Edexcel A-Level Chemistry Paper 3, November 2020: Question 2

10 marks · Medium difficulty · Short Open Response

Describe and explain flame tests for Group 1 and Group 2 metal ions, including colours, experimental procedures, and electron transitions.

Practise this question

Question

An exam question about flame tests containing multiple parts (a) to (c). Part (a) asks for flame colours of potassium and strontium ions, and why a mixture is hard to distinguish. Part (b) explores the use of nichrome wire, cleaning with acid, and the role of hydrochloric acid. Part (c) asks to explain the origin of flame colours with reference to electron transitions.
Question text

2 This question is about flame tests for Group 1 and 2 metal ions.

(a) (i) State the colour of the flame produced by separate samples of potassium and

strontium ions.

(1)

Potassium ions …

Strontium ions …

(ii) Give a reason why carrying out a flame test on a mixture of potassium chloride

and strontium chloride does not clearly show that two different metal ions

are present.

(1)

(b) In the first stage of the flame test, the nichrome wire is dipped into

concentrated hydrochloric acid and then heated in a Bunsen flame.

In the second stage, this nichrome wire is dipped into fresh hydrochloric acid and

then into the metal salt to be tested before being reheated in the Bunsen flame.

(i) Give two reasons why the wire is made of nichrome and not iron.

(2)

(ii) Give a reason why the wire is dipped into acid and then heated in the first stage.

(1)

(iii) State why fresh concentrated hydrochloric acid is used in the second stage of

the flame test.

(1)

… 4

… *P62670A0432*

(iv) State why hydrochloric acid is used in the second stage of the flame test.

(1)

(c) Explain, with reference to electron transitions, the formation of the colour in a

flame test.

(3)

(Total for Question 2 = 10 marks)

Mark scheme

Show the mark scheme The mark scheme providing the acceptable answers for question 2. It details lilac for potassium and crimson/red for strontium, masking effects, why nichrome is inert and colourless in a flame, cleaning procedures using fresh acid to remove impurities, forming volatile chlorides, and the three-mark explanation involving electron excitation, falling back to ground state, and release of energy as visible light.

How to answer it

Flame Tests for Group 1 and 2 Metal Ions

What this question tests

This question assesses your practical knowledge of qualitative analysis techniques (specifically flame tests), the specific flame colours of metal cations, the reasons behind experimental procedures (such as cleaning wires and using hydrochloric acid), and the underlying quantum theory of electron transitions responsible for emitting visible light.

Question 2 (a)

Flame Colours & Mixtures

✅ Correct Answers

  • Potassium ions: Lilac (allow pale purple)
  • Strontium ions: Crimson / red (allow scarlet)
  • Mixtures: The crimson/red colour masks, hides, or obscures the lighter lilac colour.

❌ Common Errors

  • Describing potassium as "violet" or strontium as "brick red" or "orange-red" (too imprecise for Edexcel mark schemes).
  • Stating that a "new compound" or chemical reaction occurs between the two salts in a mixture.
Marks: 1 + 1 = 2 marks total
Question 2 (b)

Practical Methodology & Technique

💡 Key Knowledge (Nichrome vs. Iron)

For part (i), give two distinct reasons:

  • Nichrome produces no colour in a Bunsen flame (whereas iron can spark or produce interference colours).
  • Nichrome is inert / stable to heat / unreactive (whereas iron reacts with oxygen/air or hydrochloric acid).

🧠 Exam Technique (Step-by-Step Practical Logic)

  • 2(b)(ii) Cleaning: The wire is dipped in acid and heated to remove residue of any previous sample.
  • 2(b)(iii) Fresh Acid: Fresh acid prevents contamination from residue left in used acid from previous tests.
  • 2(b)(iv) Hydrochloric Acid: Concentrated HCl reacts with metal compounds to form volatile chlorides, which vaporise easily in the flame.

❌ Common Errors in Practical Explanations

  • Writing that the wire is being "sterilised", "sanitised", or "disinfected" (microbiological terms do not earn chemical credit).
  • Stating that hydrochloric acid "absorbs" or "bonds" to the wire.
Marks: 2 + 1 + 1 + 1 = 5 marks total
Question 2 (c)

Explanation of Colour Formation (Electron Transitions)

✅ Marking Points (3 Marks)

  1. Electrons are excited / promoted by heat energy to higher energy levels (orbitals).
  2. Electrons fall from the excited state back down to their ground state (or lower energy levels).
  3. Energy is released/emitted as photons of (visible) light corresponding to a specific frequency/wavelength.

❌ Examiner Warning

Crucial Rule: If you omit the word "electrons" entirely from your answer, you are strictly capped at a maximum of 2 out of 3 marks, even if all other descriptions of energy states are correct!

Marks: 3 marks total

Topics

Inorganic Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 7: Identify unknown organic liquids and inorganic solids

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.