AQA GCSE Combined Science: Trilogy · 5.11

Key ideas - chemistry

50 revision questions with answers for 5.11 Key ideas - chemistry 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 · Links 5.1.1.2
Why can a mixture usually be separated without changing its particles chemically?
Answer:
Its components are not chemically bonded together, so physical differences can separate them without making new substances.
Question 2 · Links 5.2.1.3
Why is separating sodium chloride into its elements different from separating salty water?
Answer:
Sodium chloride's ions form a compound requiring chemical decomposition; water can be physically separated from dissolved salt.
Question 3 · Links 5.1.1.2
How does knowing whether a solid dissolves guide a separation method?
Answer:
An insoluble solid can be filtered; a dissolved solid requires methods such as crystallisation or solvent removal.
Question 4 · Links 5.2.2.1
Why is boiling water a physical change rather than a chemical reaction?
Answer:
Water molecules separate further but remain H₂O; no new substance forms.
Question 5 · Links 5.3.1.1
How does an atom model explain conservation of mass in reactions?
Answer:
Atoms rearrange into new combinations but are not created or destroyed, so their total mass is conserved.
Question 6 · Links 5.1.2.1
Why does periodic-table position help predict reactions?
Answer:
Group position reflects outer-shell electrons, which influence ion formation and bonding behaviour.
Question 7 · Links 5.1.2.5
Why do Group 1 and Group 7 show opposite reactivity trends down the groups?
Answer:
Greater distance and shielding make electron loss easier in Group 1 but electron gain harder in Group 7.
Question 8 · Links 5.1.1.6
Why do two isotopes of the same element have similar chemical behaviour?
Answer:
They have the same electron arrangement, despite differing neutron numbers and masses.
Question 9 · Links 5.1.1.4
An atom loses an electron. Why does its identity stay the same while its charge changes?
Answer:
Its proton number remains unchanged, so it is still the same element, but it now has more protons than electrons.
Question 10 · Links 5.2.1.2
Why must an ionic compound's formula give zero total charge?
Answer:
Positive and negative ion charges balance in the compound; the ion ratio must reflect that balance.
Question 11 · Links 5.2.2.3
How does melting explain why an ionic compound starts to conduct?
Answer:
Melting frees ions from fixed lattice positions, allowing them to carry charge through the liquid.
Question 12 · Links 5.2.2.7
Why can an alloy be harder while still conducting electricity?
Answer:
Different-sized atoms hinder layer sliding, but delocalised electrons remain able to move and carry charge.
Question 13 · Links 5.2.2.4
Why can a substance contain strong covalent bonds but have a low boiling point?
Answer:
Boiling a small molecular substance overcomes weak intermolecular forces, rather than breaking its internal covalent bonds.
Question 14 · Links 5.2.2.5
Why can joining small units into long polymer molecules change a material's properties?
Answer:
Larger molecules have stronger intermolecular attractions, often giving higher softening or melting temperatures and different mechanical properties.
Question 15 · Links 5.2.2.6
What links diamond's hardness to its high melting point?
Answer:
Strong covalent bonds extend throughout a rigid giant structure; deformation and separation require overcoming that network.
Question 16 · Links 5.2.3.2
Why do graphite and diamond have different properties despite both being carbon?
Answer:
Their carbon atoms are bonded and arranged differently, giving different layer mobility and availability of delocalised electrons.
Question 17 · Links 5.7.1.2
Why does fractional distillation not break hydrocarbons into smaller molecules?
Answer:
It changes physical state and separates by boiling point; the molecules' covalent bonds remain intact.
Question 18 · Links 5.2.1.4
How are chemical bonding and a chemical reaction connected?
Answer:
A reaction rearranges atoms, breaking existing bonds and forming new ones to produce different substances.
Question 19 · Links 5.3.1.1
Why is balancing an equation about atoms rather than matching molecule numbers?
Answer:
Reactions conserve each type of atom, but the number of molecules can change as atoms regroup.
Question 20 · Links 5.4.1.4 · Higher only
How can one reaction involve both oxidation and reduction?
Answer:
Electrons lost by one species are gained by another, so electron loss and gain occur together.
Question 21 · Links 5.4.1.4 · Higher only
Explain zinc-copper displacement using electron transfer.
Answer:
Zinc atoms lose electrons to form Zn²⁺; Cu²⁺ ions gain electrons to form copper atoms.
Question 22 · Links 5.4.3.5 · Higher only
Why is metal deposition at a cathode reduction?
Answer:
Positive metal ions gain electrons to become neutral metal atoms.
Question 23 · Links 5.4.2.5 · Higher only
Why can a concentrated weak acid be different from a dilute strong acid?
Answer:
Concentration and degree of ionisation are independent descriptions; neither term alone fully specifies hydrogen ion concentration.
Question 24 · Links 5.4.2.4
What links an acid-alkali reaction to water formation?
Answer:
Hydrogen ions from the acid react with hydroxide ions from the alkali to form water.
Question 25 · Links 5.8.2.3
How can a gas test support an explanation of mass loss in an acid-carbonate reaction?
Answer:
Milky limewater identifies carbon dioxide, supporting the explanation that escaping gas caused the measured mass decrease.
Question 26 · Links 5.8.1.3
