When NH4Cl(s) is dissolved in water, the container feels cold to touch. This implies

Step 1: Analyze the question The container feels cold when NH₄Cl dissolves. We need to decide what that tells us about the energy change in the process.

Step 2: The concept

• Heat flows from warmer to cooler. Your hand is warmer than the container, so a container that feels cold means the solution is taking in heat from its surroundings (including your hand).
• A process that absorbs heat from its surroundings is endothermic (ΔH is positive).
• A process that releases heat is exothermic (ΔH is negative), and the container would feel warm or hot.

Step 3: Solution

1. Dissolving NH₄Cl requires energy to break the ionic lattice (NH₄⁺ and Cl⁻ ions) and to separate the water molecules.
2. The energy released when the ions are hydrated is less than the energy needed to break the lattice.
3. The shortfall is drawn from the water and the surroundings, so the temperature of the solution drops and the container feels cold.

NH₄Cl(s) + water → NH₄⁺(aq) + Cl⁻(aq) ΔH > 0 (endothermic)

Step 4: Check the other options

• B. Exothermic: Wrong. An exothermic process would make the container feel warm.
• C. Saturated solution: Wrong. Feeling cold says nothing about whether the solution is saturated.
• D. Highly soluble: Wrong. Temperature change does not tell us how much dissolves. Solubility is a separate property.

Correct answer: A. The process is endothermic

Exam tip: Remember "cold = endothermic" because the process takes in heat, and "hot = exothermic" because it gives heat out. Other common endothermic dissolving examples are NH₄NO₃ and KNO₃. Strong acids and NaOH dissolving in water are exothermic.