AQA GCSE Combined Science: Trilogy · 5.4

Chemical changes

50 revision questions with answers for 5.4 Chemical changes in AQA GCSE Combined Science: Trilogy (spec 8464). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official AQA material.

All 50 questions and answers

Question 1 · Spec 5.4.1.1
Define oxidation in terms of oxygen.
Answer:
Gain of oxygen; for example, a metal reacting with oxygen forms a metal oxide.
Question 2 · Spec 5.4.1.1
Define reduction in terms of oxygen.
Answer:
Loss of oxygen from a substance.
Question 3 · Spec 5.4.1.2
Order these metals from most to least reactive: copper, potassium, zinc, magnesium, iron, sodium, calcium, lithium.
Answer:
Potassium, sodium, lithium, calcium, magnesium, zinc, iron, copper.
Question 4 · Spec 5.4.1.2
Describe lithium, sodium and potassium reacting with cold water.
Answer:
They produce hydrogen and alkaline metal hydroxide solutions; reactions become more vigorous from lithium to potassium.
Question 5 · Spec 5.4.1.2
What happens when calcium reacts with cold water?
Answer:
It fizzes, producing hydrogen and calcium hydroxide; the water can become cloudy.
Question 6 · Spec 5.4.1.2
Compare magnesium, zinc, iron and copper with dilute hydrochloric acid.
Answer:
Magnesium reacts fastest, then zinc, then iron; hydrogen and chlorides form. Copper does not react with dilute hydrochloric acid.
Question 7 · Spec 5.4.1.2
What determines how readily a metal reacts to form an ion?
Answer:
Its tendency to lose electrons and form a positive ion: a greater tendency means greater reactivity.
Question 8 · Spec 5.4.1.2
Why does zinc displace copper from copper sulfate solution?
Answer:
Zinc is more reactive, so it forms zinc ions while copper ions form copper metal.
Question 9 · Spec 5.4.1.2
A displaces B from a compound, but B cannot displace C. What can you conclude?
Answer:
A is more reactive than B; C is more reactive than B. The evidence does not establish the order of A and C.
Question 10 · Spec 5.4.1.3
Which metal oxides can usually be reduced using carbon?
Answer:
Oxides of metals less reactive than carbon. More reactive metals generally require electrolysis.
Question 11 · Spec 5.4.1.3
Why can gold occur naturally as the metal?
Answer:
It is very unreactive and does not readily form compounds.
Question 12 · Spec 5.4.1.4 · Higher only
Define oxidation and reduction in terms of electrons.
Answer:
Oxidation is loss of electrons; reduction is gain of electrons.
Question 13 · Spec 5.4.1.4 · Higher only
Write a half equation for magnesium forming Mg²⁺.
Answer:
Mg → Mg²⁺ + 2e-. Magnesium is oxidised.
Question 14 · Spec 5.4.1.4 · Higher only
Write the ionic equation for zinc displacing copper ions.
Answer:
Zn + Cu²⁺ → Zn²⁺ + Cu.
Question 15 · Spec 5.4.2.1
State the general equation for a metal reacting with dilute acid.
Answer:
Metal + acid → salt + hydrogen, provided the metal is reactive enough.
Question 16 · Spec 5.4.2.2
What are the products of acid reacting with a metal oxide or hydroxide?
Answer:
A salt and water: a neutralisation reaction.
Question 17 · Spec 5.4.2.2
What are the products when an acid reacts with a carbonate?
Answer:
A salt, water and carbon dioxide.
Question 18 · Spec 5.4.2.2
What is a base?
Answer:
A substance that neutralises an acid, including metal oxides and metal hydroxides.
Question 19 · Spec 5.4.2.4
What is an alkali?
Answer:
A base that dissolves in water and produces hydroxide ions, OH⁻.
Question 20 · Spec 5.4.2.4
What does pH show about an aqueous solution?
Answer:
Below 7 is acidic, 7 is neutral and above 7 is alkaline.
Question 21 · Spec 5.4.2.4 · Higher only
Write the ionic equation for neutralisation.
Answer:
H⁺ + OH⁻ → H₂O.
Question 22 · Spec 5.4.2.2
Which salts form from hydrochloric, sulfuric and nitric acids?
Answer:
Hydrochloric acid forms chlorides; sulfuric acid forms sulfates; nitric acid forms nitrates.
Question 23 · Spec 5.4.2.3 · Required practical
How can copper sulfate crystals be made from copper oxide and dilute sulfuric acid?
Answer:
Warm the acid, add copper oxide in excess, filter off remaining solid, concentrate the filtrate and cool to crystallise.
Question 24 · Spec 5.4.2.3 · Required practical
Why add excess copper oxide when making copper sulfate?
Answer:
To ensure all the acid reacts. Unreacted insoluble copper oxide can then be removed.
Question 25 · Spec 5.4.2.3 · Required practical
Why filter the reaction mixture before crystallisation?
Answer:
To remove excess insoluble copper oxide, leaving copper sulfate solution as the filtrate.
Question 26 · Spec 5.4.2.3 · Required practical
