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

These notes condense section 1.2 (statements 1.2.1 and 1.2.2) of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, plus the Practical Endorsement rules in section 5g. H432 is the full A level, so nothing here is labelled AS or A2. The study guide explains each technique in full, and the practice questions let you check your recall.

Course pages are on the OCR A Level Chemistry hub, the printable checklist lists every statement, and the free diagnostics show where to focus.

The endorsement in one box

Component 04, teacher assessed, reported separately: Pass or Not Classified
Minimum 12 assessed practical activities, covering PAG1 to PAG12
Every 1.2.1 skill and every 1.2.2 technique shown before the course ends
Judged against CPAC 1 to 5, "consistently and routinely"
Group work allowed; evidence of competence must be your own
Keep a record of all practical work (log book or portfolio)
A previous result can be carried forward if you retake

Practical techniques can also be tested in the written papers through 1.1.2(a); see the written examination skills notes.

The PAGs

PAG	Title	Minimum techniques
1	Moles determination	Mass; gas volume
2	Acid–base titration	Liquid volume; standard solution; burette and pipette; indicators
3	Enthalpy determination	Temperature
4	Qualitative analysis of ions	Tests for ions; qualitative observations
5	Synthesis of an organic liquid	Reflux (or in PAG6); separating funnel; distillation; risk assessment
6	Synthesis of an organic solid	Recrystallisation; melting point; TLC or paper chromatography; filtration; risk assessment
7	Qualitative analysis of organic functional groups	Organic tests; water bath or electric heater; observations
8	Electrochemical cells	Set up cells; measure voltages
9	Rates: continuous monitoring	Continuous method; time; software (or in PAG10)
10	Rates: initial rates	Initial rate method; software; identify and control variables
11	pH measurement	Measure pH
12	Research skills	Investigative approaches; research; citing sources

Safe handling of hazardous substances, 1.2.2(k), is covered across your selection of activities.

CPAC in five lines

1. Follows written procedures correctly.
2. Applies investigative approaches: uses apparatus and ICT with minimal help, works methodically, controls variables, picks equipment for accurate results.
3. Works safely: identifies hazards, assesses risks, uses safety equipment with minimal prompting.
4. Makes and records observations: accurate, precise, sufficient data with units.
5. Researches, references and reports: uses software; cites sources.

1.2.1 skills: what each one means

Statement	In practice
(a) investigative approaches	Plan your own method to answer a question
(b) safe use	Hazard = could cause harm; risk = chance it does; control matches the hazard
(c) written instructions	Follow steps in order
(d)–(e) observations, records	Dated lab book; raw data in ink; units in headers
(f) present data	Tables and graphs with labelled axes
(g) software	Spreadsheets, graph plotting, data loggers
(h) research	Books, journals and reputable websites
(i) citation	Author, title, date, URL and access date
(j) instruments	Wide range of apparatus across the course

Measurement, heating and pH, 1.2.2(a)–(c)

Quantity	Apparatus	Tip
Mass	Balance	Weigh by difference
Time	Stopwatch or data logger	Start timing as reactants mix
Volume of liquid	Burette, pipette, volumetric flask, measuring cylinder	Eye level with the meniscus bottom
Volume of gas	Gas syringe, or inverted burette or measuring cylinder over water	Pick a capacity above the predicted volume
Temperature	Thermometer or temperature probe	Stir before reading
pH	pH chart, pH meter, or probe on a data logger	Calibrate meters with buffers

Heating: water bath (gentle, to about 100 °C), electric heater such as a heating mantle (no flame), sand bath (even heat above 100 °C). Choose a flameless method for flammable organic liquids.

Useful relationships

mass for a standard solution = c x V(dm3) x M
mean titre = average of concordant titres only
gas volume at RTP (cm3) = n x 24 000
Rf = (baseline to spot centre) / (baseline to solvent front)
E(cell) = E(positive electrode) - E(negative electrode)
initial rate is proportional to 1/t (clock reactions)
% uncertainty = uncertainty / measured value x 100

1.2.2 techniques: method in steps

Standard solution, 1.2.2(e)

1. Weigh solid by difference.
2. Dissolve in distilled water in a beaker.
3. Transfer to a volumetric flask; rinse beaker, rod and funnel into it.
4. Make up so the meniscus bottom touches the line at eye level.
5. Stopper and invert to mix.

Titration, 1.2.2(d)(i)

1. Rinse the pipette and burette with their own solutions.
2. Pipette with a filler; let it drain.
3. Rough titre, then accurate titres added dropwise near the end point.
4. Mean of concordant titres only (usually within 0.10 cm³).

Recrystallisation, 1.2.2(g)(i)

Minimum hot solvent → hot filtration (fluted paper) → slow cooling, then ice → filtration under reduced pressure → wash with ice-cold solvent → dry.

Liquid purification, 1.2.2(g)(ii)

Separating funnel (vent often) → keep the organic layer → dry with an anhydrous salt → distil.

