Copper sulphate crystals available in the market are blue coloured crystals. By careful heating, they turn to white colour. Which one of the following is responsible for the blue colour?
Water
Understanding Copper Sulphate Crystals and Their Colour
Copper sulphate is a chemical compound with the formula \(\text{CuSO}_4\). When you see blue copper sulphate crystals in the market, they are not just simple \(\text{CuSO}_4\). These crystals are actually hydrated copper sulphate, meaning they contain water molecules bound within their crystal structure. The specific formula for the blue crystals is \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\), indicating that each copper sulphate unit is associated with five molecules of water. These water molecules are called 'water of crystallization'.
Why are Copper Sulphate Crystals Blue?
The blue colour of copper sulphate crystals comes from the interaction of light with the copper(II) ions (\(\text{Cu}^{2+}\)) surrounded by the water molecules within the crystal lattice. In the hydrated form, the water molecules act as ligands, coordinating with the copper ions. This coordination affects the d-orbitals of the copper ion. When visible light falls on the crystal, certain wavelengths of light are absorbed due to electronic transitions between these d-orbitals. The colour you see is the combination of the remaining wavelengths of light that are not absorbed but are reflected or transmitted. In the case of hydrated copper sulphate, wavelengths corresponding to red and orange light are absorbed, and the complementary colour, which is blue, is observed.
Heating Copper Sulphate Crystals
The question mentions that when these blue copper sulphate crystals are heated carefully, they turn white. This change in colour is a direct result of the loss of the water of crystallization. When heated, the water molecules attached to the copper sulphate are driven off as steam. The chemical equation for this process is:
\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \text{ (blue crystals) } \xrightarrow{\text{Heat}} \text{CuSO}_4 \text{ (white powder) } + 5\text{H}_2\text{O} \text{ (water vapour)}\)
The resulting substance is anhydrous copper sulphate (\(\text{CuSO}_4\)), which does not have the water molecules coordinating with the copper ions. Without the water ligands, the electronic structure of the copper ion changes, and it no longer absorbs light in the same way. Anhydrous copper sulphate is a white or off-white powder.
Analyzing the Options
Let's look at the given options and see if they are responsible for the blue colour of copper sulphate crystals:
- Oxygen: Oxygen is present in the sulphate ion (\(\text{SO}_4^{2-}\)) and also in water molecules (\(\text{H}_2\text{O}\)). While essential components, oxygen atoms themselves in isolation are not the direct cause of the colour in this compound's solid-state structure.
- Nitrogen: Nitrogen is not a component of copper sulphate crystals (\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)). Therefore, it cannot be responsible for the blue colour.
- Water: As explained above, the water molecules (water of crystallization) in the hydrated copper sulphate structure are crucial for the blue colour. They coordinate with the copper ions and influence the electronic transitions that lead to the absorption of specific light wavelengths.
- Hydrogen: Hydrogen is a component of the water molecules (\(\text{H}_2\text{O}\)) in the hydrated crystals. While present in water, it is the water molecule as a whole, acting as a ligand surrounding the copper ion, that is responsible for the colour, not the hydrogen atoms in isolation.
Based on this analysis, the presence of water molecules (water of crystallization) is the factor responsible for the blue colour of copper sulphate crystals.
| Substance | Role in \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\) | Responsible for Blue Colour? |
|---|---|---|
| Copper Ion (\(\text{Cu}^{2+}\)) | Metal centre | Yes, interacts with ligands (water) |
| Sulphate Ion (\(\text{SO}_4^{2-}\)) | Anion | Indirectly, part of the crystal structure |
| Water (\(\text{H}_2\text{O}\)) | Ligand (water of crystallization) | Yes, directly interacts with \(\text{Cu}^{2+}\) causing light absorption |
| Oxygen (element) | Part of Sulphate and Water | No (as isolated element) |
| Nitrogen (element) | Not present | No |
| Hydrogen (element) | Part of Water | No (as isolated element) |
The reversible nature of this process is also important. If you add water to the white anhydrous copper sulphate, it will reabsorb water and turn back into the blue hydrated crystals, demonstrating that the water is indeed responsible for the colour.
Revision Table: Copper Sulphate Colour Change
| Form | Formula | Colour | Water Present? | State |
|---|---|---|---|---|
| Hydrated Copper Sulphate | \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\) | Blue | Yes (Water of crystallization) | Crystalline Solid |
| Anhydrous Copper Sulphate | \(\text{CuSO}_4\) | White | No | Powder |
Additional Information: Hydrates and Colour
Many salts that crystallize from aqueous solutions incorporate water molecules into their crystal structure. These are called hydrates, and the water is known as water of crystallization or hydration. The number of water molecules associated with one formula unit of the salt is fixed and is indicated in the chemical formula (e.g., the '5' in \(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\)).
The presence of water of crystallization can significantly affect the properties of the salt, including its colour, shape, and melting point. The colour change observed upon hydration or dehydration of transition metal salts like copper sulphate is quite common. For example:
- Cobalt(II) chloride: Anhydrous \(\text{CoCl}_2\) is blue, while hydrated \(\text{CoCl}_2 \cdot 6\text{H}_2\text{O}\) is pink.
- Nickel(II) chloride: Anhydrous \(\text{NiCl}_2\) is yellow, while hydrated \(\text{NiCl}_2 \cdot 6\text{H}_2\text{O}\) is green.
These colour changes are due to the interaction of the metal ions with the water molecules acting as ligands, similar to the case of copper sulphate.
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