A SALT WHICH LOSES MASS WHEN EXPOSED TO AIR IS

• A. hygroscopic
• B. deliquescent
• C. efflorescent ✓
• D. fluorescent
• E. effervescent

 

The answer to the question is: C. efflorescent

Efflorescence is a phenomenon associated with certain crystalline substances, particularly salts, which involves the loss of water of crystallization to the atmosphere upon exposure to air. This process results in a decrease in the mass of the salt crystal as it releases its water molecules.

Understanding Efflorescent Salts

Efflorescent salts are those which contain water of crystallization in their molecular structure. These water molecules are not merely trapped physically within the crystal lattice but are a part of the chemical composition of the salt. The formula of such a salt typically includes a dot followed by the number of water molecules present per formula unit, for example, CuSO₄·5H₂O (copper(II) sulfate pentahydrate).

When efflorescent salts are exposed to air, especially air with low humidity, they tend to lose their water of crystallization due to evaporation. This process can be gradual or rapid depending on the ambient conditions and the nature of the salt. As the water evaporates, the salt’s mass decreases and it may turn into a powder or crumbly solid as it loses its crystalline structure.

Mechanism of Efflorescence

The mechanism behind efflorescence lies in the vapor pressure of the bound water molecules in comparison to the partial vapor pressure of water in the surrounding air. If the vapor pressure of water within the crystal is higher than that in the air, water molecules will tend to escape from the salt into the air until equilibrium is reached.

Examples and Consequences

Common examples of efflorescent salts include sodium sulfate decahydrate (Na₂SO₄·10H₂O), magnesium sulfate heptahydrate (MgSO₄·7H₂O), and copper(II) sulfate pentahydrate (CuSO₄·5H₂O). When these salts lose their water content, they also undergo a change in appearance and texture.

In practical applications, efflorescence can be an issue for materials like concrete or brick where soluble salts are present. When moisture moves through these materials and then evaporates at their surface, it can leave behind a white powdery residue composed of salts that have lost their hydration.

Preventing Efflorescence

To prevent or mitigate efflorescence, it is crucial to control environmental conditions such as humidity and temperature. Packaging efflorescent substances in moisture-proof containers or using desiccants can help maintain their original state by preventing them from losing moisture.

In construction, using materials with low soluble salt content, applying sealants, or designing structures that avoid trapping moisture can help reduce efflorescence on surfaces.

Conclusion

In summary, a salt which loses mass when exposed to air is indeed efflorescent, as it releases its water of crystallization upon exposure to an environment with lower humidity than that required to maintain its hydrated form.