---
title: "Cupric salts blue, cuprous colourless"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:cu2-cu1-colour"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cu2-cu1-colour
site: BCT Engineering
---

# Cupric salts blue, cuprous colourless

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

- Asked in: 2071 Magh, 2.5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cu2-cu1-colour
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Explain the following property: Cupric salts are blue in color while cuprous salts are colorless.

## Short answer

Cu²⁺ is 3d⁹, having a d-orbital vacancy that allows d–d transitions; absorption of red-orange light transmits blue, so cupric salts are blue. Cu⁺ is 3d¹⁰ with no vacant d orbital, so no d–d transition occurs and cuprous salts are colourless.

## Answer

- Cu²⁺ is 3d⁹: one d orbital has a vacancy (a "hole"). In [Cu(H₂O)₄]²⁺ an electron can jump from a lower d level to the half-filled upper level by absorbing red-orange light (d–d transition). The transmitted light is the complementary colour, **blue**. So cupric salts (CuSO₄·5H₂O) are blue.
- Cu⁺ is 3d¹⁰: all d orbitals are completely filled, so there is no vacant d orbital to accept an excited electron. No d–d transition occurs, no visible light is absorbed, and cuprous salts (CuCl) are colourless/white.

Thus the presence of an incomplete d subshell (d⁹) gives colour; the d¹⁰ configuration gives none.

## Check yourself

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

1. Why can Cu2+ show a d-d transition?
   A. It is 3d10 and full
   B. It is 3d9, so there is a vacancy (hole) in a d level to accept an excited electron
   C. It is 3d0 and empty
   D. It has no d electrons left
   Answer: B. The hole in d9 lets an electron jump from a lower d level when light is absorbed. (Key point: Cu2+ configuration)

2. Cupric salts absorb red-orange light. What colour do they appear?
   A. Red-orange
   B. Colourless
   C. Blue, the complementary colour
   D. Green-yellow
   Answer: C. The transmitted light is the complement of the absorbed red-orange: blue. (Key point: Colour seen)

3. Why are cuprous salts such as CuCl colourless?
   A. Cu+ is 3d9 with one hole
   B. Cu+ is 3d0
   C. Cu+ absorbs all visible wavelengths
   D. Cu+ is 3d10, so no vacant d orbital exists and no d-d transition occurs
   Answer: D. A full d subshell cannot accept an excited electron. (Key point: Cu+ colourless)

## Related in Engineering Chemistry

- [Zn²⁺ salts white, Cu²⁺ salts coloured](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Azn2-cu2-colour) — Engineering Chemistry, chapter 5

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