Which one of the following is the best example of desiccant?
- (a)Silica gel
- (b)Polystyrene
- (c)Sodium chloride
- (d)Sodium carbonate
Correct — A, Silica gel. A desiccant is a substance that keeps its surroundings dry by taking up moisture. Silica gel has an enormous internal surface area of tiny pores that adsorb water vapour very effectively, which is why little sachets of silica gel are packed with medicines, electronics, leather goods and food to keep them dry. It is the classic example of a desiccant.
- (b)Polystyrene — A plastic used for packaging and insulation; it does not take up moisture, so it is not a desiccant.
- (c)Sodium chloride — Common salt is only mildly hygroscopic and even cakes in damp air, but it is not used as a drying agent the way silica gel is.
- (d)Sodium carbonate — Washing soda is not used as a desiccant; silica gel is the standard drying agent.
A desiccant, or drying agent, keeps an enclosed space dry by adsorbing water vapour. Silica gel is porous silicon dioxide with a very high surface area, so it holds a large amount of moisture relative to its size. It can be dried out by heating and reused, and an indicating variety changes colour once it is saturated.
The trap is common salt, which feels damp because it is weakly hygroscopic, but it is not an effective drying agent. Silica gel adsorbs water at its surface and is the standard packaged desiccant.
- A desiccant takes up moisture to keep its surroundings dry.
- Silica gel is porous silicon dioxide with a very high surface area, making it an excellent moisture adsorbent.
- It is packed in sachets with medicines, cameras, electronics and food packaging.
- Silica gel can be regenerated by heating and reused; 'indicating' silica gel changes colour when saturated.
- Choosing common salt because it feels damp — it is only weakly hygroscopic, not a proper desiccant.
- Confusing adsorption (at a surface) with absorption (throughout the bulk).
Asked as the best example of a desiccant, or as the substance inside moisture-absorbing sachets — the answer is silica gel.
A small pouch containing silica gel is often found in bottles of medicine in tablet or powder form because silica gel
- (a) kills bacteria
- (b) kills germs and spores
- (c) absorbs moisture
- (d) absorbs all gases present inside the bottle
Answer(c) absorbs moisture
The same fact stated as an application — silica gel is packed with medicines precisely because, as a desiccant, it adsorbs moisture.
- practice — not a real PYQ
Silica gel keeps packaged goods dry by
- (a)killing bacteria
- (b)adsorbing moisture
- (c)releasing oxygen
- (d)absorbing light
Answer(b) adsorbing moisture.
- practice — not a real PYQ
Which one of the following is a common drying agent?
- (a)Polystyrene
- (b)Anhydrous calcium chloride
- (c)Sodium chloride
- (d)Glucose
Answer(b) Anhydrous calcium chloride — a common desiccant, like silica gel.