AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2022: Question 4

✅ Correct answer

halogens

💡 Key knowledge

• Group 7 elements are called halogens.
• Examples include fluorine, chlorine, bromine, iodine and astatine.

🧠 Exam technique

• If a question asks “known as”, give the group name.
• Do not list the elements unless asked.

❌ Common errors

• Writing just one element, like chlorine.
• Confusing Group 7 with Group 0 or Group 1.

✅ Correct answer

They all have 7 electrons in their outer shell.

Accept: “same number of outer electrons” or “7 outer electrons”.

💡 Key knowledge

• Elements in the same group have the same number of outer-shell electrons.
• This is why they have similar chemical properties.
• For Group 7, they all need one more electron to get a full outer shell.

🧠 Exam technique

• Use the phrase outer shell or outer electrons.
• For 1 mark, one clear sentence is enough.

❌ Common errors

• Saying “they have the same number of shells” — this is not the reason.
• Saying “they are all non-metals” — true, but not the marking point here.

✅ Correct answer

Cl₂O₇

💡 Key knowledge

• Molecular formula shows the number of each atom in one molecule.
• In the diagram, there are 2 chlorine atoms and 7 oxygen atoms.

🧠 Exam technique

• Count each type of atom separately.
• Use subscripts correctly: Cl₂O₇ .

❌ Common errors

• Writing ClO₇ or Cl₂O₃ by miscounting.
• Using the picture labels without counting the full model.

✅ Correct answer

• Extend/complete the y-axis scale correctly from −100 to −250 °C.
• Draw the fluorine bar to −220 °C.

💡 Key knowledge

• Graph scales must be sensible and evenly spaced.
• Negative temperatures must be read carefully below 0.
• Fluorine has the lowest melting point in the set shown.

🧠 Exam technique

• First check the scale already shown.
• Use the grid to place −220 °C accurately.
• The mark scheme allows a small tolerance: about ±½ a small square.

❌ Common errors

• Forgetting to include negative values on the scale.
• Putting the bar at +220 °C instead of −220 °C.
• Drawing the bar at the wrong height because the scale is uneven.

✅ Correct answer

The molecules increase in size going down the group, so the forces between the molecules increase, so the melting points increase going down the group.

💡 Key knowledge

• Going down Group 7, atoms/molecules get larger.
• There are more electrons and more shells.
• This makes intermolecular forces stronger.
• More energy is needed to separate the molecules, so melting point increases.

🧠 Exam technique

• Use the chain: bigger particles → stronger forces → higher melting point.
• For 3 marks, you need all three linked ideas, not just one.
• Refer to the pattern in Figure 4: fluorine lowest, astatine highest.

❌ Common errors

• Saying “bonds get stronger” — the question is about intermolecular forces, not covalent bonds.
• Only stating “melting point increases” with no explanation.
• Mixing up atoms and molecules: the mark scheme accepts “molecules increase in size” or “atoms increase in size”.

✅ Correct answer

(s)

💡 Key knowledge

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous

🧠 Exam technique

• Read the melting point from the graph before deciding the state.
• If the temperature is below the melting point, the substance is solid.

❌ Common errors

• Choosing (l) because bromine is often known as a liquid at room temperature.
• Confusing the state at room temperature with the state at −50 °C.

✅ Correct answer

condensation

Accept: condensing

💡 Key knowledge

• At the boiling point, liquid particles are changing between liquid and gas at the surface.
• Condensation is the reverse of evaporation/boiling.

🧠 Exam technique

• Give the exact process name: condensation.
• One mark = one correct science word.

❌ Common errors

• Writing “evaporating” or “boiling” again.
• Giving a description instead of the process name.

💡 Must-know facts

• Group 7 elements are called halogens.
• They all have 7 outer electrons.
• Melting points increase down the group.
• Bromine at −50 °C is solid.
• At the surface of a liquid, condensation can also happen.

🧠 How to score full marks

• Use the exact group name: halogens.
• For explanations, link size → forces → melting point.
• On graphs, check negative values and scale carefully.
• For state symbols, compare the temperature with the melting point.

❌ Biggest traps in this question

• Confusing intermolecular forces with covalent bonds.
• Reading the wrong sign on a temperature.
• Forgetting that Group 7 elements have the same number of outer electrons.