Which of the following compounds will undergo thermal dissociation?

Analyze the question

"Thermal dissociation" is a reversible decomposition caused by heat. The compound breaks down into simpler substances when heated, and these recombine when cooled. This is different from ordinary thermal decomposition, which is usually irreversible.

Explain the concept

The classic example in Nigerian syllabuses is ammonium chloride:

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

On heating, the white solid breaks into ammonia and hydrogen chloride gases. On cooling at the cooler part of the tube, they recombine to form solid NH₄Cl again. Because the change is reversible, it is called thermal dissociation.

Check each option

• A. AgCl: It is stable to heat and does not dissociate reversibly. It decomposes in light, not by thermal dissociation.
• B. CaCO₃: It decomposes on heating, CaCO₃ → CaO + CO₂. This is thermal decomposition. It can be reversible in a closed system, but in an open vessel the CO₂ escapes, so it is normally classed as decomposition and not as the textbook example of dissociation.
• C. K₂SO₄: It is very stable to heat and does not dissociate.
• D. NaOH: It is stable to heat and melts without decomposing.
• E. NH₄Cl: It dissociates reversibly into NH₃ and HCl. This is the standard example.

Answer

The correct option is E. NH₄Cl.

Exam tip

When you see "thermal dissociation" in a question, think of NH₄Cl first. Think of "reversible, with recombination on cooling." Similar examples are N₂O₄ ⇌ 2NO₂ and PCl₅ ⇌ PCl₃ + Cl₂. For CaCO₃, think "thermal decomposition" (used to make quicklime).