How do particle interactions explain chromatography separation?
Answer:
Components spend different proportions of time in the mobile and stationary phases, so they travel at different speeds.
Question 27 · Links 5.8.1.1
Why should purity be checked with evidence rather than a product label?
Answer:
Everyday 'pure' products can contain several substances; melting points or chromatography provide chemical evidence.
Question 28 · Links 5.8.1.2
Why can a useful formulation deliberately be impure in the chemical sense?
Answer:
It is designed as a mixture whose components combine to provide desired properties.
Question 29 · Links 5.6.1.2
Why can two reactions have different rates but equal final gas volumes?
Answer:
Different conditions change speed, while equal amounts of the limiting reactant can produce equal total gas.
Question 30 · Links 5.6.1.3
Why do many collisions fail to produce a reaction?
Answer:
The colliding particles may have insufficient energy to overcome the activation-energy barrier.
Question 31 · Links 5.6.1.4
Why does a catalyst change speed without supplying extra reactant?
Answer:
It lowers activation energy by an alternative pathway and is regenerated rather than consumed.
Question 32 · Links 5.6.1.3
Why is temperature's effect on rate more than just increased collision frequency?
Answer:
A larger proportion of particles also have energy at or above the activation energy.
Question 33 · Links 5.6.1.3
How do higher gas pressure and higher solution concentration have similar effects on rate?
Answer:
Both increase the number of reacting particles per unit volume, increasing collision frequency.
Question 34 · Links 5.6.1.3
Why can crushing a solid change reaction rate without changing the total available product?
Answer:
It exposes more surface for collisions but does not change the amount of reactant present.
Question 35 · Links 5.5.1.3 · Higher only
Why does an exothermic reaction not mean every individual step releases energy?
Answer:
Breaking reactant bonds needs energy, but forming product bonds releases a larger amount overall.
Question 36 · Links 5.5.1.3 · Higher only
What bond-energy comparison explains a cooling reaction mixture?
Answer:
More energy is needed to break reactant bonds than is released forming product bonds, so energy is taken from the surroundings.
Question 37 · Links 5.5.1.1
Why must the system boundary be clear when describing energy transfer?
Answer:
In an exothermic reaction the reacting system loses energy while its surroundings gain it; both descriptions concern the same transfer.
Question 38 · Links 5.5.1.1
Why does lower product energy not contradict conservation of energy?
Answer:
The difference has been transferred to the surroundings; total energy remains conserved.
Question 39 · Links 5.6.2.2
Why are forward and reverse reaction energy changes opposite?
Answer:
Reversing the reaction reverses the changes between the same reactant and product energy levels.
Question 40 · Links 5.6.2.6 · Higher only
Why must 'faster' and 'more product at equilibrium' be assessed separately?
Answer:
Rate concerns time; equilibrium composition concerns relative amounts. A condition can improve one while reducing the other.
Question 41 · Links 5.9.1.4
How are fossil fuels linked to earlier removal of atmospheric carbon dioxide?
Answer:
Photosynthesis stored carbon in biomass; burial formed fossil fuels, keeping some carbon out of the atmosphere for long periods.
Question 42 · Links 5.9.3.1
Why can the same hydrocarbon produce different carbon-containing combustion products?
Answer:
Oxygen supply affects completeness: sufficient oxygen favours carbon dioxide, while limited oxygen can produce carbon monoxide or soot.
Question 43 · Links 5.7.1.3
How does molecular size help determine a hydrocarbon fraction's use?
Answer:
Size influences boiling point, viscosity and flammability, making different fractions suitable for different fuel or material uses.
Question 44 · Links 5.7.1.4
How does a cracking equation demonstrate conservation of atoms?
Answer:
The carbon and hydrogen atom totals are identical before and after one large molecule splits into smaller molecules.
Question 45 · Links 5.10.1.1
Why does using oil as chemical feedstock still consume a finite resource?
Answer:
Oil is removed from reserves whether burned or converted into useful materials, so demand still depletes the resource.
Question 46 · Links 5.10.1.2
Why can filtering water improve clarity without making it potable?
Answer:
Filtration removes particles but may leave harmful microbes and dissolved chemicals that require further treatment.
Question 47 · Links 5.4.3.3
Why must the environmental assessment of aluminium include electricity generation?
Answer:
Electrolysis requires substantial electrical energy, whose production may consume resources and cause emissions.
Question 48 · Links 5.10.2.1
Why can recycling look better for one impact but worse for another?
Answer:
An option may save raw materials while requiring transport or processing energy; a full life-cycle comparison considers several impacts.
Question 49 · Links 5.10.1.4 · Higher only
How do phytomining and recycling address limited ore resources differently?
Answer:
Phytomining recovers metal from low-grade sources using plants; recycling recovers metal already present in used products.
Question 50 · Links 5.9.2.2
Why should a chemical environmental claim include evidence and uncertainty?
Answer:
Measurements, assumptions and incomplete information affect conclusions; transparent evidence allows others to assess whether the claim is justified.

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