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GCSE ChemistryAQA, Edexcel and OCR

Chemistry required practicals: GCSE questions with answers

40 questions on the chemistry required practicals. Pick your answer, then tap to see if you were right and why. Every answer comes from the Show Working science bank, checked against the GCSE content every board in England teaches.

Quiz yourself: 40 questions 6 6-mark questions

01 / Quiz yourselfTry it, then tap to check

Chemistry required practicals questions to try

Choose your answer before you open it. Then read why, so it sticks.

01 / Chemistry required practicalsBoth tiers

Put the steps for making pure, dry copper sulfate crystals from copper oxide in order.

  1. ALeave the solution to cool so crystals form
  2. BAdd copper oxide, stirring, until some is left over
  3. CWarm some dilute sulfuric acid
  4. DFilter off the excess copper oxide
  5. EPat the crystals dry with filter paper
  6. FHeat the filtrate to evaporate some of the water
Show the answer

The right order:

  1. Warm some dilute sulfuric acid
  2. Add copper oxide, stirring, until some is left over
  3. Filter off the excess copper oxide
  4. Heat the filtrate to evaporate some of the water
  5. Leave the solution to cool so crystals form
  6. Pat the crystals dry with filter paper

The base goes in until no more reacts, so all the acid is used up. Filtering removes the leftover solid. Some water is evaporated, then the solution cools and crystals of the salt form.

Checked against AQA GCSE Chemistry 8462 specification, 8.2.1 Required practical activity 1 (with 4.4.2.3 Soluble salts)

02 / Chemistry required practicalsBoth tiers

Copper oxide is added to warm dilute sulfuric acid. How do you know all the acid has reacted?

  1. ABlack solid stays undissolved
  2. BThe solution turns colourless
  3. CA white precipitate forms
  4. DThe mixture starts to fizz hard
Show the answer

Answer: A Black solid stays undissolved

Copper oxide is a black powder that does not dissolve in water. While acid is left, it reacts to give a blue solution of copper sulfate. When black solid stays in the beaker, the acid has all been used up.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

03 / Chemistry required practicalsBoth tiers

Why is the dilute acid warmed before the copper oxide is added?

  1. ATo make the reaction faster
  2. BTo turn the acid into an alkali
  3. CTo boil off all of the acid
  4. DTo make the acid more dilute
Show the answer

Answer: A To make the reaction faster

Warming speeds up the reaction between the acid and the insoluble base. The leftover base does not dissolve, so filtering removes it and leaves a solution of only the salt and water.

Checked against AQA GCSE Chemistry 8462 specification, 8.2.1 Required practical activity 1 (with 4.4.2.3 Soluble salts)

04 / Chemistry required practicalsBoth tiers

When should you stop heating the copper sulfate solution?

  1. AWhen the solid left turns white
  2. BAs soon as the solution turns blue
  3. CWhen crystals start to form at the edge
  4. DWhen the solution first starts to boil
Show the answer

Answer: C When crystals start to form at the edge

Heating to dryness gives a powder and can drive the water out of hydrated crystals. Instead, heat until the solution is saturated, then let it cool. The salt is less soluble when cold, so crystals form.

Checked against AQA GCSE Chemistry 8462 specification, 8.2.1 Required practical activity 1 (with 4.4.2.3 Soluble salts)

05 / Chemistry required practicalsBoth tiers

How should the copper sulfate crystals be dried?

  1. ARinse them with lots of tap water
  2. BHeat them strongly in a crucible
  3. CLeave them in the solution overnight
  4. DPat them between filter papers
Show the answer

Answer: D Pat them between filter papers

Hydrated copper sulfate crystals contain water of crystallisation, which gives them their blue colour. Strong heat drives this water out and leaves white anhydrous copper sulfate, so the crystals are dried gently.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

06 / Chemistry required practicalsBoth tiers

Why should eye protection be worn while evaporating the salt solution?

  1. ACopper sulfate gives off chlorine
  2. BThe crystals glow very brightly
  3. CHot solution can spit out
  4. DThe steam is poisonous
Show the answer

Answer: C Hot solution can spit out

Hot salt solution can spit out of the evaporating basin, so wear eye protection and heat gently. Copper sulfate is harmful if swallowed and irritates the eyes and skin.

Checked against PubChem, Copper sulfate

07 / Chemistry required practicalsBoth tiers

A soluble salt is made from an insoluble metal carbonate and dilute acid. When do you stop adding the carbonate?

