Hygroscopic and deliquescent substances
Hygroscopic substances absorb moisture but stay solid. Deliquescent substances absorb so much moisture that they dissolve in it and form a solution.
Hygroscopic vs deliquescent
Both kinds are used as drying agents (desiccants). Deliquescent substances are kept in airtight bottles. Your textbook lists anhydrous calcium chloride under both: it first absorbs moisture, and in damp air it goes on to dissolve.
Uses of drying agents
| Substance | Type | Use |
|---|---|---|
| Silica gel | Hygroscopic | Small packets keep medicines, shoes and electronics dry |
| Concentrated sulphuric acid | Hygroscopic | Dries gases in the laboratory |
| Quicklime (CaO) | Hygroscopic | Drying agent, for example to dry ammonia gas |
| Anhydrous calcium chloride | Deliquescent | Keeps air dry in a desiccator and in the rusting experiment |
| Sodium hydroxide pellets | Deliquescent | Must be stored in airtight bottles or they turn liquid |
Activity: two watch glasses in open air
- 1Put a little silica gel on one watch glass and a few pieces of anhydrous calcium chloride on another.
- 2Weigh each watch glass with its substance and note the masses.
- 3Leave both open to the air for a few hours, preferably on a humid day.
- 4Weigh again and look closely.Both weigh more. Silica gel is still a dry solid; calcium chloride has become wet and turned into a solution.
- 5★Conclusion: both absorb moisture. Silica gel is hygroscopic; calcium chloride is deliquescent.
★ key step
Why common salt becomes damp in the rainy season
Pure sodium chloride does not absorb much moisture. But ordinary common salt contains small amounts of magnesium chloride and calcium chloride. These are deliquescent. In humid weather they absorb water vapour and dissolve, so the salt becomes damp and sticky. Refined salt stays drier, and some brands add an anti-caking agent.
In the monsoon, kitchen salt becomes damp because of its MgCl₂ and CaCl₂ impurities. Many families keep a few rice grains in the salt shaker to soak up moisture, and silica gel sachets keep medicines and new shoes dry.
1 marks: “A substance that absorbs moisture from the air and dissolves in it is called ______.” Deliquescent.
2 marks: “Common salt becomes wet during the rainy season. Why?” It contains magnesium chloride and calcium chloride as impurities. These are deliquescent: they absorb moisture from humid air and dissolve.
4 marks: “How is a hygroscopic substance different from a deliquescent one? Name two examples of each.” Hygroscopic: absorbs moisture, no change of state (conc. H₂SO₄, silica gel). Deliquescent: absorbs moisture and dissolves (CaCl₂, NaOH). Both are drying agents.
Hygroscopic holds, deliquescent dissolves. Both drink water from the air.