CAPF AC 2021 Q13: Science & Technology – Chemistry
Packets of potato chips are usually flushed with which one of the following gases to prevent rancidity?
- (a) Carbon dioxide
- (b) Hydrogen
- (c) Nitrogen
- (d) Oxygen
Answer: (c) Nitrogen
Explanation
Gas in packets of chips
How to think: rancidity is the oxidation of fats and oils.
So the packet needs a gas that keeps oxygen out and does not react with the food.
That is nitrogen, an unreactive and cheap gas.
Rancidity: when fats and oils in food are oxidised, the food smells and tastes bad.
| Way to prevent rancidity | How it works |
|---|---|
| Flushing with nitrogen | Removes oxygen from the packet |
| Adding antioxidants | Slow down oxidation |
| Airtight containers | Keep air out |
| Refrigeration | Slows the reaction |
The options checked:
| Gas | Verdict |
|---|---|
| Carbon dioxide | Not the gas used for chips in the NCERT account |
| Hydrogen | Catches fire easily |
| Nitrogen | Unreactive; used |
| Oxygen | The very gas that causes rancidity |
Answer: (c) Nitrogen.
Why each option is right or wrong
- (a) Incorrect. Carbon dioxide is used in fizzy drinks, not for flushing chips packets.
- (b) Incorrect. Hydrogen is highly flammable and unsafe for food packets.
- (c) CORRECT. Nitrogen is unreactive and keeps oxygen away from the oil in the chips.
- (d) Incorrect. Oxygen oxidises the oil and causes rancidity.
Key point
Rancidity = oxidation of fats and oils. Prevented by nitrogen flushing, antioxidants, airtight containers and refrigeration.
Remember
Chips packets: nitrogen, to stop rancidity.
Common mistakes
- Thinking the packet is filled with ordinary air; air has oxygen in it.
- Marking oxygen, which is the cause of rancidity and not the cure.
- Confusing rancidity with corrosion, which is the oxidation of metals.
Exam tip
Educated guess: rancidity is oxidation, so oxygen is out. Hydrogen burns. Nitrogen makes up 78% of air, so it is the cheap, safe choice.
Sources
- NCERT Class X Science — Chemical Reactions and Equations (rancidity)
- NCERT Exemplar Class X Science — Chemical Reactions and Equations