Edexcel A-Level Chemistry AS Paper 1, November 2021: Question 7
12 marks · Hard difficulty · Synoptic Questions
State the meaning of an ionic bond, calculate the percentage by mass of strontium in strontium nitrate, analyze flame test chemistry for group 1 and 2 compounds, and devise a procedure to identify lithium, rubidium, and strontium nitrates.
Practise this questionQuestion
Question text
7 The nitrates of lithium, rubidium and strontium are all white solids. The compounds
are held together by ionic bonds.
(a) State the meaning of the term ‘ionic bond’.
(2)
(b) What is the percentage by mass of strontium in strontium nitrate?
(1)
A 38.0%
B 41.4%
C 58.6%
D 74.5%
(c) These three compounds cannot be identified with certainty from a flame test as
the colours seen are similar.
Concentrated hydrochloric acid is used in a flame test procedure.
(i) Which of the following is a reason for dipping the flame test wire in
concentrated hydrochloric acid during a flame test procedure?
(1)
A it dissolves metal ions from the wire
B it neutralises hydroxide ions that might colour the flame
C it reduces the metal ions to metal atoms
D it reacts with the compounds to form volatile chlorides
(ii) The flame colour given by these three solids in the flame test are shades of
(1)
A green
B lilac
C red
D yellow
(iii) What is the best explanation for why metal ions produce different
flame colours?
20 (1)
Different wavelengths of light energy are*P67083A02024*
A required to promote electrons to higher energy levels
B released because electrons move from lower to higher energy levels
C released due to different gaps between energy levels
D required for electron transfer from non-metal ions to metal ions
*(d) Devise a procedure to identify the nitrates of lithium, rubidium and strontium using the
effect of heat on the three solids and any precipitation reactions of the compounds.
Practical details are not required, but you should give the observations expected in
each case.
(6)
… *P67083A02124*
… 22
… *P67083A02224*
(Total for Question 7 = 12 marks)
Mark scheme
Show the mark scheme
Question Answer Additional Guidance Mark
Number
7(a) An answer that makes reference to the following points: (2)
• (strong) electrostatic attraction
• between oppositely charged ions
Question Answer
Mark
Number
7(b) The only correct answer is B (41.4%) (1)
A is not correct because this uses the atomic number in the calculation instead of the relative atomic mass
C is not correct because this assumes the formula of strontium nitrate is SrNO3
D is not correct because this assumes the formula of strontium nitrate is SrNO
Mark
Number
7(c)(i) The only correct answer is D (it reacts with the compounds to form volatile chlorides) (1)
A is not correct because hydrochloric acid doesn’t dissolve the metal
B is not correct because there are no hydroxide ions present and they don’t colour the flame
C is not correct because hydrochloric acid doesn’t reduce the metal ions
Mark
Number
7(c)(ii) The only correct answer is C (red) (1)
A is not correct because ions giving green shades include barium ions
B is not correct because ions giving lilac shades include potassium ions
D is not correct because ions giving yellow shades include sodium ions
Mark
Number
7(c)(iii) The only correct answer is C (released due to different gaps between energy levels) (1)
A is not correct because promoting electrons in a flame test uses heat energy
B is not correct because promoting electrons does not release light
D is not correct because electrons are promoted within the metal atoms energy levels not transferred to non-metal ions
Question
Acceptable Answer Additional Guidance Mark
Number
*7(d) This question assesses a student’s ability to show a Guidance on how the mark scheme should be (6)
coherent and logically structured answer with linkages and applied:
fully-sustained reasoning.
The mark for indicative content should be
Marks are awarded for indicative content and for how the added to the mark for lines of reasoning.
answer is structured and shows lines of reasoning.
For example, an answer with five indicative
The following table shows how the marks should be marking points, which is partially structured
awarded for indicative content. with some linkages and lines of reasoning,
scores 4 marks (3 marks for indicative
Number of indicative Number of marks awarded content and 1 mark for partial structure and
marking points seen in for indicative marking some linkages and lines of reasoning).
answer points
64 If there are no linkages between points, the
5–4 3 same five indicative marking points would
3–2 2 yield an overall score of 3 marks (3 marks for
11 indicative content and no marks for linkages).
Question
Acceptable Answer Additional Guidance Mark
Number
*7(c)(ii) The following table shows how the marks should be
contd awarded for structure and lines of reasoning.
Number of marks awarded
In general it would be expected
for structure of answer and
that 5 or 6 indicative points would get
sustained line of reasoning
2 reasoning marks, and 3 or 4
Answer shows a coherent and logical 2
indicative points would get 1 mark
structure with linkages and fully
for reasoning, and 0, 1 or 2
sustained lines of reasoning
indicative points would score zero
demonstrated throughout.
marks for reasoning.
