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How to plan a fair test, handle hazards, and carry out the core chemistry practicals: making salts, neutralisation and pH, temperature change, electrolysis and rates of reaction.
Specification references this topic covers: AQA 5.3.1, 5.4.2; Edexcel C0.5, C0.6, C3.6, C3.15, C3.17, C3.31, C7.1, C7.2.
Chemistry is an experimental science. You are expected to know how to do experiments properly, how to keep them safe, and how to make sense of the results. The core practicals are the experiments every student should have done.
This matters because exam questions often describe an experiment you have not seen and ask you to improve it, explain a step or spot an error. If you know the reasons behind each step you can answer these.
By the end you will know the skills for planning and processing results, and the method and reasons for the main chemistry practicals.
The independent variable is the one you change. The dependent variable is the one you measure. Control variables are all the other things you keep the same so that the test is fair.
Repeat the experiment and find the mean, but leave out any anomalous result (one that does not fit the pattern). Results that are repeatable are the same when the same person repeats the experiment. Results that are reproducible are the same when a different person or method is used. Accurate results are close to the true value. Precise results are close to each other.
The uncertainty in a mean can be estimated as half the range. For results of 23.0, 25.0 and 26.0 seconds the range is 3.0, so the uncertainty is ±1.5 seconds. A measuring instrument with a smaller resolution (smallest change it can show) gives better readings.
A hazard is something that could cause harm. A risk is how likely the harm is, and how serious. Hazard symbols on containers warn about the dangers: flammable, corrosive, toxic or harmful, oxidising, and harmful to the environment.
To evaluate the risks in a practical, name the hazard, say what could go wrong, and suggest a precaution. For example, dilute acid is an irritant, so wear eye protection and wash splashes off straight away. Heating a liquid in a test tube can spit, so point the tube away from people. Hot glassware should be left to cool before handling. Flammable liquids must be kept away from flames.
Good precautions are specific: eye protection, gloves, a fume cupboard for toxic gases, a water bath instead of a flame, small amounts.
To make copper sulfate crystals, warm some dilute sulfuric acid in a beaker. Add copper oxide a little at a time, stirring, until no more dissolves. The copper oxide is insoluble, so it is easy to add in excess. This makes sure all the acid has been used up. The solution turns blue.
Filter the mixture to remove the excess copper oxide. The filtrate is a solution of copper sulfate. Pour it into an evaporating basin and heat gently to evaporate about half of the water, until crystals start to form on the edge. Do not boil it dry.
Leave the solution to cool and crystallise slowly. Filter off the crystals and dry them between filter papers or in a warm oven. The equation is CuO + H₂SO₄ → CuSO₄ + H₂O. If the solid base is replaced by a soluble alkali, you must use a titration instead, because you cannot filter off an excess of a soluble substance.
To investigate pH, put a fixed volume of dilute hydrochloric acid in a beaker. Add small, measured amounts of calcium hydroxide powder, stirring each time. Measure the pH with a pH meter or universal indicator paper after each addition. The pH rises from acidic towards neutral and then into the alkaline range once the acid has been used up. Calcium hydroxide is an irritant, so wear eye protection.
To investigate temperature change, use a polystyrene cup with a lid. Polystyrene is a good insulator, so little heat is lost to the surroundings. Measure a fixed volume of acid and record its starting temperature. Add the alkali or metal, stir, and record the highest temperature. The temperature change is the highest temperature minus the starting temperature.
If the temperature goes up the reaction is exothermic. Neutralisation is exothermic. A temperature fall shows an endothermic change.
Put copper sulfate solution in a beaker with two electrodes connected to a d.c. power supply. With inert electrodes (graphite), copper is deposited on the negative electrode as a pink-brown solid. Bubbles of oxygen form at the positive electrode. The blue colour fades as copper ions are used up.
With copper electrodes, the positive electrode loses mass as copper dissolves into the solution. The negative electrode gains the same mass as copper is deposited. The blue colour stays the same because the copper ions leaving and arriving balance.
To investigate electrolysis of other solutions, use inert electrodes and test the gases. Hydrogen gives a squeaky pop. Oxygen relights a glowing splint. Chlorine bleaches damp litmus paper.
