Number of molecules of water of crystallization in copper sulphate, sodium carbonate and Gypsum are
5, 10 and 2 respectively
Understanding Water of Crystallization in Salts
Many crystalline salts contain a definite number of water molecules loosely attached to their crystal structure. This associated water is known as water of crystallization. When these hydrated salts are heated, they lose their water of crystallization and become anhydrous.
Let's examine the three compounds mentioned in the question:
- Copper Sulphate: The hydrated form of copper sulphate is commonly known as Blue Vitriol. Its chemical formula is \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \). This means one molecule of copper sulphate is associated with 5 molecules of water of crystallization.
- Sodium Carbonate: The most common hydrated form of sodium carbonate is known as Washing Soda. Its chemical formula is \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \). This indicates that each molecule of sodium carbonate has 10 molecules of water of crystallization attached.
- Gypsum: Gypsum is a hydrated mineral composed of calcium sulphate. Its chemical formula is \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \). This formula shows that one molecule of calcium sulphate is associated with 2 molecules of water of crystallization.
To summarize the number of water molecules of crystallization for each compound:
| Compound | Common Name | Chemical Formula (Hydrated) | Number of Water Molecules |
|---|---|---|---|
| Copper Sulphate | Blue Vitriol | \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \) | 5 |
| Sodium Carbonate | Washing Soda | \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \) | 10 |
| Gypsum | \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \) | 2 |
The question asks for the number of molecules of water of crystallization in copper sulphate, sodium carbonate, and Gypsum in that specific order. Based on our findings, the numbers are 5, 10, and 2 respectively.
Comparing this sequence (5, 10, 2) with the given options, we can determine the correct choice.
- Option 1: 5, 10 and 2 respectively
- Option 2: 10, 2 and 5 respectively
- Option 3: 5, 2 and 10 respectively
- Option 4: 2, 5 and 10 respectively
The sequence 5, 10, and 2 matches Option 1.
Revision Table: Hydrated Salts
| Salt | Hydrated Formula | Water Molecules |
|---|---|---|
| Copper(II) Sulphate | \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \) | 5 |
| Sodium Carbonate | \( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \) | 10 |
| Calcium Sulphate (Gypsum) | \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \) | 2 |
| Calcium Sulphate (Plaster of Paris) | \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \) | 0.5 |
| Iron(II) Sulphate | \( \text{FeSO}_4 \cdot 7\text{H}_2\text{O} \) | 7 |
| Magnesium Sulphate | \( \text{MgSO}_4 \cdot 7\text{H}_2\text{O} \) | 7 |
| Zinc Sulphate | \( \text{ZnSO}_4 \cdot 7\text{H}_2\text{O} \) | 7 |
| Aluminium Potassium Sulphate (Alum) | \( \text{KAl(SO}_4)_2 \cdot 12\text{H}_2\text{O} \) | 12 |
Additional Information: Properties Related to Water of Crystallization
Understanding water of crystallization is important as it affects the physical properties (like colour and shape) of the crystal. Losing water of crystallization can change these properties significantly.
- Anhydrous Salt: A salt that has lost its water of crystallization. For example, anhydrous copper sulphate (\( \text{CuSO}_4 \)) is white, unlike hydrated copper sulphate which is blue.
- Efflorescence: The process where a hydrated salt spontaneously loses its water of crystallization to the atmosphere when exposed to air, especially if the atmospheric humidity is lower than the vapour pressure of the hydrate. Washing soda (\( \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \)) is efflorescent.
- Deliquescence: The process where a substance absorbs moisture from the atmosphere and dissolves in the absorbed water, forming a solution. This is distinct from efflorescence. Examples include calcium chloride and sodium hydroxide.
- Hygroscopy: The ability of a substance to attract and hold water molecules from the surrounding environment. Many substances are hygroscopic without necessarily dissolving (like deliquescence) or forming a defined hydrate (like water of crystallization).
These concepts highlight the dynamic nature of water association with salts.
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