Rates, 1.2.2(l)

Initial rate (PAG10)	Continuous monitoring (PAG9)
Clock reaction: time t for fixed small extent	Measure gas volume, mass or colour against time
Rate ∝ 1/t	Gradient of curve gives rate
Repeat at several concentrations	Constant half-life means first order

Must-know distinctions

Pair	Difference
Reflux vs distillation	Reflux: vertical condenser, vapour returns. Distillation: condenser leads to collector, thermometer at side arm
Fluted paper vs reduced pressure	Fluted: fast gravity, hot, removes insoluble impurities. Reduced pressure: collects and dries crystals
Hazard vs risk	The property vs the chance of harm in your method
Water bath vs sand bath	Water bath to about 100 °C; sand bath for higher temperatures; both heat evenly
pH chart vs pH meter	Chart: rough. Meter or probe: precise after calibration with buffers
Pure vs impure solid	Pure: sharp melting point. Impure: lower and over a range

Indicator choice, 1.2.2(f)

Acid	Base	Indicator
Strong	Strong	Methyl orange or phenolphthalein
Weak	Strong	Phenolphthalein
Strong	Weak	Methyl orange
Weak	Weak	No sharp end point; use a pH meter

Tests to recall, 1.2.2(d)(iii)

Test for	Reagent	Positive result
Carbonate	Dilute acid	Effervescence
Sulfate	Barium ions	White precipitate
Halide	Silver nitrate, then aqueous ammonia	White, cream or yellow precipitate
Ammonium	NaOH, warm	Gas turns damp red litmus blue
Alkene	Bromine water	Orange to colourless
Aldehyde	Tollens’ reagent	Silver mirror
Carbonyl	2,4-DNP	Orange or yellow precipitate
Primary/secondary alcohol or aldehyde	Acidified dichromate, warm	Orange to green
Carboxylic acid	Carbonate	Effervescence

Ion test order: carbonate, then sulfate, then halide.

Cells, 1.2.2(j)

Two half-cells, salt bridge, high-resistance voltmeter. E(cell) = E(positive) − E(negative). The salt-bridge solution must not react with either half-cell; avoid chloride with silver ions.

Quick self-test

1. Distinguish a hazard from a risk.
2. Calculate the mass of sodium carbonate (M = 106.0 g mol⁻¹) for 100.0 cm³ of 0.150 mol dm⁻³ solution.
3. Titres: rough 19.20; then 18.65, 18.90, 18.60, 18.70 cm³. Find the mean titre.
4. Choose an indicator for ethanoic acid against sodium hydroxide.
5. On a paper chromatogram the solvent front is 5.5 cm from the baseline and a spot is 3.3 cm from it. Find Rf.
6. Use Fe²⁺/Fe −0.44 V and Ag⁺/Ag +0.80 V to find the standard cell potential.
7. In a clock reaction, doubling one concentration cuts the time from 50 s to 25 s. State the order.
8. A pH meter reads 2.00 for 0.0100 mol dm⁻³ HCl. Is this the expected value?
9. Why is fluted filter paper used for hot filtration?
10. Which PAG covers research skills, and what must you do with sources?
11. Predict the hydrogen volume at RTP from 1.50 × 10⁻³ mol of zinc reacting with excess acid (1 : 1).
12. State the heating method for refluxing a mixture containing ethanol, and why.

Answers

1. Hazard: the property that could cause harm. Risk: the chance of harm from the way you use it.
2. 0.150 × 0.1000 × 106.0 = 1.59 g.
3. Concordant: 18.65, 18.60, 18.70 → 18.65 cm³ (18.90 is not concordant).
4. Phenolphthalein (weak acid, strong base).
5. 3.3 / 5.5 = 0.60.
6. 0.80 − (−0.44) = +1.24 V.
7. 1/t doubles from 0.020 to 0.040 s⁻¹: first order.
8. Yes: pH = −log(0.0100) = 2.00 for a strong acid.
9. It filters fast, so the product does not crystallise in the funnel.
10. PAG12; cite them correctly.
11. 1.50 × 10⁻³ × 24 000 = 36.0 cm³.
12. A water bath or electric heater: ethanol is flammable, so no naked flame.

Where marks are usually lost

• Writing a generic control measure (“take care”) that is not linked to the named hazard.
• Including the rough titre, or a non-concordant titre, in the mean.
• Leaving the rough titre unrecorded; your record should show every reading.
• Putting the thermometer bulb in the liquid during distillation instead of level with the side arm.
• Suggesting an indicator for a weak acid against a weak base.
• Using excess hot solvent when recrystallising, then wondering why the yield is low.
• Saying an impure solid melts at a higher temperature; it melts lower and over a range.
• Measuring the Rf distance to the top of the plate, not to the marked solvent front.
• Forgetting to calibrate a pH meter with buffers before use.
• Calling a clock reaction a continuous monitoring method.

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.