Rate is how fast the amount of reactant falls or the amount of product rises. There are three common ways to follow a reaction. Collect a gas in a gas syringe or upturned measuring cylinder and record the volume at intervals. Put the reaction on a balance and record the loss in mass as a gas escapes. Or measure the time for a cross under the flask to disappear as a solid forms and turns the mixture cloudy.
Change one variable at a time: concentration, temperature, surface area of a solid (large lumps or powder) or a catalyst, such as manganese dioxide with hydrogen peroxide. Keep everything else the same.
Mean rate = amount of product formed (or reactant used) ÷ time. For the cross method a shorter time means a faster rate, and you can use 1 ÷ time as a measure of the rate. Plot the results as a graph and draw a line of best fit.
Independent variable: change it. Dependent variable: measure it. Control variables: keep them the same. Find the mean, spot anomalies, estimate uncertainty as half the range.
Salt practical: excess insoluble base, filter, evaporate part way, crystallise, dry. Temperature change practical: polystyrene cup, highest temperature minus starting temperature.
Electrolysis: copper on the negative electrode, oxygen at a positive inert electrode, the positive copper electrode loses mass. Rates: gas volume, loss in mass or cross disappearing.
A student adds magnesium to acid in a polystyrene cup. The starting temperature is 20.5 °C and the highest temperature is 27.5 °C. Calculate the temperature change.
Answer: 7.0 °C
Describe how to make a pure, dry sample of copper sulfate crystals from copper oxide and dilute sulfuric acid.
Answer: Add excess copper oxide to warm acid, filter, evaporate part of the water, cool to crystallise, filter and dry
A student measures the volume of gas given off by magnesium and acid. In 30 s the volume is 60 cm³. A second experiment with a higher temperature gives 60 cm³ in 15 s. Calculate both mean rates and say how many times faster the second one is.
Answer: 2.0 cm³/s and 4.0 cm³/s. The second is 2 times faster
Required practicals and working scientifically in chemistry practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Which variable do you change in an investigation?
Answer: independent variable
What is the name for a variable that you keep the same?
Answer: control variable
Which hazard symbol warns that a substance can burn skin and eyes?
Answer: corrosive
What is added in excess to dilute sulfuric acid to make copper sulfate?
Answer: copper oxide
Why is a polystyrene cup used for a temperature change experiment? It is a good …
Answer: insulator
In making copper sulfate, the copper oxide is in excess so that all of which substance is used up?
Answer: the acid
How is the excess copper oxide removed from the copper sulfate solution?
Answer: filtration
The starting temperature is 21.5 °C and the highest temperature is 29.0 °C. Calculate the temperature change in °C.
Answer: 7.5 °C
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Describe how you would make a pure, dry sample of copper sulfate crystals from copper oxide and dilute sulfuric acid. [6 marks]
Answer: Warm the dilute sulfuric acid in a beaker. Add copper oxide a little at a time until no more dissolves, so it is in excess. Filter to remove the excess copper oxide. Heat the filtrate in an evaporating basin to evaporate some of the water. Leave it to cool so that crystals form. Filter off the crystals and dry them with filter paper.
(a) A student investigates how the concentration of acid affects the rate of reaction with magnesium. Name two control variables. [2 marks] (b) Give one safety precaution when using dilute hydrochloric acid. [1 mark] (c) State why the student should repeat the experiment. [1 mark]
Answer: (a) the mass of magnesium and the temperature (b) wear eye protection (c) to find a mean and spot anomalous results
Mixing up independent and dependent variables
Independent: the one you change. Dependent: the one you measure.
Saying the excess solid is added to use up the alkali
In salt making the insoluble solid is in excess to use up all the acid.
Boiling the solution dry when making crystals
Evaporate only some of the water, then let the solution cool so crystals form.
Saying all repeat results must be included in the mean
Leave out anomalous results before finding the mean.
Writing temperature change as start minus end
Temperature change = highest (or final) temperature − starting temperature.
Required practicals and working scientifically in chemistry is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 5.
Mixing up independent and dependent variables. Independent: the one you change. Dependent: the one you measure.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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