Which of the following salts will NOT have 10 molecules of water of crystallisation?
Understanding Water of Crystallization in Salts
Water of crystallization refers to the fixed number of water molecules chemically bound to a crystal structure of a salt. These water molecules are an integral part of the crystal lattice and give the salt its characteristic crystalline shape and properties. When these salts are heated, the water of crystallization is usually driven off, leaving behind the anhydrous salt.
Analyzing Common Hydrated Salts
Let's examine the chemical composition and hydration state of each salt listed in the options to determine which one does not have 10 molecules of water of crystallization.
- Glauber's salt: This is the common name for sodium sulfate decahydrate. Its chemical formula is $\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}$. As the formula indicates, Glauber's salt contains 10 molecules of water of crystallization per formula unit.
- Washing soda: This is the common name for sodium carbonate decahydrate. Its chemical formula is $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$. Washing soda also contains 10 molecules of water of crystallization per formula unit.
- Borax: This is the common name for sodium tetraborate decahydrate. Its chemical formula is $\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$. Borax contains 10 molecules of water of crystallization per formula unit.
- Epsom salt: This is the common name for magnesium sulfate heptahydrate. Its chemical formula is $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$. Epsom salt contains 7 molecules of water of crystallization per formula unit.
Comparing Water Molecules in Salts
Based on the analysis, we can clearly see the number of water molecules associated with each salt:
| Salt Name | Chemical Formula | Molecules of Water of Crystallization |
|---|---|---|
| Glauber's salt | $\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}$ | 10 |
| Washing soda | $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ | 10 |
| Borax | $\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$ | 10 |
| Epsom salt | $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$ | 7 |
Glauber's salt, Washing soda, and Borax all have 10 molecules of water of crystallization. Epsom salt, however, has only 7 molecules of water of crystallization.
Conclusion
The salt that will NOT have 10 molecules of water of crystallization is Epsom salt, as it contains 7 molecules of water of crystallization.
Revision Table: Hydrated Salts
| Common Salt Name | Chemical Name | Chemical Formula | Water Molecules |
|---|---|---|---|
| Glauber's salt | Sodium sulfate decahydrate | $\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}$ | 10 |
| Washing soda | Sodium carbonate decahydrate | $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ | 10 |
| Borax | Sodium tetraborate decahydrate | $\text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O}$ | 10 |
| Epsom salt | Magnesium sulfate heptahydrate | $\text{MgSO}_4 \cdot 7\text{H}_2\text{O}$ | 7 |
Additional Information: Water of Crystallization Concepts
Understanding water of crystallization is important in chemistry. Here are a few related points:
- Anhydrous Salts: These are salts that have lost their water of crystallization, usually by heating. For example, anhydrous copper(II) sulfate ($\text{CuSO}_4$) is white, while hydrated copper(II) sulfate ($\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$) is blue.
- Hydrated Salts: These are salts that contain water of crystallization. The number of water molecules is fixed for a specific salt and temperature/pressure conditions.
- Efflorescence: This is the process where a hydrated salt loses its water of crystallization when exposed to air. This happens when the vapor pressure of water in the hydrated salt is higher than the partial pressure of water vapor in the air. Washing soda ($\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$) is a good example of an efflorescent substance, slowly losing water to form the monohydrate ($\text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O}$) and eventually the anhydrous salt ($\text{Na}_2\text{CO}_3$).
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