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Chemistry · Chapter 2: Separation and Purification
Sublimation
Solids that change directly into gas, and separating ammonium chloride or iodine from a mixture by sublimation.
Key termRememberExampleWatch out
01Concept
Sublimation is the change of a solid directly into a gas on heating, without becoming a liquid. When the gas is cooled, it changes straight back into a solid; this is called deposition (some books call it desublimation).
Substances that sublime: ammonium chloride (NH₄Cl), iodine, naphthalene (mothballs), camphor and solid carbon dioxide (dry ice).
Separating a mixture by sublimation
Sublimation can separate a solid that sublimes from one that does not. Example: ammonium chloride and common salt.
- Put the mixture in a china dish and cover it with an inverted funnel. Plug the stem of the funnel with cotton to stop the vapour escaping.
- Heat the dish gently.
- Ammonium chloride turns into vapour, rises, and turns back into solid on the cool walls of the funnel.
- Common salt does not sublime, so it stays in the dish.
- Scrape the solid ammonium chloride from the funnel.
02Real World Examples
Mothballs
Naphthalene balls in wardrobes get smaller and smaller without leaving any liquid: they sublime.
Dry ice
Dry ice (solid CO₂) is used to keep ice cream cold during transport and to make stage "smoke". It never melts into a puddle.
Camphor (kafoor)
Camphor tablets left in a room slowly disappear by sublimation, spreading their smell.
03Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Define sublimation. Give three examples of substances that sublime.Show answer
Answer
Sublimation is the direct change of a solid into a gas on heating, without passing through the liquid state.
Examples: ammonium chloride, iodine, naphthalene.
Q2.How would you separate a mixture of ammonium chloride and common salt?Show answer
Answer
By sublimation. Heat the mixture in a china dish covered with an inverted funnel. Ammonium chloride sublimes and collects on the cool funnel; salt stays in the dish.
Q3.Why is the stem of the funnel plugged with cotton?Show answer
Answer
To stop the vapour escaping into the air, so that it turns back into solid on the funnel.
Q4.Why do mothballs in a cupboard get smaller over time?Show answer
Answer
Mothballs are made of naphthalene, which sublimes at room temperature. The solid slowly turns into vapour, which also keeps insects away.
Q5.What is deposition?Show answer
Answer
Deposition is the direct change of a gas into a solid on cooling, the reverse of sublimation.
Long questions
Q6.Describe the separation of a mixture of iodine and sand.Show answer
Answer
- Place the mixture in a china dish and cover it with an inverted funnel with a cotton plug. Work in a fume cupboard.
- Heat gently. Iodine sublimes to a violet vapour.
- The vapour turns back into shiny grey-black solid iodine on the cool funnel walls.
- Sand does not sublime and remains in the dish.
- Collect the iodine by scraping it off the funnel.