed in a water bath, turns green with primary and secondary alcohols and aldehydes; 2,4-DNP precipitates carbonyls; Tollens’ reagent gives a silver mirror with aldehydes; carbonates fizz with carboxylic acids; warm aqueous silver nitrate in ethanol precipitates halide from haloalkanes.

Electrochemical cells, 1.2.2(j). Join two half-cells with a salt bridge and a high-resistance voltmeter. Clean electrodes with emery paper.

Worked example 4: cell voltage

Use these values: Ag⁺/Ag +0.80 V; Cu²⁺/Cu +0.34 V.

E(cell) = E(positive) - E(negative) = 0.80 - 0.34 = +0.46 V

Soak the salt bridge in potassium nitrate, not potassium chloride, which would precipitate silver chloride.

Rates by two methods, 1.2.2(l). An initial rate method such as a clock reaction (PAG10) times how long a fixed small amount of product takes to form, so rate is proportional to 1/t.

Worked example 5: clock reaction

[reactant] / mol dm⁻³	t / s	1/t / s⁻¹
0.020	118	0.00847
0.040	60	0.0167
0.080	29	0.0345

Doubling concentration roughly doubles 1/t (ratios 1.97 and 2.07): first order.

A continuous monitoring method (PAG9) follows gas volume, mass loss or colour against time. If the final gas volume is 64.0 cm³, with 32.0 cm³ at 38 s and 48.0 cm³ at 76 s, the half-life is a constant 38 s, so the reaction is first order.

Related pages

Amount of substance, Acids and exam preparation.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, published by OCR. Sections 1.2.1, 1.2.2 and 5g.