SSC
A change of state directly from solid to gas, without passing through the liquid state, is called:
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Answer: Option A — Sublimation
Detailed Solution
Sublimation is the process in which a solid changes directly into a gas (vapour) on heating, without first melting into a liquid. The reverse process, in which a gas changes directly into a solid without becoming a liquid, is called deposition (or desublimation). The solid formed by deposition from vapour is sometimes called the sublimate.
Sublimation happens when the molecules of a solid gain enough energy to escape directly into the gas phase. It usually occurs in substances whose triple point lies above normal atmospheric pressure, or which have weak forces between their particles and a high vapour pressure.
Common examples of substances that sublime:
- Camphor: it slowly disappears when left open, and its vapour gives a smell.
- Naphthalene: mothballs in cupboards slowly become smaller and finally disappear, leaving no liquid.
- Iodine: when heated, solid iodine turns directly into violet vapour.
- Ammonium chloride: used to demonstrate sublimation in school laboratories.
- Dry ice (solid carbon dioxide): it changes directly into carbon dioxide gas at –78.5°C at normal pressure, producing a white fog effect used on stage.
Sublimation is used to separate a sublimable substance from a mixture. For example, a mixture of common salt (sodium chloride) and ammonium chloride, or of salt and naphthalene, can be separated by heating: the ammonium chloride or naphthalene sublimes and is collected on a cool surface, leaving the salt behind.
Frost forming on cold surfaces is an example of deposition, where water vapour turns directly into ice crystals. Snow can also sublime directly into water vapour in dry, windy, sunny conditions.
Freeze-drying of food and medicines uses sublimation of ice under low pressure.
Why not the other options?
- ✗Evaporation is the change of a liquid into vapour at its surface, below its boiling point.
- ✗Condensation is the change of vapour into liquid.
- ✗Fusion (melting) is the change of solid into liquid.
Quick tip
Remember: Sublimation = solid to gas directly (camphor, naphthalene, iodine, ammonium chloride, dry ice); deposition = gas to solid (frost); used to separate salt from ammonium chloride.
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Matter & Its States — Preparation
This question tests your understanding of Matter & Its States, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers matter and its classification: the states of matter, changes of state (melting, evaporation, sublimation and condensation), elements, compounds and mixtures, solutions, colloids and suspensions, and methods of separating mixtures. Exam questions often ask about examples of sublimation (camphor, naphthalene and dry ice), which substances are colloids (milk, fog and blood), and which method separates which mixture, such as chromatography for the colours in ink and fractional distillation for crude oil. Know the difference between a physical change and a chemical change. Practise the matter and its states questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.