  1. AWhen a white precipitate forms
  2. BWhen no more fizzing happens
  3. CWhen the solution turns red
  4. DWhen the solution starts to boil
Show the answer

Answer: B When no more fizzing happens

A carbonate fizzes as it reacts with acid, giving off carbon dioxide. When adding more carbonate gives no more fizzing and solid is left over, the acid has all reacted.

Checked against AQA GCSE Chemistry 8462 specification, 8.2.1 Required practical activity 1 (with 4.4.2.3 Soluble salts)

08 / Chemistry required practicalsBoth tiers · separate only

What is the rough titration for?

  1. ATo rinse the burette out with acid
  2. BTo find roughly where the end point is
  3. CTo warm the solutions up first
  4. DTo make the acid more concentrated
Show the answer

Answer: B To find roughly where the end point is

The rough titration shows roughly how much acid is needed. Later runs go quickly to just before that volume, then drop by drop, so the end point is not overshot. Concordant titres are averaged.

Checked against AQA GCSE Chemistry 8462 specification, 4.4.2.5 Titrations and Required practical 2

09 / Chemistry required practicalsBoth tiers · separate only

Why is a pipette used to put the alkali into the conical flask?

  1. AIt stops the alkali from reacting
  2. BIt measures any volume very quickly
  3. CIt measures one fixed volume accurately
  4. DIt shows the volume of acid added
Show the answer

Answer: C It measures one fixed volume accurately

A volumetric pipette delivers one exact volume, such as 25.0 cm³, of solution. A burette lets you add the other solution a drop at a time and read how much was added. A measuring cylinder is not accurate enough.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

10 / Chemistry required practicalsBoth tiers · separate only

Why is the conical flask stood on a white tile?

  1. ATo make the reaction faster
  2. BTo see the colour change clearly
  3. CTo absorb any extra acid
  4. DTo keep the flask warm
Show the answer

Answer: B To see the colour change clearly

The white tile makes the indicator colour change easy to see. Swirling mixes each addition in. Near the end point one drop can change the colour, so adding drop by drop stops you adding too much.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

11 / Chemistry required practicalsBoth tiers · separate only

How should a burette be read?

  1. AAt eye level, at the top of the meniscus
  2. BLooking up from below, at the bottom of the meniscus
  3. CAt eye level, at the bottom of the meniscus
  4. DFrom above, at the top of the meniscus
Show the answer

Answer: C At eye level, at the bottom of the meniscus

Read the burette with your eye level with the liquid, at the bottom of the curved meniscus, or the reading will be off. Burettes are read to the nearest 0.05 cm³, so readings have two decimal places.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

12 / Chemistry required practicalsBoth tiers · separate only

Which titres are concordant?

  1. AThe first and the last titres
  2. BTitres very close to each other
  3. CThe rough titre and the largest
  4. DTitres that differ the most
Show the answer

Answer: B Titres very close to each other

Concordant titres agree closely with each other. The rough titre and any titre far from the rest are left out, and the mean of the concordant titres is used to work out the concentration.

Checked against OCR Twenty First Century Chemistry B J258 specification, C5.4 outcomes 7 and 8 (PAG6)

13 / Chemistry required practicalsBoth tiers · separate only

Which indicator could be used to titrate a strong acid against a strong alkali?

  1. ALimewater
  2. BPhenolphthalein
  3. CLitmus
  4. DUniversal indicator
Show the answer

Answer: B Phenolphthalein

A titration needs an indicator with one sharp colour change, such as phenolphthalein or methyl orange. Phenolphthalein is pink in alkali and colourless in acid, so it goes colourless as the alkali is neutralised.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, 3.3 Indicators

14 / Chemistry required practicalsBoth tiers · separate only

Why is the burette rinsed with the acid before it is filled?

  1. ASo the scale is easier to read
  2. BSo leftover water does not dilute it
  3. CSo the burette warms up
  4. DSo the acid reacts faster
Show the answer

Answer: B So leftover water does not dilute it

Water left in the burette would dilute the acid, so more would be needed. Water in the conical flask does not matter: the pipette still delivers the same amount of alkali, and that is what the acid neutralises.

Checked against AQA GCSE Chemistry 8462 specification, 4.4.2.5 Titrations and Required practical 2

15 / Chemistry required practicalsBoth tiers · separate only

Which of these would make a titre too large?

