Revision notes for OCR GCSE Chemistry Practical skills. Open the guide for explanations and worked examples. Written against the OCR GCSE Chemistry (J248) specification, so the content matches what's examinable rather than general Chemistry background.
Practical skills
What you'll learn
- Why practical work matters in OCR Gateway GCSE Chemistry (J248).
- How to plan safe, fair and accurate experiments.
- How to record, process and evaluate practical results.
- What the eight Chemistry Practical Activity Groups practise.
Why practical skills matter
Practical chemistry is about using evidence. You do not just “do experiments”; you plan them, control variables, measure carefully, observe changes, process data, and decide how much you trust the results.
In OCR GCSE Chemistry, practical skills are assessed in the written exam papers. About 15% of the marks test practical understanding, such as choosing apparatus, interpreting results, spotting errors, or improving a method.
Definition
Practical Activity Group (PAG)
A Practical Activity Group, or PAG, is a required group of practical techniques that your centre must give you the opportunity to complete during the course.
For J248 Chemistry, you must have the opportunity to complete at least eight practical activities covering the required apparatus and techniques in Topic C7. Your centre keeps records and signs a practical science statement; you do not get a separate practical exam, but you can be asked about these skills in the written papers.
Key Idea
The big idea
Practical work is not just about remembering methods. In exams, you often need to explain why a method is valid, safe, accurate, or reliable.
Planning an investigation
Before you begin, you need to know what you are investigating.
Definition
Variables
An independent variable is the factor you deliberately change. A dependent variable is the factor you measure. Control variables are factors kept the same so the test is fair.
For example, if you investigate how concentration affects the rate of reaction between hydrochloric acid and marble chips, the concentration is the independent variable, and the volume of gas produced per minute could be the dependent variable.
A fair test is an investigation where only the independent variable is changed. A valid investigation is one that actually tests what it is meant to test.
Example
Choosing variables in a rates practical
You are planning an investigation into how the surface area of calcium carbonate affects the rate of reaction with hydrochloric acid.
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Choose the independent variable: use different surface areas of calcium carbonate, such as large chips, small chips, and powder.
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Choose the dependent variable: measure the volume of carbon dioxide gas produced in a fixed time, because gas production shows how quickly the reaction is happening.
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Choose control variables: keep the mass of calcium carbonate, volume of acid, concentration of acid, and temperature the same, so any change in rate is due to surface area only.
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Decide how to improve reliability: repeat each surface area and calculate a mean result, ignoring any clearly anomalous result only if there is a good reason.
Common Mistake
Changing two variables at once
If you change both the concentration of acid and the mass of solid, you cannot tell which change caused the difference in the results.
Safety: hazards, risks and control measures
Chemistry practicals involve chemicals, glassware, heat, gases and electrical equipment. You must think about safety before choosing the method.
Definition
Hazard, risk and control measure
A hazard is something that could cause harm, a risk is the chance of harm happening, and a control measure is an action that reduces the risk.
Examples include wearing eye protection when using acids or alkalis, using a water bath instead of a Bunsen burner for some flammable liquids, and keeping electrical equipment away from water.
Safety does not mean “nothing dangerous is present”. It means the risks are understood and controlled.
Measuring accurately
A lot of practical marks come from choosing suitable apparatus and reading it correctly.
Common measuring apparatus includes:
- a balance for mass, usually in grams (g)
- a measuring cylinder for approximate volumes
- a pipette for accurately measuring a fixed volume
- a burette for accurately delivering a variable volume
- a thermometer for temperature in degrees Celsius (°C)
- a stopwatch for time in seconds (s)
- a gas syringe or upside-down measuring cylinder for gas volume
Definition
Accuracy, precision and resolution
Accuracy means closeness to the true value. Precision means repeated readings are close together. Resolution is the smallest scale division an instrument can measure.
When reading a liquid volume, read the bottom of the meniscus, which is the curved surface of the liquid, at eye level.
Common Mistake
Reading the scale from an angle
Looking down at a measuring cylinder or burette causes a parallax error. Put your eye level with the meniscus before taking the reading.
Recording results properly
A good results table has:
- the independent variable in the first column
- repeated measurements where possible
- units in column headings, not repeated in every box
- calculated values such as means or rates in separate columns
- observations written clearly, such as colour change, gas produced, precipitate formed, or temperature change
A contemporaneous record means a record made at the time of the practical, not reconstructed later from memory.
Drawing scientific apparatus diagrams
In exams, you may be asked to draw or complete apparatus diagrams. Use simple lines, label important pieces of apparatus, and avoid artistic shading. Show how the apparatus is connected and where substances are placed.
Tip
Diagram checklist
Label the apparatus that matters: measuring equipment, heat source, collection vessel, delivery tube, electrodes, or condenser. The labels should help someone understand the method.
Separation and purification techniques
Many Chemistry practicals involve separating mixtures or purifying products.
Filtration separates an insoluble solid from a liquid using filter paper. The solid left behind is the residue and the liquid passing through is the filtrate.
Evaporation removes a solvent by heating, leaving dissolved solid behind.
Crystallisation produces crystals by forming a hot concentrated solution, then allowing it to cool.
Chromatography separates substances such as dyes because they move different distances through a stationary material with a solvent.
Distillation separates a liquid from a mixture by evaporating it and then condensing it back into a liquid.
This distillation setup shows how vapour is cooled in a condenser and collected as a distillate.
Titration skills
A titration is a practical technique used to find the concentration of an acid or alkali by reacting it with a solution of known concentration.
