The correct option is **D. CaCl2**. ### Explanation of the Correct Answer **Calcium Chloride (CaCl2)** is a hygroscopic compound, meaning it has the ability to absorb moisture from the air. When exposed to air, CaCl2 can attract water vapor and dissolve in it, forming a solution. This property is due to the ionic nature of the compound and its strong affinity for water molecules. #### Step-by-Step Breakdown: 1. **Hygroscopic Nature**: - Calcium chloride is hygroscopic, which means it can absorb water from the environment. This is a key characteristic of many salts, particularly those that are ionic and have a high lattice energy. - When CaCl2 absorbs moisture, it can dissolve in the water it collects, leading to the formation of a solution. 2. **Dissolution Process**: - When CaCl2 absorbs water, it dissociates into its constituent ions: \[ \text{CaCl}_2 (s) + \text{H}_2O (g) \rightarrow \text{Ca}^{2+} (aq) + 2 \text{Cl}^- (aq) \] - This process results in a solution of calcium ions and chloride ions in water. ### Why the Other Options are Incorrect or Weaker **A. Na2CO3.10H2O (Sodium Carbonate Decahydrate)**: - While Na2CO3.10H2O is a hydrate and contains water, it does not actively absorb moisture from the air to form a solution. Instead, it is stable in its hydrated form and will not dissolve further unless heated or mixed with additional water. **B. NaNO3 (Sodium Nitrate)**: - Sodium nitrate is not hygroscopic to the same extent as CaCl2. It can dissolve in water, but it does not absorb moisture from the air to form a solution. It remains solid unless it is directly mixed with water. **C. CuSO4 (Copper(II) Sulfate)**: - Copper(II) sulfate is also a hydrate (commonly found as CuSO4.5H2O) and can absorb some moisture from the air. However, it does not readily form a solution just by exposure to air. It requires the addition of water to dissolve effectively. ### Common Pitfalls - **Confusing Hygroscopic with Hydrated**: Many students may confuse compounds that are hydrated (like Na2CO3.10H2O) with those that are hygroscopic (like CaCl2). Hydrated compounds contain water in their structure but do not necessarily absorb moisture from the air. - **Assuming All Salts are Hygroscopic**: Not all salts have the ability to absorb moisture from the air. Understanding the specific properties of each compound is crucial. ### Revision Summary - **CaCl2 is hygroscopic** and can absorb moisture from the air, forming a solution. - **Na2CO3.10H2O** and **NaNO3** do not actively absorb moisture to form solutions. - **CuSO4** can absorb moisture but does not readily dissolve in air exposure alone. - Understanding the properties of compounds (hygroscopic vs. hydrated) is essential for solving similar questions.