Why avoid evaporating the salt solution completely to dryness?
Answer:
Concentrating then cooling allows crystals to form; heating to dryness can cause spitting or damage some hydrated salts.
Question 27 · Spec 5.4.2.3 · Required practical
What precaution is needed when heating acid for salt preparation?
Answer:
Use eye protection and gentle controlled heating to reduce splashing; allow hot apparatus to cool before handling.
Question 28 · Spec 5.4.2.5 · Higher only
How do strong and weak acids differ?
Answer:
Strong acids ionise completely in water; weak acids ionise only partially.
Question 29 · Spec 5.4.2.5 · Higher only
How do concentrated and dilute differ from strong and weak?
Answer:
Concentration describes the amount of acid per volume; strength describes its degree of ionisation.
Question 30 · Spec 5.4.2.5 · Higher only
How does hydrogen ion concentration change from pH 4 to pH 2?
Answer:
It increases by 10 × 10 = 100 times.
Question 31 · Spec 5.4.2.5 · Higher only
At the same concentration, why does a strong acid generally have a lower pH than a weak acid?
Answer:
More acid particles ionise, producing a higher hydrogen ion concentration.
Question 32 · Spec 5.4.3.1
What is an electrolyte?
Answer:
A molten or dissolved ionic substance with mobile ions that can carry electrical charge.
Question 33 · Spec 5.4.3.1
Which ions move to the cathode in electrolysis?
Answer:
Positive ions move to the negative cathode.
Question 34 · Spec 5.4.3.1
Which ions move to the anode in electrolysis?
Answer:
Negative ions move to the positive anode.
Question 35 · Spec 5.4.3.2
Why must an ionic solid be melted before electrolysis if it is not dissolved?
Answer:
Its ions must be free to move; in the solid they are fixed in a lattice.
Question 36 · Spec 5.4.3.2
What are the products of electrolysis of molten lead bromide?
Answer:
Lead forms at the cathode; bromine forms at the anode.
Question 37 · Spec 5.4.3.3
Why is aluminium extracted using electrolysis rather than carbon reduction?
Answer:
Aluminium is more reactive than carbon, so carbon cannot extract it from aluminium oxide.
Question 38 · Spec 5.4.3.3
Why is aluminium oxide dissolved in molten cryolite?
Answer:
It lowers the operating temperature compared with melting aluminium oxide alone, reducing energy demand.
Question 39 · Spec 5.4.3.3
Why must carbon anodes be replaced during aluminium extraction?
Answer:
Oxygen produced at the anodes reacts with the hot carbon, forming carbon dioxide and wearing them away.
Question 40 · Spec 5.4.3.4
What is the GCSE rule for the cathode product from an aqueous metal salt?
Answer:
A metal less reactive than hydrogen is deposited; if the metal is more reactive, hydrogen is produced.
Question 41 · Spec 5.4.3.4
What is the GCSE rule for the anode product with inert electrodes?
Answer:
A halogen forms when halide ions are present; otherwise oxygen is normally produced.
Question 42 · Spec 5.4.3.4
Predict the products of aqueous sodium chloride electrolysis with inert electrodes.
Answer:
Hydrogen at the cathode and chlorine at the anode, using the specification's product rules.
Question 43 · Spec 5.4.3.4
Predict the products of aqueous copper sulfate electrolysis with inert electrodes.
Answer:
Copper at the cathode and oxygen at the anode.
Question 44 · Spec 5.4.3.4 · Required practical
Suggest a hypothesis for comparing aqueous copper chloride and sodium chloride electrolysis.
Answer:
Both produce chlorine at the anode, but copper chloride produces copper at the cathode while sodium chloride produces hydrogen.
Question 45 · Spec 5.4.3.4 · Required practical
What should be kept comparable when comparing electrolysis solutions?
Answer:
Electrode material and area, solution volume and concentration, applied potential difference and electrolysis time.
Question 46 · Spec 5.4.3.4 · Required practical
How can hydrogen and oxygen electrode gases be identified?
Answer:
Hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint.
Question 47 · Spec 5.4.3.5 · Higher only
Write the cathode half equation for copper deposition.
Answer:
Cu²⁺ + 2e- → Cu. This is reduction.
Question 48 · Spec 5.4.3.5 · Higher only
Write the anode half equation for chloride ions.
Answer:
2Cl⁻ → Cl₂ + 2e-. This is oxidation.
Question 49 · Spec 5.4.3.5 · Higher only
Write the anode half equation for oxygen from hydroxide ions.
Answer:
4OH⁻ → O₂ + 2H₂O + 4e-.
Question 50 · Spec 5.4.3.1
Why is electrolysis described as decomposition?
Answer:
Electrical energy causes an ionic substance to break down into simpler substances at the electrodes.

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