The solution in the burette is added to a measured volume in the conical flask until the end point, where the indicator just changes colour. The volume added from the burette is the titre.
This is the main titration apparatus you need to recognise and explain.
In a typical acid-alkali titration:
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Example
Finding an acid concentration by titration
25.0 cm³ of sodium hydroxide solution with concentration 0.100 mol/dm³ is neutralised by 20.0 cm³ of hydrochloric acid. Find the concentration of the acid.
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Convert the alkali volume into cubic decimetres, because concentration is in moles per cubic decimetre:
V=25.0÷1000=0.0250 dm3V = 25.0 \div 1000 = 0.0250 \text{ dm}^3V=25.0÷1000=0.0250 dm3 -
Calculate the moles of sodium hydroxide using n=c×Vn = c \times Vn=c×V:
n=0.100×0.0250=0.00250 moln = 0.100 \times 0.0250 = 0.00250 \text{ mol}n=0.100×0.0250=0.00250 mol -
Use the equation ratio. HCl and NaOH react in a 1 to 1 ratio, so the moles of hydrochloric acid are also 0.00250 mol.
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Convert the acid titre into cubic decimetres:
V=20.0÷1000=0.0200 dm3V = 20.0 \div 1000 = 0.0200 \text{ dm}^3V=20.0÷1000=0.0200 dm3 -
Calculate the acid concentration using c=nVc = \frac{n}{V}c=Vn:
c=0.002500.0200=0.125 mol dm−3c = \frac{0.00250}{0.0200} = 0.125 \text{ mol dm}^{-3}c=0.02000.00250=0.125 mol dm−3
Common Mistake
Forgetting to convert cm³ to dm³
In titration calculations, volumes must be in dm³ when using n=c×Vn = c \times Vn=c×V. Divide cm³ by 1000.
Identification of substances
Some practicals are about identifying unknown substances using qualitative tests. A qualitative test tells you what is present, rather than measuring how much is present.
You should be familiar with techniques such as:
- gas tests, for example testing for hydrogen, oxygen, carbon dioxide or chlorine
- flame tests for some metal ions
- precipitation reactions for some positive and negative ions
- pH testing using indicators or pH probes
The important practical skill is not just naming the test, but linking the observation to the conclusion. For example, a white precipitate may suggest a particular ion only when the correct reagent has been used.
Electrolysis practicals
Electrolysis uses electricity to break down ionic substances. In practical work, you need to recognise the cell setup: electrodes, electrolyte, power supply, and sometimes apparatus to collect gases.
An electrode is a conductor placed in the electrolyte. The electrolyte is the liquid or solution containing mobile ions. Products form at the electrodes, so observations near each electrode matter.
Measuring rates of reaction
A reaction rate tells you how quickly reactants are used up or products are made.
You can measure rate by:
- collecting gas in a gas syringe
- measuring loss of mass when a gas escapes
- timing how long it takes for a solution to become cloudy
- recording a colour change
Example
Calculating rate from gas volume
A reaction produces 48 cm³ of gas in 120 s. Calculate the mean rate of gas production.
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Choose the correct relationship:
mean rate=volume of gas producedtime taken\text{mean rate} = \frac{\text{volume of gas produced}}{\text{time taken}}mean rate=time takenvolume of gas produced -
Substitute the values with units:
mean rate=48 cm3120 s\text{mean rate} = \frac{48 \text{ cm}^3}{120 \text{ s}}mean rate=120 s48 cm3 -
Calculate the rate:
mean rate=0.40 cm3/s\text{mean rate} = 0.40 \text{ cm}^3\text{/s}mean rate=0.40 cm3/s
The eight Chemistry PAGs
Your school may use OCR suggested practicals or equivalent centre-devised practicals. The key point is that, across the course, you experience the required apparatus and techniques.
| PAG | Main practical focus | Example activity |
|---|---|---|
| 1 | Reactivity trends | Displacement reactions of Group 7 elements |
| 2 | Electrolysis | Electrolysis of aqueous sodium chloride or copper sulfate solution |
| 3 | Separation techniques | Chromatography of inks or dyes |
| 4 | Distillation | Distilling a liquid mixture |
| 5 | Identification of species | Flame tests, gas tests and ion tests |
| 6 | Titration | Finding the concentration of an acid or alkali |
| 7 | Production of salts | Making a pure, dry salt sample |
| 8 | Measuring rates of reaction | Investigating concentration or surface area |
Evaluating practical work
To evaluate a practical, ask: “How good is the evidence?”
A result is repeatable if the same person using the same method gets similar results. It is reproducible if different people or different equipment get similar results.
A random error causes readings to vary unpredictably, such as slight differences in judging an end point. A systematic error shifts all readings in the same direction, such as a balance that is not zeroed.
Good improvements usually target a specific weakness: use more repeats, use apparatus with smaller scale divisions, control temperature with a water bath, or collect gas with a gas syringe instead of counting bubbles.
Exam technique
In the exam
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Link every method choice to a reason: accuracy, safety, validity, reliability, or control of variables.
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When calculating from practical data, carry units through and check whether volumes need converting from cm³ to dm³.
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For evaluation questions, name the error or weakness, then suggest a practical improvement that directly fixes it.
Self review
Check yourself
- Can you identify the independent, dependent and control variables in a practical method?
- Can you explain why a pipette and burette are used in titration instead of measuring cylinders?
- Can you suggest one improvement to make a rates experiment more reliable?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
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