Key Stage 4 science · Working scientifically

GCSE science practical skills: planning and carrying out experiments

GCSE science expects students to work like scientists: to turn a scientific idea into a testable hypothesis, plan a method, choose the right apparatus, carry the work out safely and accurately, record what they see, and judge how good the method was. These skills are taught through the biology, chemistry and physics content, and questions about practical work appear in written exams as well as in the lab.

Experimental skills and strategies, explained

Everything starts with a hypothesis: a possible explanation for something, based on scientific theory, that can be tested. A prediction is what you expect to happen if the hypothesis is right. For example, “plants need light energy to photosynthesise” is a hypothesis, and “pondweed will give off oxygen bubbles faster when a lamp is moved closer” is a prediction that an experiment can test. Students are expected to use the theory they know to propose hypotheses, and then plan experiments to test them, make observations or explore something they do not yet understand.

Planning means deciding what to change, what to measure and what to keep the same. The independent variable is the one the experimenter changes, the dependent variable is the one that is measured, and control variables are everything else that could affect the result and must be kept constant. Students also decide how many different values to test, over what range, and how many times to repeat each one. They choose apparatus that suits the job: for example, a measuring cylinder rather than a beaker for a volume, a balance that reads to 0.01 g for a small mass, or quadrats and a tape measure for fieldwork. The smallest division of a measuring instrument limits how precisely it can read.

Carrying out the experiment well is a skill in itself. It includes setting up apparatus correctly, reading scales at eye level, such as the bottom of the curved surface (the meniscus) of a liquid in a measuring cylinder, and starting and stopping timers consistently. Students are expected to think about health and safety before they begin, by identifying hazards, judging the risk, and deciding what control measures to use. In a written exam, a vague answer such as “be careful” earns nothing, while a specific one, such as “tie back long hair and wear eye protection because of splashes”, does. Schools carry out a set of required practical activities across the course, and the written exams ask about them.

Sampling matters in biology and environmental science, where it is impossible to measure every individual. A sample must be representative of the whole, which means chosen without bias and large enough to reflect the whole area or population. A common method is to use random sampling with quadrats, where coordinates are chosen using random numbers rather than by placing the quadrat in the places that look most interesting. Students also learn to choose the right technique for fieldwork, such as a transect to see how a habitat changes along a line.

Observations and measurements are recorded in tables with clear headings and units, and the number of decimal places should match the resolution of the instrument. Finally, students evaluate their own method. They look for what limited the quality of the data and suggest improvements that would make a real difference, such as using a more precise instrument, taking more readings or controlling a variable better, and suggest further investigations that the results point to. Evaluation is covered in more detail in the analysis topic.

Words your child will hear

Hypothesis: A possible explanation, based on scientific theory, that can be tested. Predictions are made from it.
Independent variable: The variable you deliberately change in an experiment.
Dependent variable: The variable you measure to see the effect of the change.
Control variable: A variable you keep the same so that it does not affect the result.
Resolution: The smallest change an instrument can show, such as 1 mm on a ruler.
Random sample: A sample where every individual has an equal chance of being chosen, so the choice is not biased.
Risk assessment: Identifying hazards in an activity, judging the risk and deciding how to reduce it.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should develop understanding and first-hand experience of using scientific theories and explanations to develop hypotheses
Students should develop understanding and first-hand experience of planning experiments to make observations, test hypotheses or explore phenomena
Students should develop understanding and first-hand experience of applying a knowledge of a range of techniques, apparatus, and materials to select those appropriate both for fieldwork and for experiments
Students should develop understanding and first-hand experience of carrying out experiments appropriately, having due regard to the correct manipulation of apparatus, the accuracy of measurements and health and safety considerations
Students should develop understanding and first-hand experience of recognising when to apply a knowledge of sampling techniques to ensure any samples collected are representative
Students should develop understanding and first-hand experience of making and recording observations and measurements using a range of apparatus and methods
Students should develop understanding and first-hand experience of evaluating methods and suggesting possible improvements and further investigations

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: planning a rate of reaction investigation

Plan an investigation into how temperature affects the rate of the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which turns the mixture cloudy. Say how the results should be recorded.