  1. AOvershooting the end point
  2. BSwirling the flask
  3. CReading the burette at eye level
  4. DStanding the flask on a white tile
Show the answer

Answer: A Overshooting the end point

Adding acid past the colour change makes the titre bigger than the true value. Titres that agree closely are precise, but they can still all be wrong in the same way, so precise does not have to mean accurate.

Checked against OCR Twenty First Century Chemistry B J258 specification, C7 Ideas about Science 2.9 and 2.10

16 / Chemistry required practicalsBoth tiers

Why are graphite electrodes used to electrolyse a salt solution?

  1. AThey are inert, so they do not react
  2. BThey give off hydrogen themselves
  3. CThey dissolve to colour the solution
  4. DThey react to make the ions needed
Show the answer

Answer: A They are inert, so they do not react

Inert electrodes, such as graphite, carry the current without taking part in the reaction. A low-voltage direct current makes positive ions move to the cathode and negative ions to the anode, and it is safe to use.

Checked against AQA GCSE Chemistry 8462 specification, 4.4.3.4 Electrolysis of aqueous solutions and Required practical 3

17 / Chemistry required practicalsBoth tiers

A salt solution is electrolysed with inert electrodes. When is hydrogen given off at the cathode?

  1. AWhen the solution contains halide ions
  2. BWhen the metal is more reactive than hydrogen
  3. CWhen the metal is less reactive than hydrogen
  4. DWhen the solution contains sulfate ions
Show the answer

Answer: B When the metal is more reactive than hydrogen

At the cathode, a metal less reactive than hydrogen, such as copper, is deposited; otherwise hydrogen is given off. At the anode, a halogen forms if halide ions are present; otherwise oxygen is given off.

Checked against AQA GCSE Chemistry 8462 specification, 4.4.3.4 Electrolysis of aqueous solutions and Required practical 3

18 / Chemistry required practicalsBoth tiers

Sodium chloride solution is electrolysed with inert electrodes. What forms at the cathode?

  1. AHydrogen
  2. BOxygen
  3. CSodium
  4. DChlorine
Show the answer

Answer: A Hydrogen

Sodium is more reactive than hydrogen, so hydrogen is given off at the cathode, not sodium. Chloride ions are present, so chlorine is given off at the anode.

Checked against OCR Gateway Chemistry A J248 specification, C3.4c Electrolysis of aqueous solutions (PAG C2)

19 / Chemistry required practicalsBoth tiers

The gas from the anode bleaches damp litmus paper. Which gas is it?

  1. AHydrogen
  2. BOxygen
  3. CCarbon dioxide
  4. DChlorine
Show the answer

Answer: D Chlorine

A test tube full of the solution is held over an electrode so the gas pushes the liquid out and collects. Chlorine bleaches damp litmus paper, which turns white.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.2 Identification of common gases

20 / Chemistry required practicalsBoth tiers

A gas from the cathode burns with a pop when a burning splint is held to it. Which gas is it?

  1. AHydrogen
  2. BOxygen
  3. CCarbon dioxide
  4. DChlorine
Show the answer

Answer: A Hydrogen

Hydrogen burns rapidly with a pop when a burning splint is held at the mouth of the tube. Oxygen relights a glowing splint. These tests show which gas each electrode gave off.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.2 Identification of common gases

21 / Chemistry required practicalsBoth tiers

Why is sodium chloride solution electrolysed in a well-ventilated room, and only for a short time?

  1. AThe solution boils over
  2. BSodium catches fire in water
  3. CHydrogen is a toxic gas
  4. DChlorine is a toxic gas
Show the answer

Answer: D Chlorine is a toxic gas

Chlorine is toxic and irritates the lungs and eyes. Running the electrolysis for a short time keeps the amount small, and good ventilation stops it building up.

Checked against PubChem, Chlorine

22 / Chemistry required practicalsBoth tiers

Copper sulfate solution is electrolysed with graphite electrodes. What is seen at the anode?

  1. AA white precipitate
  2. BA pink-brown coating
  3. CBubbles of gas
  4. DA yellow precipitate forms
Show the answer

Answer: C Bubbles of gas

With inert electrodes, copper ions are removed as a pink-brown coating of copper on the cathode, and oxygen bubbles off at the anode. As copper ions are used up, the blue colour fades.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, 3.25 and 3.31 Electrolysis of copper sulfate solution

23 / Chemistry required practicalsBoth tiers

Why must the bung go in as soon as the solid is added?

