---
title: "Zn²⁺ salts white, Cu²⁺ salts coloured"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:zn2-cu2-colour"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/zn2-cu2-colour
site: BCT Engineering
---

# Zn²⁺ salts white, Cu²⁺ salts coloured

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2075 Baisakh, 1.5 marks
- Video: [Zn²⁺ salts are white while Cu²⁺ salts are coloured. Why?](https://www.youtube.com/watch?v=23KqUV5epsA) — AnswerXpert
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/zn2-cu2-colour
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Explain why Zn²⁺ salts are white while Cu²⁺ salts are coloured.

## Short answer

Zn²⁺ (3d¹⁰) has completely filled d orbitals, preventing d–d transitions; its salts are white. Cu²⁺ (3d⁹) has a vacancy, allowing d–d absorption of red-orange light, so the hydrated ion appears blue.

## Answer

- **Zn²⁺** has the configuration 3d¹⁰. All d orbitals are full, so no d–d transition can take place and no visible light is absorbed: Zn²⁺ salts are white.
- **Cu²⁺** has 3d⁹, with one vacancy (hole) in the d subshell. An electron can be excited from the t₂g to e_g level by absorbing red-orange visible light, and the complementary blue/green colour is seen: hydrated Cu²⁺ is blue.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are Zn2+ salts white?
   A. 3d9 has a hole
   B. 3d0 is empty but absorbs
   C. They absorb all light
   D. 3d10 is full, so no d–d transition
   Answer: D. A full d subshell has no vacancy. (Key point: Zn2+)

2. Why are Cu2+ salts coloured?
   A. 3d9 has one vacancy allowing a d–d transition
   B. 3d10 is full
   C. 3d0
   D. No d electrons
   Answer: A. An electron can be excited from t2g to eg. (Key point: Cu2+)

3. Hydrated Cu2+ is
   A. Colourless
   B. Blue
   C. Pink
   D. Violet
   Answer: B. It absorbs red-orange light. (Key point: Hydrated Cu2+)

## Related in Engineering Chemistry

- [Electrode potential origin; Zn-Cu cell notation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aelectrode-potential-zn-cu) — Engineering Chemistry, chapter 1
- [Cupric salts blue, cuprous colourless](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acu2-cu1-colour) — Engineering Chemistry, chapter 5