CAPF AC 2022 Q35: Science & Technology – Chemistry
Borax is prepared from
- (a) calcium carbonate
- (b) magnesium carbonate
- (c) potassium carbonate
- (d) sodium carbonate
Answer: (d) sodium carbonate
Explanation
How borax is made
How to think: borax is a compound of sodium.
Its name is sodium tetraborate decahydrate, Na2B4O7.10H2O.
So the carbonate used to make it must be the sodium one.
The process: the mineral colemanite (a calcium borate) is boiled with a solution of sodium carbonate.
Calcium carbonate settles out, and borax crystallises from the liquid.
| Fact | Detail |
|---|---|
| Formula | Na2B4O7.10H2O |
| Made from | Colemanite + sodium carbonate |
| Natural form | Tincal, found in dry lake beds |
| Uses | Heat-resistant glass (borosilicate), flux in soldering, antiseptic, borax bead test |
Answer: (d) sodium carbonate.
Why each option is right or wrong
- (a) Incorrect. Calcium carbonate is a by-product of the process, not the reagent.
- (b) Incorrect. Magnesium carbonate is not used; borax contains no magnesium.
- (c) Incorrect. Potassium carbonate would give a potassium salt, not borax.
- (d) CORRECT. Borax is made by treating colemanite with sodium carbonate.
Key point
Borax: Na2B4O7.10H2O, sodium tetraborate decahydrate; from colemanite + sodium carbonate, or from the mineral tincal. Uses: borosilicate glass, flux, antiseptic, borax bead test for coloured metal ions.
Remember
Borax is a sodium salt, made with sodium carbonate.
Common mistakes
- Marking calcium carbonate because the ore colemanite contains calcium.
- Mixing up borax with boric acid (H3BO3), a different boron compound.
- Forgetting that the Na in the formula of borax stands for sodium.
Exam tip
Educated guess: borax is a sodium salt. Only one option can supply sodium.
Sources
- NCERT Class XI Chemistry — The p-Block Elements (some important compounds of boron: borax)
- J. D. Lee, Concise Inorganic Chemistry — Group 13 elements