---
title: "Double salt vs complex salt; Werner's theory"
subject: "Engineering Chemistry"
chapter: 6
key: "semester_2/engineering_chemistry:double-complex-werner"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/double-complex-werner
site: BCT Engineering
---

# Double salt vs complex salt; Werner's theory

Engineering Chemistry · Chapter 6: Coordination Complexes

- Asked in: 2075 Baisakh, 3+2 marks; 2072 Magh, 2+3 marks; 2070 Bhadra, 2+3 marks; 2070 Magh, 1+4 marks; 2068 Bhadra, 1+4 marks (asked 5 times)
- Video: [Double salts and complex salts | werner theory | IIT JEE & NEET | Vineet Khatri Sir | ATP STAR Kota](https://www.youtube.com/watch?v=fv9aTfpwDx0) — IIT JEE Prep by iQuanta
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/double-complex-werner
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-6

## Question

Differentiate between double salt and complex salt. Write the application of Werner's theory of co-ordination compound.

## Short answer

Double salts dissociate into constituent ions in solution, e.g., Mohr's salt. Complex salts retain identity through coordinate bonds, e.g., K₄[Fe(CN)₆]. Werner's theory explains primary and secondary valency, coordination number, conductivity, isomerism, and why ions inside coordination sphere do not precipitate with AgNO₃.

## Answer

#### Double salt vs complex salt
| Basis | Double salt | Complex salt |
|---|---|---|
| Formation | Two simple salts crystallise together | Metal ion combines with ligands by coordinate bonds |
| Existence | Exists only in solid; breaks up in water | Retains identity in solid and solution |
| Ions in water | Gives the ions of both salts | Gives a complex ion and counter ions |
| Test for metal | Metal ions respond to normal tests | Central metal ion does not respond to usual tests |
| Example | Mohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O; potash alum | K₄[Fe(CN)₆], [Cu(NH₃)₄]SO₄ |

#### Applications of Werner's theory
- Explains why metals have two types of valency, **primary (ionisable)** and **secondary (non-ionisable)**, and shows that the secondary valency equals the coordination number.
- Explains structure: octahedral for CN 6, square planar or tetrahedral for CN 4.
- Explains the conductivity of complexes (from the number of ions formed).
- Explains isomerism (geometrical and optical) in complexes.
- Explains why some chlorides do not precipitate with AgNO₃: they are inside the coordination sphere. For example, all 3 Cl⁻ of [Co(NH₃)₆]Cl₃ precipitate with AgNO₃, but only 2 Cl⁻ of [Co(NH₃)₅Cl]Cl₂ do.

## Check yourself

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

1. How does a double salt behave in water?
   A. It keeps a complex ion intact
   B. It breaks up into the ions of both salts
   C. It does not dissolve
   D. It forms a gas
   Answer: B. Double salts exist only in solid state. (Key point: Behaviour in water)

2. Which is a complex salt?
   A. Mohr's salt
   B. Potash alum
   C. K4[Fe(CN)6]
   D. KCl
   Answer: C. Mohr's salt and alum are double salts. (Key point: Complex salt)

3. In Werner's theory, secondary valency equals
   A. the oxidation state
   B. the atomic number
   C. the number of counter ions
   D. the coordination number
   Answer: D. Primary valency is the oxidation state. (Key point: Valencies)

4. How many Cl- precipitate with AgNO3 in [Co(NH3)5Cl]Cl2?
   A. 2
   B. 3
   C. 1
   D. 0
   Answer: A. Only ionisable Cl- outside the sphere precipitates; one Cl is inside. (Key point: AgNO3 test)

## Related in Engineering Chemistry

- [Complex salt; main postulates of Werner's theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acomplex-salt-werner-postulates) — Engineering Chemistry, chapter 6
- [Define co-ordination number with example](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adefine-coordination-number) — Engineering Chemistry, chapter 6
- [Werner's theory and structure of complexes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awerner-theory) — Engineering Chemistry, chapter 6
- [Complex compounds vs double salts](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acomplex-vs-double) — Engineering Chemistry, chapter 6, asked 2×
- [Complex vs double salts; EAN of Fe(CN)₆](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acomplex-double-ean) — Engineering Chemistry, chapter 6
- [Complex ion and ligands in [Co(NH₃)₆]Cl₃](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aidentify-complex-ion-ligands) — Engineering Chemistry, chapter 6

## Related in other subjects

- [Define co-ordination in protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adefine-coordination) — Switchgear and Protection, chapter 1