Water of crystallisation

A hydrated salt is a crystalline compound in which a definite, fixed number of water molecules is combined with every formula unit of the salt. This number is not a rough average or a variable amount of dampness — it is exact and characteristic of that particular salt, which is why it is written straight into the chemical formula using a raised dot:

where: the number before $H_2O$ is the water of crystallisation of that salt — five for copper sulphate, seven for ferrous sulphate, ten for both washing soda and Glauber's salt — and it never varies from crystal to crystal of the same pure compound. A substance with no water of crystallisation at all in its formula is called anhydrous. Sodium nitrate ($NaNO_3$), potassium chloride ($KCl$), lead(II) chloride ($PbCl_2$) and potassium sulphate ($K_2SO_4$) are standard ICSE examples — none of them carry a fixed water molecule count, because none of them are hydrated salts. Heating a hydrated salt drives the water out and converts it to the anhydrous form, usually with a visible colour change:

Add water back to the white anhydrous residue and it turns blue again — a reversible change that is used as a standard chemical test for detecting water in an unknown liquid.

The water molecules in a hydrated salt are not trapped by accident — they are chemically bonded into the crystal, usually attaching around the positive metal ion (for example, $Cu^{2+}$ in copper sulphate) or fitting into fixed positions within the crystal's repeating pattern. Because these positions are a fixed feature of the crystal's internal geometry, only one particular number of water molecules can fit per formula unit; that is why the ratio is a whole number and the same for every crystal of that pure compound, rather than a variable amount that depends on how "wet" a sample happens to be. This also explains why the water contributes to the crystal's shape and, in salts such as copper sulphate, to its colour: the water molecules sit close to the metal ion and influence how it interacts with light. When the salt is heated, thermal energy breaks these bonds and the water molecules escape as vapour, leaving the anhydrous salt behind. Because the underlying ionic bonds of the salt itself ($Cu^{2+}$ and $SO_4^{2-}$, for instance) are unaffected, the process is reversible: exposing the anhydrous salt to water lets the same fixed number of water molecules re-attach, restoring both the hydrated formula and the original colour. This reversibility — and the sharpness of the colour change — is exactly what makes anhydrous copper sulphate such a dependable test reagent: a colour-neutral, dry white powder that turns unmistakably blue the moment it meets water, and nothing else.

Question: A sample of white crystalline anhydrous copper sulphate ($CuSO_4$) is used to test an unknown colourless liquid suspected of containing water. A few drops of the liquid are added to the powder. (a) What observation would confirm the liquid contains water, and what is the resulting compound and its formula? (b) If the test is positive, what happens if the resulting solid is then strongly heated, and why is this change described as reversible?

Step 1. Identify the expected observation. If the liquid contains water, the white anhydrous copper sulphate powder turns deep blue, because it regains its water of crystallisation and becomes hydrated copper sulphate.

Step 2. Write the resulting compound and formula. The blue solid formed is hydrated copper sulphate, $CuSO_4\cdot 5H_2O$ — the same salt described earlier in the core idea, now re-formed from its anhydrous state by combining with exactly five water molecules per formula unit.

Step 3. Describe what strong heating does next. Strongly heating the blue $CuSO_4\cdot 5H_2O$ drives off the five water molecules as steam, leaving white/grey anhydrous $CuSO_4$ behind again — the same starting material.

Step 4. Explain the reversibility. Because the salt simply gains or loses its fixed water of crystallisation without any other part of the compound changing, the cycle white to blue to white can be repeated indefinitely by alternately adding water and heating. The test is positive because the powder turns blue, forming $CuSO_4\cdot 5H_2O$; heating reverses this back to white $CuSO_4$, confirming the change is a reversible gain and loss of water of crystallisation, not a permanent chemical transformation.

• Confusing water of crystallisation with ordinary surface dampness — it is a fixed, definite number of molecules per formula unit built into the crystal structure, not a variable amount of moisture on the surface.
• Thinking a hydrated salt feels wet to the touch — hydrated crystals like $CuSO_4\cdot 5H_2O$ are dry, stable solids; the water is locked inside the crystal, not sitting on it.
• Writing a hydrated formula with the wrong number of water molecules, such as $CuSO_4\cdot H_2O$ instead of $CuSO_4\cdot 5H_2O$ — each hydrated salt has its own specific, fixed number that must be recalled correctly.
• Assuming every salt is naturally hydrated — common salts such as $NaNO_3$, $KCl$, $PbCl_2$ and $K_2SO_4$ are anhydrous and contain no water of crystallisation in their formula at all.

Ask the tutor: want to practise matching more hydrated salts to their exact water-of-crystallisation numbers, or run through the copper sulphate test again step by step?