  1. ASo the reaction stops at once
  2. BSo the acid does not evaporate
  3. CSo no gas escapes before it is measured
  4. DSo the flask stays cool
Show the answer

Answer: C So no gas escapes before it is measured

The gas is collected in a gas syringe or an upturned measuring cylinder full of water. The bung must go in straight away so none of the gas escapes, and the volume is read at regular times.

Checked against AQA GCSE Chemistry 8462 specification, 4.6.1.1 Calculating rates of reactions

24 / Chemistry required practicalsBoth tiers

On a graph of gas volume against time, why does the line level off?

  1. AThe reaction has sped up
  2. BThe gas syringe has broken
  3. COne reactant has been used up
  4. DThe catalyst has run out
Show the answer

Answer: C One reactant has been used up

The gradient of the line shows the rate. It is steepest at the start, when the reactants are most concentrated. As they are used up it gets less steep, and it is flat once one reactant has run out.

Checked against AQA GCSE Chemistry 8462 specification, 4.6.1.1 Calculating rates of reactions

25 / Chemistry required practicalsBoth tiers

A pupil changes the concentration of the acid and measures the volume of gas made. What is the independent variable?

  1. AThe mass of the solid
  2. BThe temperature of the acid
  3. CThe concentration of the acid
  4. DThe volume of gas made
Show the answer

Answer: C The concentration of the acid

Only the acid concentration is changed, and the gas volume is measured. Temperature, acid volume, and the mass and size of the solid are kept the same, because each of these also affects the rate.

Checked against AQA GCSE Chemistry 8462 specification, 8.2.5 Required practical activity 5

26 / Chemistry required practicalsBoth tiers

What happens to the rate when the acid is more concentrated?

  1. AIt increases: particles have more energy
  2. BIt increases: collisions are more frequent
  3. CIt stays the same: the temperature is fixed
  4. DIt decreases: there are fewer collisions
Show the answer

Answer: B It increases: collisions are more frequent

A more concentrated acid has more acid particles in the same volume, so they collide with the solid more often. More frequent collisions mean a faster rate. The particles do not gain energy.

Checked against AQA GCSE Chemistry 8462 specification, 4.6.1.3 Collision theory and activation energy

27 / Chemistry required practicalsBoth tiers

Why must the same person watch the cross in every run?

  1. APeople judge “disappeared” differently
  2. BSo the acid does not evaporate
  3. CSo the reaction goes faster
  4. DSo less sulfur is made
Show the answer

Answer: A People judge “disappeared” differently

Sodium thiosulfate reacts with acid to make a precipitate of sulfur, which slowly hides the cross. The same observer and the same cross keep the judgement of when it has disappeared as fair as possible.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 3.17, 3.31, 5.9C and 7.1

28 / Chemistry required practicalsBoth tiers

Why should the room be well ventilated during the disappearing cross experiment?

  1. AHydrogen gas is given off
  2. BThe sulfur catches fire
  3. CSulfur dioxide gas is given off
  4. DChlorine gas is given off
Show the answer

Answer: C Sulfur dioxide gas is given off

The reaction also makes sulfur dioxide, a toxic gas that irritates the lungs, especially for people with asthma. Small volumes, a ventilated room and quickly diluting the mixture afterwards keep the risk low.

Checked against PubChem, Sulfur dioxide

29 / Chemistry required practicalsBoth tiers

Put the steps for making a paper chromatogram of some inks in order.

  1. AMark the solvent front and let the paper dry
  2. BTake the paper out before the solvent reaches the top
  3. CDraw a pencil line near the bottom of the paper
  4. DStand the paper in solvent below the line
  5. EPut small spots of each ink on the line
Show the answer

The right order:

  1. Draw a pencil line near the bottom of the paper
  2. Put small spots of each ink on the line
  3. Stand the paper in solvent below the line
  4. Take the paper out before the solvent reaches the top
  5. Mark the solvent front and let the paper dry

The start line is drawn in pencil and the inks are spotted on it. The paper stands in a little solvent, below the line, and is taken out before the solvent reaches the top. The solvent front is marked at once.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.1.3 Chromatography and Required practical 6

30 / Chemistry required practicalsBoth tiers

Why is a lid put on the beaker while the chromatogram runs?

