CAPF AC 2020 Q8: Science & Technology – Chemistry
Which one of the following ingredients does NOT constitute paint?
- (a) Turpentine oil (thinner)
- (b) Poly-hydroxy phenols (anti-skinning agent)
- (c) Thiourea (accelerator)
- (d) Titanium dioxide (pigment)
Answer: (c) Thiourea (accelerator)
Explanation
Constituents of paint
How to think: name the four parts of a paint: pigment, binder, thinner, drier.
Then find the option that fits none of them.
A paint is a mix of a pigment, a binder, a thinner and small amounts of additives.
| Constituent | Job | Example |
|---|---|---|
| Pigment | Gives colour and hiding power | Titanium dioxide (white), zinc oxide |
| Binder (vehicle) | Forms the film that holds the pigment | Linseed oil, resins |
| Thinner | Lowers thickness so the paint spreads | Turpentine oil |
| Drier | Speeds up drying | Compounds of cobalt, manganese and lead |
| Anti-skinning agent | Stops a skin forming in the can | Poly-hydroxy phenols |
The options checked:
| Option | Verdict |
|---|---|
| (a) Turpentine oil (thinner) | A constituent of paint |
| (b) Poly-hydroxy phenols (anti-skinning agent) | A constituent of paint |
| (c) Thiourea (accelerator) | Not a constituent of paint |
| (d) Titanium dioxide (pigment) | A constituent of paint |
Thiourea is not an ingredient of paint.
It is known as an accelerator in rubber vulcanisation.
Answer: (c) Thiourea (accelerator).
Why each option is right or wrong
- (a) Incorrect. Turpentine oil is the common thinner in oil paints.
- (b) Incorrect. Poly-hydroxy phenols are added as anti-skinning agents.
- (c) CORRECT. Thiourea does not constitute paint; paint uses driers, not this accelerator.
- (d) Incorrect. Titanium dioxide is the most widely used white pigment.
Key point
Paint = pigment (titanium dioxide) + binder (linseed oil) + thinner (turpentine) + drier (cobalt, manganese compounds) + anti-skinning agent (poly-hydroxy phenols).
Remember
Paint: pigment, binder, thinner, drier. Thiourea is not in it.
Common mistakes
- Rejecting poly-hydroxy phenols only because the name is unfamiliar.
- Assuming accelerator means the same as the drier used in paint.
- Forgetting that titanium dioxide has replaced white lead as the white pigment.
Exam tip
Educated guess: thinner, anti-skinning agent and pigment are all paint words. Accelerator is a rubber word.
Sources
- Jain and Jain — Engineering Chemistry, chapter on Paints (constituents of paint)
- Encyclopaedia Britannica — paint