Question

Which of the following compound releases heat when dissolved in water?

• A. Barium chloride
• B. Ammonium chloride
• C. Lead chloride
• D. Calcium chloride

Hint:

$CaCl_2$ $\rightarrow$ Hot (Exothermic).
$NH_4Cl$ $\rightarrow$ Cold (Endothermic).
Think of "Heat Packs" ($CaCl_2$) vs. "Instant Cold Packs" ($NH_4NO_3$ or $NH_4Cl$).

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks which of the listed chemical salts undergoes an exothermic dissolution process (releasing heat) when mixed with water.
Detailed Explanation:

• Enthalpy of Solution:
Dissolving a salt involves two energy steps: breaking the ionic lattice (requires energy) and hydrating the ions (releases energy).
If the energy released by hydration is greater than the lattice energy, the process is exothermic (releases heat).

• Calcium Chloride ($CaCl_2$):
When anhydrous calcium chloride is dissolved in water, the hydration energy is extremely high. The temperature of the water rises significantly. This is why $CaCl_2$ is used in commercial "self-heating" pads or for melting ice on roads.

• Ammonium Chloride ($NH_4Cl$):
In contrast, this salt has a positive enthalpy of solution (it is endothermic). Dissolving it makes the water feel cold.

• Other options:
Barium chloride and Lead chloride do not have the same strongly exothermic profile as Calcium chloride. Lead chloride is also poorly soluble in cold water.

Step 2: Final Answer:
Calcium chloride releases substantial heat upon dissolution in water.