  1. ATo make the dyes dissolve faster
  2. BTo keep light off the dyes
  3. CTo stop the paper getting wet
  4. DTo stop the solvent evaporating
Show the answer

Answer: D To stop the solvent evaporating

A lid keeps the solvent from evaporating off the paper as it rises. The solvent front is needed for Rf values, and it disappears as the paper dries, so it is marked in pencil straight away.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.1.3 Chromatography and Required practical 6

31 / Chemistry required practicalsBoth tiers

Why are the ink spots kept small?

  1. ASo the dyes separate into clear spots
  2. BSo the ink does not dissolve
  3. CSo the Rf values get bigger
  4. DSo the solvent moves faster
Show the answer

Answer: A So the dyes separate into clear spots

Big spots spread out and overlap, so the dyes do not separate clearly. For an Rf value, measure from the start line to the centre of the spot, and from the start line to the solvent front.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.1.3 Chromatography and Required practical 6

32 / Chemistry required practicalsBoth tiers

An unknown ink gives two spots, level with dyes A and C on the same chromatogram. What does this suggest?

  1. AIt contains only dye B
  2. BIt contains dyes A and C
  3. CIt is a pure substance
  4. DIt contains no dyes at all
Show the answer

Answer: B It contains dyes A and C

Spots that travel the same distance as known dyes, on the same paper and in the same solvent, are probably the same substances. A pure substance gives a single spot in every solvent.

Checked against AQA GCSE Chemistry 8462 specification, 4.8.1.3 Chromatography and Required practical 6

33 / Chemistry required practicalsBoth tiers

Why does the cold water go into the condenser at the bottom and out at the top?

  1. ASo the cooling water flows faster under gravity
  2. BSo the condenser fills completely with water
  3. CSo the dyes dissolve in the water
  4. DSo the distillate comes out hot
Show the answer

Answer: B So the condenser fills completely with water

Water boils off the ink or salty water, cools in the condenser and drips into the beaker as pure, colourless water; the dyes or salt stay behind. Feeding cold water in at the bottom keeps the condenser jacket full, so it cools the vapour well.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 2.11 and 3.6

34 / Chemistry required practicalsBoth tiers

Where should the thermometer bulb be in simple distillation?

  1. ADipping into the boiling liquid
  2. BAt the bottom end of the condenser
  3. CInside the collecting beaker
  4. DLevel with the side arm to the condenser
Show the answer

Answer: D Level with the side arm to the condenser

The thermometer measures the temperature of the vapour going into the condenser, so its bulb sits at the side arm. While water distils, the vapour is at the boiling point of water, about 100 °C.

Checked against OCR Gateway Chemistry A J248 specification, C2.1f Separation techniques (PAG C4)

35 / Chemistry required practicalsBoth tiers

Why are anti-bumping granules added to the flask?

  1. ATo soak up the dyes in the ink
  2. BSo the liquid boils at a lower temperature
  3. CSo the liquid boils smoothly
  4. DTo stop the condenser leaking
Show the answer

Answer: C So the liquid boils smoothly

Anti-bumping granules give bubbles somewhere to form, so the liquid boils smoothly instead of in sudden violent bursts. Heating stops before the flask is dry, as a dry flask can crack and the residue can burn.

Checked against Pearson Edexcel GCSE Chemistry 1CH0 specification, Appendix: core practicals 2.11 and 3.6

36 / Chemistry required practicalsBoth tiers

What is the difference between a hypothesis and a prediction?

  1. AThey mean exactly the same thing in science
  2. BA hypothesis explains; a prediction says what will happen
  3. CA hypothesis is always proved right by the results
  4. DA hypothesis is a result; a prediction is a method
Show the answer

Answer: B A hypothesis explains; a prediction says what will happen

A hypothesis is a possible explanation based on scientific ideas. A prediction says what should happen in a test if the hypothesis is right. Sodium is more reactive than hydrogen; copper and silver are not.

Checked against AQA GCSE Chemistry 8462 specification, WS 2.1 Develop hypotheses

37 / Chemistry required practicalsBoth tiers

In an investigation, what is the dependent variable?

  1. AA variable you keep the same
  2. BThe variable you change
  3. CThe variable you measure
  4. DA variable you cannot measure
Show the answer

Answer: C The variable you measure

The independent variable is the one you change, the dependent variable is the one you measure, and control variables are kept the same. Then any change in the result is caused by the independent variable.

