When a solid substance disappears completely as a gas on heating, the substance is said to have undergone?

Step 1: Analyze the question A solid is heated and turns directly into a gas, with no liquid stage in between. We need the term for this process.

Step 2: Explain the concepts

• Sublimation: a solid changes directly to a gas on heating, without passing through the liquid state. Examples are ammonium chloride, iodine, and dry ice (solid CO₂).
• Crystallization: the formation of solid crystals from a solution as it cools or as the solvent evaporates.
• Distillation: a liquid is boiled and the vapour is condensed back to a liquid. It is used to separate liquids from solutions or to separate liquid mixtures.
• Evaporation: a liquid changes to a gas at a temperature below its boiling point.

Step 3: Apply to the question The question starts with a solid that disappears completely as a gas. Only sublimation describes a solid going straight to a gas.

• Crystallization starts from a solution, so it is out.
• Distillation involves a liquid, so it is out.
• Evaporation involves a liquid turning to a gas, so it is out.

Step 4: Answer The correct option is A. Sublimation.

Exam tip: Remember "Sublimation = Solid → Gas (skips Liquid)". Ammonium chloride (NH₄Cl) and iodine are the classic JAMB examples. In a mixture of NH₄Cl and sand, heating makes the NH₄Cl sublime and leaves the sand behind.