AQA GCSE Chemistry Chemistry Paper 1 (Higher), June 2025: Question 4

✅ Correct Answer

Any value in the range: -70 °C to -20 °C

Examiner actual value: -34 °C

🧠 Exam Technique

• Check the key: Boiling points are represented by white bars.
• Look at the trend: Boiling point increases down the group from fluorine (-188 °C) to bromine (+59 °C).
• Chlorine's boiling point must be between fluorine's and bromine's, and comfortably above chlorine's own melting point (-101 °C).

❌ Common Errors

• Reading the shaded bar (melting point: -101 °C) instead of boiling point.
• Forgetting the minus sign (writing 34 °C instead of -34 °C ).
• Predicting a value lower than its melting point (a substance cannot boil at a lower temperature than it melts).

✅ Correct Answer

Solid (or allow (s) )

💡 Key Knowledge: State Rules

• Temperature < Melting Point → Solid
• Temperature between MP and BP → Liquid
• Temperature > Boiling Point → Gas

Fluorine melts at approx. -220 °C . Because -250 °C is lower (colder) than its melting point, it must be a solid.

❌ Common Errors

• Confusing negative numbers: thinking -250 is higher than -220.
• Writing "gas" purely from recall that fluorine is a gas at room temperature, without checking the temperature given in the question.

✅ Correct Answers

04.3: 7

04.4: HAt

💡 Periodic Table Patterns

• Group number = number of outer shell electrons. Astatine is in Group 7, so it has 7 outer electrons.
• Because all Group 7 halogens need 1 electron to complete their octet, they form 1 single covalent bond with hydrogen:
  HF, HCl, HBr, HI → therefore HAt.

❌ Common Errors

• Writing total electrons instead of outer shell electrons.
• Incorrect capitalisation or stoichiometry: e.g. H₂At , Hat , or HAt₂ .

✅ Model 4-Mark Response

Point 1: Going down Group 7, reactivity decreases.

Point 2: The atoms get larger / have more electron shells, so the outer shell is further from the nucleus.

Point 3: There is weaker electrostatic attraction between the nucleus and the incoming outer electron (due to greater distance and more shielding).

Point 4: Therefore, it is harder / more difficult to gain an electron.

🧠 How to Structure 4-Mark Periodic Explanations

1. Trend statement: State clearly whether reactivity increases or decreases.
2. Atomic size: Mention atomic radius / number of shells / distance from nucleus.
3. Nuclear attraction: Describe attraction to the incoming electron (mention shielding).
4. Electron transfer: Group 7 elements react by gaining an electron, so state how easily this happens.

❌ Common Errors & Misconceptions

• Confusing Group 1 and Group 7: Stating that halogens "lose an electron" to react. Group 7 elements gain an electron!
• Vague statements like "it has less pull" without specifying what is pulling what (nucleus pulls outer electron).
• Saying "reactivity increases down the group" (which is true for Group 1 metals, but the opposite for Group 7).

✅ Balanced Chemical Equation

Br₂ + 2KI → I₂ + 2KBr

📐 Step-by-Step Construction

1. Identify reactants:
   Bromine is a diatomic element → Br₂
   Potassium iodide consists of K⁺ and I⁻ → KI
2. Identify products:
   Bromine is more reactive than iodine, displacing it.
   Displaced iodine forms diatomic molecules → I₂
   Potassium bromide salt forms → KBr
3. Balance the atoms:
   2 bromine atoms on left require 2KBr on right.
   2 iodine atoms on right require 2KI on left.

❌ Common Calculation & Formula Traps

• Writing halogens as monatomic: Br instead of Br₂ , or I instead of I₂ .
• Inventing incorrect salt formulae to force balancing without coefficients: e.g. writing KBr₂ or K₂I . K is always +1, halides are always -1!
• Incorrect casing: ki or kbr will lose marks.