Checked against AQA GCSE Chemistry 8462 specification, WS 2.2 to 2.7 Experimental skills and strategies

38 / Chemistry required practicalsBoth tiers

The times for a cross to disappear are 32 s, 34 s, 61 s and 33 s. What should be done with 61 s?

  1. AInclude it when working out the mean
  2. BUse it as the only result
  3. CLeave it out when working out the mean
  4. DCount it twice when working out the mean
Show the answer

Answer: C Leave it out when working out the mean

A result far from the others is anomalous. Check it for a cause, and if it came from a poor measurement, leave it out of the mean. A mean of repeats reduces random error, but it cannot remove a systematic error.

Checked against AQA GCSE Chemistry 8462 specification, WS 3.7 Evaluating data

39 / Chemistry required practicalsBoth tiers

What is a hazard?

  1. AA result that does not fit the pattern
  2. BA step taken to make work safe
  3. CThe chance that harm will happen
  4. DSomething that could cause harm
Show the answer

Answer: D Something that could cause harm

A hazard is something that could cause harm, such as an irritant acid. The risk is the chance of harm happening. Precautions, such as wearing eye protection, reduce the risk.

Checked against AQA GCSE Chemistry 8462 specification, WS 2.4 Health and safety

40 / Chemistry required practicalsBoth tiers

How should a line of best fit be drawn through rate results?

  1. AA smooth line or curve with points either side
  2. BThrough the first and last points only
  3. CDot to dot, joining every single point
  4. DAlways a straight line drawn through the origin
Show the answer

Answer: A A smooth line or curve with points either side

A line of best fit is a single smooth straight line or curve that follows the trend, with points scattered evenly either side. It ignores anomalous points and does not join the dots.

Checked against OCR Twenty First Century Chemistry B J258 specification, C7 Ideas about Science 2.7 Displaying data

02 / Longer answersMarked by levels

6-mark questions on this topic

Write your answer first, on paper. Then open it and tick off the points you made. How 6-mark questions are marked.

6 marks / Making a pure dry saltBoth tiers

Copper(II) oxide is a black solid that does not dissolve in water. It reacts with dilute sulfuric acid to make copper(II) sulfate, a soluble salt, and water.

Describe how you would make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

What a full answer includes
  • Warm a measured volume of dilute sulfuric acid in a beaker, with goggles on.
  • Add copper(II) oxide a little at a time, stirring, until some black solid is left over.
  • The oxide is in excess, so all of the acid has reacted.
  • Filter the mixture to remove the excess copper(II) oxide, keeping the blue filtrate.
  • Heat the filtrate in an evaporating basin, for example over a water bath, to evaporate some of the water.
  • Stop heating when crystals start to form, and do not heat to dryness.
  • Leave the solution to cool so crystals form.
  • Filter off the crystals and dry them between filter paper or in a warm oven.

Model answer

Wearing goggles, I would measure about 50 cm³ of dilute sulfuric acid into a beaker and warm it gently, because the reaction is faster in warm acid. Then I would add copper(II) oxide a spatula at a time, stirring after each addition, until black powder stays at the bottom and no more seems to react. This means the copper(II) oxide is in excess, so all of the acid has been used up and the solution contains only copper(II) sulfate and water. Next I would filter the mixture through a filter funnel to remove the leftover copper(II) oxide, collecting the blue filtrate in an evaporating basin. I would heat the filtrate over a water bath to evaporate some of the water until the solution is saturated and small crystals start to appear at the edge. I would not boil it dry, because that would spoil the crystals. I would leave the basin somewhere cool for the copper(II) sulfate to crystallise, then filter off the blue crystals and pat them dry between pieces of filter paper.

Where marks slip

Many answers heat the solution until all the water has gone, or forget to add the oxide in excess and filter it off. Say why the oxide is in excess and stop heating once crystals start to form.

6 marks / Carrying out an accurate titrationBoth tiers

A student wants to find the volume of dilute hydrochloric acid that exactly neutralises 25.0 cm³ of sodium hydroxide solution.

Describe how the student should carry out a titration to get accurate results.

What a full answer includes
  • Use a volumetric pipette with a filler to measure 25.0 cm³ of sodium hydroxide solution into a conical flask.
  • Add a few drops of a suitable indicator, such as phenolphthalein or methyl orange.
  • Fill a burette with the acid and record the starting reading, reading the bottom of the meniscus at eye level.
  • Add the acid to the flask while swirling, standing the flask on a white tile.
  • Add the acid drop by drop near the end point.
  • Stop when the indicator changes colour for good and record the final reading.
  • The titre is the final reading take away the starting reading.
  • Repeat until two or more titres are concordant (agree closely with each other).
  • Take the mean of the concordant titres, leaving out the rough titration.

