The correct option is **C. hygroscopy**. ### Explanation of the Correct Answer **Hygroscopy** refers to the ability of a substance to absorb moisture from the air. Salts that are hygroscopic can attract and hold water vapor, making them effective drying agents. When these salts are exposed to humid air, they absorb water, which helps to reduce the moisture content in the surrounding environment. This property is particularly useful in various applications, such as in desiccators, where maintaining a dry atmosphere is crucial. #### How Hygroscopic Salts Work: 1. **Absorption of Water**: Hygroscopic salts, such as calcium chloride (CaCl₂) or magnesium sulfate (MgSO₄), have a strong affinity for water molecules. When placed in a humid environment, they draw in water vapor from the air. 2. **Formation of Hydrates**: Upon absorbing water, these salts often form hydrates. For example, calcium chloride can absorb water and form calcium chloride dihydrate (CaCl₂·2H₂O). 3. **Drying Effect**: As these salts continue to absorb moisture, they effectively lower the humidity level in their vicinity, making them useful for drying purposes. ### Why the Other Options Are Incorrect **A. Efflorescence**: - Efflorescence is the process where a hydrated salt loses water to the atmosphere, often resulting in the formation of a powdery residue. While this process involves water loss, it is not a property that aids in drying; rather, it indicates a loss of moisture. Therefore, efflorescence does not contribute to the drying capability of salts. **B. High Melting Point**: - While some salts do have high melting points, this property is not directly related to their ability to act as drying agents. A high melting point indicates thermal stability but does not imply that the salt can absorb moisture effectively. Thus, this option does not address the fundamental property that makes a salt useful for drying. **D. Low Solubility**: - Low solubility in water means that a salt does not dissolve easily in water. While some drying agents may be low in solubility, this property alone does not guarantee that a salt can absorb moisture from the air. In fact, many hygroscopic salts are quite soluble. Therefore, low solubility is not a defining characteristic of effective drying agents. ### Summary of Key Points - **Hygroscopy** is the ability of a substance to absorb moisture from the air, making it essential for drying agents. - Salts like calcium chloride and magnesium sulfate are effective because they can attract and hold water vapor. - **Efflorescence** indicates moisture loss, **high melting point** relates to thermal stability, and **low solubility** does not ensure moisture absorption, making these options less relevant to the property of drying agents. ### Revision Summary - **Hygroscopic salts** absorb moisture from the air, making them effective drying agents. - Common hygroscopic salts include **calcium chloride** and **magnesium sulfate**. - **Efflorescence** is moisture loss, not absorption; **high melting point** and **low solubility** do not relate to drying properties. - Understanding the properties of salts is crucial for their application in drying and moisture control.