AQA A-Level Chemistry Paper 3, 2017: Question 31

1 mark · Medium difficulty · Multiple Choice

Identify which aqueous transition metal ion is present in a solution that absorbs light with wavelengths corresponding to red, yellow, and green.

Practise this question

Question

Question 31 asks: 'A solution absorbs light with wavelengths corresponding to red, yellow and green light. Which ion is most likely to be in the solution?' followed by four multiple choice options: A Cr2O7 2-(aq), B Fe2+(aq), C Fe3+(aq), and D Cu2+(aq).
Question text

31 A solution absorbs light with wavelengths corresponding to red, yellow and green light.

Which ion is most likely to be in the solution?

[1 mark]

A Cr O 2−(aq)

B Fe2+(aq)

C Fe3+(aq)

D Cu2+(aq)

Mark scheme

Show the mark scheme Mark scheme table row showing question number 31 with the correct answer D.

31 D

How to answer it

Absorbed vs Transmitted Light & Transition Metal Ion Colours

📌 What this question tests

This question evaluates your understanding of the relationship between absorbed light frequencies and the observed colour of aqueous transition metal complexes, alongside recall of key aqueous ion colours specified in the AQA syllabus.

  • Understanding complementary colours: the observed colour is the colour of transmitted light, not absorbed light.
  • Recalling standard aqueous transition metal ion colours: Cr₂O₇²⁻(aq) , Fe²⁺(aq) , Fe³⁺(aq) , and Cu²⁺(aq) .
Question 31

Identifying an Aqueous Ion from Absorption Wavelengths

Multiple Choice — 1 Mark

✅ Correct Answer: D

D: Cu²⁺(aq)

1 Mark awarded for: Selecting option D.

A solution containing Cu²⁺(aq) (specifically [Cu(H₂O)₆]²⁺ ) is blue. Because it absorbs light in the red, yellow, and green regions of the visible spectrum, only the blue wavelengths are transmitted to the observer's eye.

💡 Key Knowledge

1. Transmitted vs. Absorbed Light:

  • White light = Red + Orange + Yellow + Green + Blue + Indigo + Violet.
  • When red, yellow, and green light are absorbed by d-d electron transitions, the remaining transmitted frequencies combine to produce blue light.

2. Standard AQA Aqueous Colours:

  • Cr₂O₇²⁻(aq) : Orange
  • Fe²⁺(aq) : Green (pale green)
  • Fe³⁺(aq) : Yellow / Brown (amber)
  • Cu²⁺(aq) : Blue

🧠 Exam Technique: The Two-Step Elimination

  1. Step 1: Deduce the observed colour:
    List the visible spectrum: R - O - Y - G - B - I - V .
    Cross out what is absorbed: Red , Yellow , Green .
    What is left over and transmitted? Blue!
  2. Step 2: Match to the known transition metal ion:
    Which option gives a blue solution in water?
    • Cr₂O₇²⁻ is orange ❌
    • Fe²⁺ is pale green ❌
    • Fe³⁺ is yellow-brown ❌
    • Cu²⁺ is blue ✅

❌ Common Misconceptions & Traps

  • The "Absorbed = Seen" Trap: Students often mistakenly pick B ( Fe²⁺ ) because the question mentions "green light". Remember: an ion cannot appear green if it absorbs green light! An ion appears the colour of light that it transmits (or reflects).
  • Confusing Fe²⁺ and Fe³⁺: In acidic aqueous conditions, aqueous [Fe(H₂O)₆]²⁺ is pale green and [Fe(H₂O)₆]³⁺ appears yellow/brown. Neither absorbs red, yellow, and green to leave a pure blue colour.
  • Dichromate charge transfer: Dichromate ( Cr₂O₇²⁻ ) is intensely orange because it absorbs blue/violet light via ligand-to-metal charge transfer, the exact opposite of what is described here.

Topics

Inorganic Chemistry · 3.2.5 Transition Metals · 3.2.6 Reactions of Ions in Aqueous Solution

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.