Model answer

First I would rinse a burette with the hydrochloric acid, fill it and record the starting volume, reading the bottom of the meniscus at eye level. Using a volumetric pipette and a pipette filler, I would measure exactly 25.0 cm³ of sodium hydroxide solution into a conical flask and add a few drops of phenolphthalein, which is pink in the alkali. I would put the flask on a white tile so the colour change is easy to see. Then I would run acid from the burette into the flask, swirling all the time. The first run is a rough titration to find roughly where the end point is. In the next runs I would add the acid quickly at first, then drop by drop near the end point, stopping as soon as the solution turns colourless and stays colourless. I would record the final burette reading and work out the titre by subtracting the starting reading. I would repeat until I had at least two concordant titres, which agree closely with each other, and take their mean, leaving out the rough titre.

Where marks slip

Using a measuring cylinder or beaker instead of a pipette costs marks, and so does universal indicator, which changes colour gradually and hides the end point. So does putting the rough titre into the mean.

6 marks / Electrolysis of salt solutionsBoth tiers

Plan an investigation to find out which products form at each electrode when copper(II) sulfate solution and sodium chloride solution are electrolysed using inert electrodes.

What a full answer includes
  • Pour the solution into a beaker and dip in two graphite (inert) electrodes that do not touch.
  • Connect the electrodes to a low-voltage direct current supply and switch on.
  • Fill a small test tube with the solution and hold it upside down over each electrode to collect any gas.
  • Look for a solid coating on the negative electrode (cathode) and bubbles at either electrode.
  • Test the gases: a lit splint gives a squeaky pop with hydrogen, a glowing splint relights in oxygen, and damp litmus paper is bleached by chlorine.
  • Wash the apparatus and repeat with the second solution, keeping the voltage, electrodes and volume of solution the same.
  • Copper(II) sulfate gives copper at the cathode and oxygen at the anode.
  • Sodium chloride solution gives hydrogen at the cathode and chlorine at the anode.
  • Do the chlorine test in a well-ventilated room because chlorine is toxic.

Model answer

I would pour copper(II) sulfate solution into a beaker and put in two graphite electrodes, making sure they do not touch, then connect them to a low-voltage d.c. supply. I would fill two small test tubes with the solution and hold one upside down over each electrode to collect any gas. After a few minutes I would look at each electrode. A pink-brown coating on the negative electrode shows copper has formed, because copper is less reactive than hydrogen. Bubbles at the positive electrode can be tested with a glowing splint, which relights if the gas is oxygen. Then I would wash everything and repeat with sodium chloride solution, using the same voltage, electrodes and volume. This time there are bubbles at both electrodes. The gas from the negative electrode gives a squeaky pop with a lit splint, so it is hydrogen, because sodium is more reactive than hydrogen. The gas from the positive electrode bleaches damp litmus paper, so it is chlorine. I would keep the room well ventilated because chlorine is toxic.

Where marks slip

Pupils often put hydrogen at the positive electrode or say sodium forms from the solution. Metals and hydrogen form at the negative electrode, and sodium is too reactive to form from a solution.

6 marks / How concentration affects rateBoth tiers

Marble chips (calcium carbonate) react with dilute hydrochloric acid to give carbon dioxide gas.

Plan an investigation to find out how the concentration of the hydrochloric acid affects the rate of this reaction.

What a full answer includes
  • Measure a fixed volume of acid, for example 50 cm³, into a conical flask with a measuring cylinder.
  • Add a fixed mass of marble chips, connect a gas syringe with a bung and start a stopwatch at once.
  • Record the volume of gas in the syringe at regular times, for example every 10 seconds.
  • Repeat with at least four other concentrations of acid.
  • Keep the same: volume of acid, mass and size of chips, and temperature.
  • Repeat each concentration and work out a mean.
  • Plot volume of gas against time for each concentration and compare the slopes.
  • A steeper start, or a larger mean rate (volume of gas divided by time), shows a faster reaction.

