Edexcel A-Level Chemistry AS Paper 1, June 2017: Question 1

6 marks · Medium difficulty · Short Open Response

Identify metal and anion ions using flame tests, thermal decomposition, and chemical reactions with acid and limewater.

Practise this question

Question

Question 1 asks about the identification of ions using a Bunsen burner flame. Part (a)(i) asks to describe how to carry out a flame test on an unknown solid (2 marks). Part (a)(ii) is a multiple-choice question asking which ion does not give a red flame among barium, calcium, lithium, and strontium (1 mark). Part (a)(iii) asks to identify an anion that gives off a brown gas and relights a glowing splint upon heating, with options Br-, NO2-, NO3-, and O2- (1 mark). Part (b) states a flame test on a white powder gave a lilac flame, and reaction with dilute hydrochloric acid produced a gas turning limewater cloudy, and asks for a possible formula for the white powder (2 marks).
Question text

1 The presence of some ions in compounds can be identified using a Bunsen burner flame.

(a) (i) Some metal ions give characteristic colours in a flame test.

Describe how to carry out a flame test on an unknown solid.

(2)

(ii) Which of the following ions does not give a red flame?

(1)

A barium

B calcium

C lithium

D strontium

(iii) Some anions can also be identified by heating in a Bunsen burner flame.

A compound heated in a test tube in a Bunsen burner flame gave off a

brown gas and caused a glowing splint to relight. The formula of the ion

responsible is

(1)

A Br–

B NO–

C NO–

D O2–

(b) A flame test on a white powder gave a lilac flame colour. Dilute hydrochloric acid

was added to a second sample of the same powder in a boiling tube and the gas

produced bubbled into limewater. The limewater turned cloudy.

Give a possible formula for the white powder.

(2)

*P49858A0224*

(Total for Question 1 = 6 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 1 providing acceptable answers and additional guidance for parts (a)(i) through (b). Part (a)(i) requires mentioning the use of a nichrome/platinum wire/ceramic/silica rod, conc/dilute HCl, and placing the sample in a blue Bunsen flame. Part (a)(ii) lists answer A (barium). Part (a)(iii) lists answer C (NO3-). Part (b) accepts K2CO3 or KHCO3, awarding marks separately for a correct potassium-containing part and a correct carbonate/hydrogencarbonate part.

Question

Acceptable Answer Additional Guidance Mark

Number

1(a)(i) A description that makes reference to the following points: Allow splint, spray method for both (2)

marks

use of a nichrome / platinum wire / ceramic / silica rod Reject just ‘nichrome’,

(1) nickel/chromium, inoculation loop,

spatula, capillary tubing

use of (conc.) HCl/HCl(aq)/dilute HCl Reject other acids, just ‘acid’

AND

dip into the sample and place in / over a (blue) Bunsen Assume blue/roaring flame if not stated

burner flame (1) but reject use of yellow/safety flame

Question

Answer Mark

Number

1(a)(ii) 1(a)(ii). The only correct answer is A (1)

B is not correct because this would give a red flame (brick red)

C is not correct because this would give a red flame (carmine red)

D is not correct because this would give a red flame (crimson red)

Question

Answer Mark

Number

1(a)(iii) 1(a)(iii). The only correct answer is C (1)

A is not correct because bromine is a brown gas, but bromide does not decompose to give it.

B is not correct because nitrate(III) not nitrate(V) and does not give NO2 by decomposing

D is not correct because O2- does not decompose in this way

Question

Acceptable Answer Additional Guidance Mark

Number

1(b) K2CO3 / KHCO3 For 1 mark allow names (2)

K+ with any anion (1) Award 1 mark for a correct formula

+ − 2−

containing K , HCO3 or CO3 , eg KCl,

CO 2- / HCO - with any cation (1) or Na CO

33 2 3

Award 1 mark for an incorrect formula

containing both potassium and

carbonate/hydrogencarbonate e.g.

KCO3

Do not award any marks for KCO2

Ignore equations even if incorrect, but

award marks for the compound as a

reactant.

(Total for Question 1 = 6 marks)

How to answer it

Identification of Ions and Qualitative Analysis

What this question tests

This question assesses your knowledge of qualitative inorganic analysis, specifically standard laboratory procedures for flame tests, characteristic flame colours for Group 1 and 2 metal ions, thermal decomposition reactions of nitrates, and identification tests for gases (carbon dioxide and oxygen).

Part (a)(i)

Describing a flame test procedure

✅ Expected Answer

1. Use a nichrome, platinum, ceramic, or silica wire (or a soaked wooden splint).

2. Dip the wire into hydrochloric acid (conc. or dilute) and then into the sample solid, before placing it directly into a blue (roaring) Bunsen burner flame.

🧠 Exam Technique

Be extremely precise with apparatus naming. Examiners accept nichrome or platinum wire, but will reject vague terms like "just nichrome", "spatula", or "capillary tubing". Always specify a blue/roaring Bunsen flame.

❌ Common Errors

Students frequently lose marks by failing to mention dipping the wire in hydrochloric acid beforehand, or by suggesting a yellow/safety Bunsen flame where the characteristic colours cannot be clearly seen.

Part (a)(ii)

Identifying ions that do not give a red flame

✅ Correct Answer: A (barium)

Barium gives an apple-green flame, not a red flame.

💡 Key Knowledge

  • Calcium (B): Brick-red flame
  • Lithium (C): Carmine/crimson-red flame
  • Strontium (D): Crimson-red flame

Part (a)(iii)

Identifying an anion from thermal decomposition products

✅ Correct Answer: C (NO₃⁻)

Nitrate(V) ions decompose upon heating to produce brown nitrogen dioxide gas (NO₂) and oxygen gas (O₂), which relights a glowing splint.

❌ Common Misconceptions

Option B ( NO₂⁻ , nitrate(III)) does not decompose to give brown NO₂ gas in this manner. Option A ( Br⁻ ) produces bromine vapour (which is brown), but bromide ions do not cause a glowing splint to relight.

Part (b)

Deduining a white powder formula from analytical tests

✅ Correct Answers

K₂CO₃ or KHCO₃ (Potassium carbonate or potassium hydrogencarbonate)

💡 Mark Breakdown & Logic

  • Lilac flame: Confirms the presence of potassium ions ( K⁺ ) — [1 mark].
  • Acid + limewater turns cloudy: Confirms the gas is carbon dioxide ( CO₂ ), proving the presence of carbonate ( CO₃²⁻ ) or hydrogencarbonate ( HCO₃⁻ ) ions — [1 mark].

❌ Common Errors

Watch out for formula writing rules: charges must balance correctly. K₂CO₃ has a 2+ total positive charge balanced by a 2- carbonate ion. Writing incorrect stoichiometry like KCO₃ will lose the formula mark.

Topics

Inorganic Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.