CAPF AC 2020 Q12: Science & Technology – Chemistry
- (a) Ozone
- (b) Peroxyacetyl nitrate
- (c) Polyaromatic hydrocarbons
- (d) Nitric oxide
Answer: (c) Polyaromatic hydrocarbons
Explanation
Photochemical smog
How to think: recall the NCERT list of five: ozone, nitric oxide, acrolein, formaldehyde, PAN.
The option outside the list is the answer.
Photochemical smog forms when sunlight acts on nitrogen oxides and unburnt hydrocarbons from vehicles.
It occurs in a warm, dry and sunny climate.
It is called oxidising smog.
| Component of photochemical smog | In the options? |
|---|---|
| Ozone | Yes, option (a) |
| Peroxyacetyl nitrate (PAN) | Yes, option (b) |
| Nitric oxide | Yes, option (d) |
| Acrolein | Not listed |
| Formaldehyde | Not listed |
Polyaromatic hydrocarbons are not on the NCERT list.
They come from incomplete burning of coal, wood, fuel and tobacco, and are found in soot.
Two kinds of smog:
| Feature | Classical smog | Photochemical smog |
|---|---|---|
| Climate | Cool and humid | Warm, dry and sunny |
| Made of | Smoke, fog and sulphur dioxide | Ozone, PAN, nitrogen oxides |
| Nature | Reducing | Oxidising |
Answer: (c) Polyaromatic hydrocarbons.
Why each option is right or wrong
- (a) Incorrect. Ozone is a main component of photochemical smog.
- (b) Incorrect. Peroxyacetyl nitrate (PAN) is a component and a strong eye irritant.
- (c) CORRECT. Polyaromatic hydrocarbons are not listed as a component of photochemical smog.
- (d) Incorrect. Nitric oxide from vehicle exhaust is a component and starts the reactions.
Key point
Photochemical smog (oxidising; warm, dry, sunny): ozone, nitric oxide, acrolein, formaldehyde, PAN. Classical smog (reducing; cool, humid): smoke, fog, sulphur dioxide.
Remember
Photochemical smog = ozone + PAN + nitrogen oxides, in sunlight.
Common mistakes
- Mixing up classical smog (sulphur dioxide) and photochemical smog (ozone, PAN).
- Assuming any hydrocarbon name in the options must be a smog component.
- Thinking ozone is always good; at ground level it is a pollutant.
Exam tip
NCERT list of five: ozone, nitric oxide, acrolein, formaldehyde, PAN. Anything else is the odd one.
Sources
- NCERT Class XI Chemistry — Environmental Chemistry (smog)
- Central Pollution Control Board — National Ambient Air Quality Standards (November 2009)