Model answer

I would measure 50 cm³ of hydrochloric acid of the first concentration into a conical flask using a measuring cylinder. I would weigh out 2.0 g of marble chips, all of a similar size, add them to the flask, quickly put in a bung connected to a gas syringe and start a stopwatch. I would read the volume of carbon dioxide in the syringe every 10 seconds for two minutes. Then I would repeat this with four more concentrations of acid. To make it a fair test I would keep the volume of acid, the mass and size of the chips and the temperature the same, so that only the concentration changes. I would do each concentration three times and calculate a mean volume at each time, ignoring any anomalous result. For each concentration I would plot a graph of volume of gas against time. The steeper the curve at the start, the faster the reaction, and I could work out a mean rate in cm³/s by dividing the volume of gas by the time. I would expect higher concentrations to give faster rates.

Where marks slip

Saying “keep everything the same” earns nothing; name the variables you control. Writing “syringe” alone, or “gas meter”, is not credited: say gas syringe.

6 marks / Temperature change in neutralisationBoth tiers

When sodium hydroxide solution is added to dilute hydrochloric acid, the mixture gets warmer.

Plan an investigation to find out how the volume of sodium hydroxide solution added affects the temperature of the mixture.

What a full answer includes
  • Put a fixed volume of acid, for example 25 cm³, into a polystyrene cup standing in a beaker.
  • Measure the starting temperature of the acid with a thermometer.
  • Add the sodium hydroxide solution in small equal portions, for example 5 cm³ at a time.
  • Stir and record the highest temperature after each portion.
  • Fit a lid to cut down energy transferred to the surroundings.
  • Keep the concentrations of both solutions and the volume of acid the same.
  • Repeat the whole experiment and work out mean temperatures.
  • Plot temperature against volume of alkali added: the highest point shows when the acid is just neutralised.

Model answer

I would measure 25 cm³ of hydrochloric acid with a measuring cylinder and pour it into a polystyrene cup standing in a beaker, so the cup does not tip over. Polystyrene is a good insulator, so less energy is transferred to the surroundings, and I would cover the cup with a lid that has a hole for the thermometer. I would record the starting temperature of the acid. Then I would add 5 cm³ of sodium hydroxide solution, stir, and record the highest temperature reached. I would keep adding 5 cm³ portions, recording the highest temperature each time, until I had added 40 cm³. I would keep the concentrations of the acid and the alkali and the volume of acid the same throughout. I would repeat the whole experiment and calculate mean temperatures. Then I would plot a graph of temperature against volume of sodium hydroxide added and draw two lines of best fit. The temperature rises while the acid is reacting and then falls once excess alkali is added, so the peak shows the volume that just neutralises the acid.

Where marks slip

Pupils often record the temperature at a fixed time instead of the highest temperature, or forget the insulation and lid. Say what you record after each addition and why the cup is insulated.

6 marks / Paper chromatography of inksBoth tiers

A black ink may contain some of four known dyes, A, B, C and D.

Describe how you would use paper chromatography to find out which dyes are in the ink.

What a full answer includes
  • Draw a start line in pencil about 2 cm from the bottom of the chromatography paper.
  • Put a small spot of the ink and of each known dye on the line, spaced apart.
  • Stand the paper in a beaker of solvent, such as water, with the solvent below the start line.
  • Cover the beaker and leave the solvent to move up the paper.
  • Take the paper out before the solvent reaches the top and mark the solvent front in pencil.
  • Measure the distance from the start line to the centre of each spot and to the solvent front.
  • Rf equals the distance moved by the spot divided by the distance moved by the solvent.
  • The ink contains a dye if one of its spots is at the same height, with the same Rf value, as that dye.

Model answer

I would draw a horizontal line in pencil about 2 cm above the bottom of a piece of chromatography paper. Pencil is used because it does not dissolve and run. I would put a small spot of the black ink on the line and a spot of each known dye next to it, leaving space between them. Then I would hang the paper in a beaker containing a little water as the solvent, making sure the water is below the pencil line so the spots do not wash away, and cover the beaker so the solvent does not evaporate. The solvent soaks up the paper, which is the stationary phase, and the dyes move different distances depending on how they divide between the paper and the solvent. Just before the solvent reaches the top I would remove the paper, mark the solvent front in pencil and let it dry. I would measure from the start line to the centre of each spot and to the solvent front, and work out each Rf value by dividing the spot distance by the solvent distance. A dye is in the ink if one of the ink spots matches its height and Rf value.

Where marks slip

Drawing the start line in pen, or letting the solvent cover the start line, loses marks. Measure distances from the start line, not from the bottom edge of the paper.

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