Difference Between a Double Salt and a Complex Compound
Chemistry · Coordination Compounds · NEET
Both are "addition compounds" (two simple salts joined together). The one difference: when you put them in water, a double salt breaks apart completely into all its simple ions, but a complex compound keeps a stable complex ion that does not break apart. Memory hook: "Double DIES in water, Complex STAYS together."
In water a double salt (potash alum) splits into all its simple ions, while a complex (potassium ferrocyanide) keeps one stable complex ion joined together.
Your doubts, answered
What is the difference between a double salt and a complex compound?
Both are made by joining two simple salts, so both look similar on paper. The difference shows only in water. A double salt breaks apart 100% into all of its simple ions and behaves like a plain mixture of the original salts. A complex compound keeps a stable complex ion that does NOT break apart, so it behaves like a new substance with new properties. For NEET, remember: the test is what happens in water.
Does a double salt exist in solution or does it break?
It breaks. When you dissolve a double salt in water, it dissociates completely and gives all its simple ions freely. For example potash alum KAl(SO4)2.12H2O gives K+, Al3+ and SO4^2- ions, exactly the same ions you would get from potassium sulfate and aluminium sulfate separately. The double salt only exists as a solid crystal; it loses its identity in water.
Is KAl(SO4)2.12H2O a double salt or a complex?
It is a DOUBLE SALT (the classic example, called potash alum). In water it gives K+, Al3+ and SO4^2- as free simple ions. There is no stable complex ion. Compare it with a complex like K4[Fe(CN)6]: that one gives K+ and one stable [Fe(CN)6]^4- ion, and it does NOT give free Fe2+ or free CN- ions. NEET 2023 used alum exactly as the double salt example.
Why does a complex compound not give all its ions in water?
Because inside a complex the central metal and the ligands are held by strong coordinate bonds. This group stays joined as one unit called the complex ion, written inside square brackets. So K4[Fe(CN)6] releases only K+ and the whole [Fe(CN)6]^4- ion. It cannot release free Fe2+ or free CN-, so it does not show the normal tests for Fe2+ or CN-. That stability is the whole point of a complex.
What is common between a double salt and a complex compound?
Both are addition (molecular) compounds. This means both are formed when two or more stable simple salts combine in a fixed ratio and crystallise together, for example salt A + salt B -> A.B. So in the SOLID state both look the same. The difference appears only when you dissolve them in water. NEET likes to test this 'same on paper, different in water' idea.
⚠️ The NEET trap ✗ KAl(SO4)2.12H2O is a complex because it has a metal (Al) joined to sulfate and water, so it must contain a complex ion. ✓ KAl(SO4)2.12H2O is a DOUBLE SALT. In water it breaks completely into K+, Al3+ and SO4^2- as free simple ions, with no stable complex ion. A complex like K2PtCl2.2NH3 keeps a stable complex ion in water. NEET 2023 marked exactly this pair. 🧠 Do not judge by the formula looking crowded. Ask one question: in water, does it give a stable complex ion (complex) or all free simple ions (double salt)?
Real NEET questions
NEET 2023 (Phase 1)
Which of the following forms a set of a complex and a double salt, respectively?
A · K2PtCl2.2NH3 and KAl(SO4)2.12H2O ✓
B · NiCl2.6H2O and NiCl2(H2O)4
C · CuSO4.5H2O and CuCl2.4NH3
D · PtCl2.2NH3 and PtCl4.2HCl
Solution: The question asks for one complex first, then one double salt. Test in water. KAl(SO4)2.12H2O (potash alum) is the classic double salt: in water it breaks completely into K+, Al3+ and SO4^2- as free simple ions, so it shows the properties of both original salts. K2PtCl2.2NH3 contains a stable platinum-ammine-chloride complex ion that does not fully break apart in water, so it is a coordination (complex) compound. Complex + double salt = option A.
NEET 2018
The type of isomerism shown by the complex [CoCl2(en)2] is:
A · Ionization isomerism
B · Coordination isomerism
C · Geometrical isomerism ✓
D · Linkage isomerism
Solution: The square brackets tell you this is a true complex (a stable complex ion, not a double salt). It is octahedral with two bidentate en ligands and two Cl ligands. The two Cl can sit next to each other (cis) or opposite each other (trans), giving geometrical (cis/trans) isomers, option C. This PYQ reminds you that when a compound is written inside brackets, it is a complex whose ligands stay bonded to the metal, which is why it can even show cis-trans isomerism.
Solved Coordination Compounds NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Give one example each of a double salt and a complex compound.
Double salt: potash alum KAl(SO4)2.12H2O, which gives K+, Al3+ and SO4^2- in water. Complex compound: potassium ferrocyanide K4[Fe(CN)6], which gives K+ and the stable [Fe(CN)6]^4- ion but no free Fe2+ or CN-.
Are double salts and complex compounds the same thing?
No. Both are addition compounds and both look similar as solids, but a double salt breaks fully into simple ions in water while a complex keeps a stable complex ion. That water test is the only real difference NEET asks about.
How do I quickly decide double salt or complex in an exam?
Ask: in water does it give a stable complex ion? If it keeps one joined group (often shown inside square brackets), it is a complex. If it splits into all its simple ions like the original salts, it is a double salt.
Why does a complex ion not show the usual tests of its metal ion?
Because the metal is locked inside the complex ion by strong coordinate bonds. For example [Fe(CN)6]^4- does not release free Fe2+, so it does not give the normal Fe2+ tests. This hidden or 'masked' behaviour is a key sign of a complex.
