When NH 4CI was dissolved in water, the container was cold to touch. This implies that

Step 1: Analyze the question When ammonium chloride (NH₄Cl) dissolves in water, the container feels cold. We need to decide what this tells us about the energy change.

Step 2: The concept

• Endothermic process: absorbs heat from the surroundings, so the surroundings (the water and the container) get colder.
• Exothermic process: releases heat to the surroundings, so the container feels warm or hot.

Dissolving involves two energy steps:

1. Breaking the ionic lattice (needs energy).
2. Hydration of the ions (releases energy).

For NH₄Cl, the energy needed to break the lattice is greater than the energy released by hydration. The net result is that heat is absorbed from the surroundings, so ΔH is positive.

Step 3: Apply to the options

• A. The process is endothermic. Correct. The container feels cold because heat is drawn from the water and the container into the dissolving salt.
• B. The process is exothermic. Wrong. That would make the container warm.
• C. NH₄Cl is highly soluble in water. Temperature change tells us nothing about how much dissolves.
• D. NH₄Cl forms a saturated solution. Also unrelated to the temperature change.

Answer: A. the process is endothermic

Exam tip: Remember "cold to touch means endothermic" (heat goes in from the surroundings). "Hot to touch means exothermic" (heat goes out). Other common endothermic dissolving examples are NH₄NO₃ and KNO₃. Examples of exothermic dissolving are NaOH and concentrated H₂SO₄.