If a salt weighs 2g and upon exposure to the atmosphere weighs 1.5g, this is as a result of

Analysis

The question describes a process where a salt loses mass (from 2g to 1.5g) after being exposed to the atmosphere. This indicates that the substance has lost water to the surroundings.

Concept Explanation

In chemistry, the interaction between salts and atmospheric water vapor is categorized into three main phenomena:

1. Efflorescence: This occurs when a hydrated salt loses its water of crystallization to the atmosphere because its vapor pressure is higher than that of the surrounding air. As the salt loses water, it loses mass and often crumbles into a powder.
2. Hygroscopy: This is the property of a substance that absorbs moisture from the atmosphere without necessarily dissolving in it. The mass of the substance increases.
3. Deliquescence: This occurs when a substance absorbs so much moisture from the atmosphere that it eventually dissolves in the absorbed water to form a solution. The mass of the substance increases significantly.
4. Effervescence: This is the escape of gas from an aqueous solution (like the fizzing of a soda), which is unrelated to the absorption or loss of water by a solid salt.

Step-by-Step Solution

• Initial state: The salt weighs 2g.
• Final state: After exposure, it weighs 1.5g.
• Observation: There is a decrease in mass (2g - 1.5g = 0.5g loss).
• Reasoning: Since the salt lost mass, it must have released water molecules into the atmosphere. Among the options provided, only efflorescence involves the loss of water of crystallization to the atmosphere, resulting in a decrease in the mass of the salt.

Conclusion

The correct phenomenon is efflorescence.

Exam Tip

Think of the letters:

• Deliquescence = Damp (it absorbs water and becomes wet/liquid).
• Efflorescence = Exits (water exits the crystal, causing it to lose mass).