Answer is partially structured with 1
some linkages and lines of reasoning.
Reasoning marks may be reduced for
Answer has no linkages between 0
extra incorrect chemistry
points and is unstructured.
The identification should be a logical
deduction of unknowns.
If the candidate assumes they know
which is which of the solids and prove
it deduct one reasoning mark
Candidates may start with either
heating or solutions (IP1/2 or IP3)
Indicative content:
Allow RbNO3 only gives off oxygen
which relights a glowing splint
• IP1 no brown gas on heating with RbNO3
Allow equations giving NO2
• IP2 brown gas on heating for LiNO3 and/or Sr(NO3)2
• IP3 dissolve / make a solution in deionised / distilled water Allow even if added to solids
• IP4 Add a solution of NaOH / Na2SO4 / H2SO4 / Na2CO3 Allow even if test is done on solids
• IP5 White precipitate for Sr(NO3)2 Allow even if test is done on solids
• IP6 No precipitate for LiNO3 and/or RbNO3
Logical deductions may not require
description of the results for both
substances in IP2 or IP6 but they
should mention one at least
(Total for Question 7 = 12 marks)
How to answer it
Properties and Identification of Group 1 and Group 2 Nitrates
What this question tests
This question assesses core Inorganic and Physical Chemistry concepts from AS Level, specifically: definitions of ionic bonding, percentage composition calculations, flame test practical procedures and theory (electron transitions and volatile chlorides), thermal stability trends of Group 1 and Group 2 nitrates, and analytical chemical testing strategies combining thermal decomposition and precipitation reactions.
Definition of Ionic Bonding
✅ Correct Answer
Strong electrostatic attraction between oppositely charged ions.
💡 Key Knowledge
Both key phrases ( electrostatic attraction and oppositely charged ions ) are essential mnemonic pillars for definition questions in chemistry.
❌ Common Errors
Students often lose marks by omitting the word "electrostatic" or vaguely referring to "attraction between atoms/molecules" instead of ions.
Percentage by Mass Calculation
✅ Correct Answer
B (41.4%)
📐 Step-by-Step Calculation
- Formula for strontium nitrate: Sr(NO₃)₂
- Molar mass of Sr = 87.6 g mol⁻¹
- Molar mass of N = 14.0 × 2 = 28.0 g mol⁻¹
- Molar mass of O = 16.0 × 6 = 96.0 g mol⁻¹
- Total molar mass = 87.6 + 28.0 + 96.0 = 211.6 g mol⁻¹
- Percentage = (87.6 / 211.6) × 100 = 41.4%
❌ Common Errors
Distractor A uses the atomic number instead of relative atomic mass. Distractors C and D assume incorrect chemical formulas for strontium nitrate ( SrNO₃ or SrNO ).
Flame Test Procedure: Hydrochloric Acid
✅ Correct Answer
D — it reacts with the compounds to form volatile chlorides.
💡 Key Knowledge
Volatile chlorides vaporize readily in the non-luminous Bunsen burner flame, ensuring distinct and visible characteristic flame colors are produced.
Flame Colours
✅ Correct Answer
C — red
💡 Key Knowledge
Lithium ions give a crimson/red flame. (Note: Barium gives green, potassium gives lilac, and sodium gives yellow).
Origin of Flame Colours
✅ Correct Answer
C — released due to different gaps between energy levels
🧠 Exam Technique
Energy is absorbed to promote electrons to higher energy levels, but light energy is released when electrons fall back down. The specific frequency/wavelength of light depends directly on the size of the energy level gap ( ΔE = hf ).
Analytical Strategy: Identifying Nitrates
💡 Key Knowledge & Indicative Content
- IP1: RbNO₃ (Group 1) decomposes on heating to give oxygen gas only (relights a glowing splint), with no brown gas produced.
- IP2: LiNO₃ and Sr(NO₃)₂ decompose on heating to release brown gas ( NO₂ ).
- IP3 & IP4: Dissolve the remaining unknown solids in water to form aqueous solutions, then add a sulfate source (e.g., Na₂SO₄ or H₂SO₄ ) or sodium hydroxide ( NaOH ).
- IP5: Sr²⁺ forms a white precipitate with sulfates ( SrSO₄ ).
- IP6: Li⁺ produces no precipitate under these conditions.
🧠 Exam Technique & Marking Structure
This is a levels-marked QWC (Quality of Written Communication) question worth 6 marks total:
- Indicative Content (up to 4 marks): Awarded based on how many of the key chemical observations (IP1 to IP6) are included.
- Structure & Reasoning (up to 2 marks): Awarded for a logical, step-by-step deductive pathway without assuming identities prematurely.
Topics
Physical Chemistry · Inorganic Chemistry · Organic Chemistry · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 1: Atomic Structure and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.