Use theory to write a hypothesis and a prediction. Collision theory says particles move faster at higher temperatures, so they collide more often and more of the collisions have enough energy to react. The prediction that follows is: “the higher the temperature, the less time it will take for the mixture to turn cloudy.”
Identify the variables. Independent: the temperature of the sodium thiosulfate solution, for example 20, 30, 40, 50 and 60 °C. Dependent: the time taken for a cross drawn on paper under the flask to disappear from view. Control: the volume and concentration of both solutions, the same flask, the same cross and the same person judging when it has gone.
Choose apparatus and method. Measure the solutions with measuring cylinders, warm the sodium thiosulfate in a water bath, check its temperature with a thermometer, then add the acid, swirl and start the stopwatch straight away. Stop it when the cross can no longer be seen from above.
Plan repeats. Repeat each temperature three times and calculate a mean. If one time is clearly out of line with the others, repeat it rather than include it.
Deal with safety. Wear eye protection to guard against splashes of acid. The reaction gives off a small amount of sulfur dioxide, which can trigger asthma, so work in a well-ventilated room and pour the mixture away promptly as instructed. Use a water bath rather than heating the flask directly.
Plan the table. Use columns for temperature (°C), each repeat time (s) and the mean time (s), with the unit in every heading and the same number of decimal places throughout.

A good plan uses collision theory to justify the prediction, changes only temperature, measures the time for the cross to disappear, keeps volumes, concentrations, flask, cross and observer the same, repeats each temperature three times, gives specific safety precautions and records the results in a clearly headed table. A natural improvement to suggest afterwards is a light sensor and data logger, because judging when a cross “disappears” varies from person to person.

Common mistakes and how to help

Writing a prediction and calling it a hypothesis

In earlier years, “I think the bigger ball will bounce higher” was enough.

What helps: Ask “because…?” A GCSE answer links the prediction to a scientific idea that explains it. Practise by finishing the sentence “I think … will happen because…”, using a theory from the topic.

Giving “the equipment” or “the experiment” as a control variable

Students know that the test must be fair but have not identified which factors actually matter.

What helps: Ask “what else could change the result?” and list specific factors, such as the temperature, the volume used, the starting angle or the time left. Each one should be something that can be measured and held constant.

Placing the quadrat where it is easiest or most interesting

It feels natural to choose a good spot, and random placement seems wasteful.

What helps: Explain that choosing spots introduces bias, so the sample no longer represents the whole area. Using random numbers as coordinates, and enough quadrats, gives a fairer picture.

Giving vague safety points and vague improvements, such as “be more careful” or “do it more accurately”

The phrases feel sensible but do not name a hazard or a change.

What helps: Make each point specific: what is the hazard, what could happen and what exactly would be done about it. For improvements, ask which measurement was weakest and what instrument or step would improve it.

How to help at home

Plan a fair test in the kitchen. Ask which of three kitchen rolls soaks up the most water. Together, decide what to change, what to measure (the volume of water left over, using a measuring jug) and what to keep the same, then carry it out and record the results in a table.
Read scales properly. Pour water into a clear measuring jug and read it with your eyes level with the surface. Try reading it from above and from below to see how much the apparent reading changes. Then agree on the smallest division the jug can show, which is its resolution.
Sample the lawn. Count the daisies in a garden or park by throwing a hula hoop or marking a square at random spots, rather than choosing the thickest patches. Compare ten random squares with ten chosen ones and see how the estimates differ.
Talk through the risks. Before an activity, ask your teenager to name the hazards, how likely and how serious the harm would be, and what they would do about each one. It builds the habit of specific risk assessment that exam questions look for.

Builds on

Key Stage 3: Scientific attitudes
Key Stage 3: Planning science investigations
Key Stage 3: Analysing and evaluating data
Key Stage 3: Units, equations and calculations