Fuels and Energy Sources – NDA Chemistry PYQs

Q1. NDA 2010-II
Which one of the following statements is not correct?
(a) Diesel burns with air in a diesel engine at a high temperature at which nitrogen and oxygen from air can combine whereas petrol burns at a lower temperature at which nitrogen and oxygen cannot combine
(b) Diesel contains some nitrogenous compounds whereas petrol does not
(c) Nitrogen from air is easily dissolved in diesel but it is not soluble in petrol
(d) A catalyst is used in diesel engines which contains nitrogen, whereas petrol engines do not need such catalysts
Answer: (c) Nitrogen from air is easily dissolved in diesel but it is not soluble in petrol
This is false. Nitrogen is not significantly dissolved in either petrol or diesel. The real reason diesel engines produce more NOₓ is that diesel burns at higher temperatures, at which N₂ and O₂ from air react to form NO. Option (a) correctly states this. Options (b) and (d) are also incorrect but less directly tested here.

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Q2. NDA 2012-I
Which one among the following is an electrochemical cell that cannot be charged?
(a) Electrolytic cell (b) Storage cell (c) Primary cell (d) Fuel cell
Answer: (d) Fuel cell
A fuel cell converts chemical energy directly to electricity through an electrochemical reaction. It cannot be “recharged” in the way a storage cell is. It works continuously as long as fuel (hydrogen) is supplied. A storage cell (battery) can be recharged. An electrolytic cell uses electricity to drive chemical reactions.

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Q3. CDS 2009-I
Which one of the following petroleum refinery products has the lowest boiling point?
(a) Kerosene (b) Diesel (c) Gasoline (d) Lubricating oil
Answer: (c) Gasoline
Gasoline (petrol) is the lightest liquid fraction from petroleum distillation. It has the lowest boiling point (30-200°C) among the liquid petroleum products. The boiling point order is: Gasoline < Kerosene < Diesel < Lubricating oil.

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Q4. CDS 2011-I
Which one among the following is a foul smelling agent added to LPG cylinder to help the detection of gas leakage?
(a) Ethanol (b) Thioethanol (c) Methane (d) Chloroform
Answer: (b) Thioethanol
Pure LPG (butane and propane) is colourless and odourless. A gas leak would be impossible to detect by smell. Thioethanol (ethanethiol, C₂H₅SH) is a sulphur-containing organic compound with an extremely strong, unpleasant smell. Even tiny amounts in the air are immediately detectable.

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Q5. CDS 2014-II
Addition of ethylene dibromide to petrol:
(a) increases the octane number of fuel (b) helps elimination of lead oxide (c) removes the sulphur compound in petrol (d) serves as a substitute of tetraethyl lead
Answer: (b) helps elimination of lead oxide
Tetraethyl lead (TEL) was added to petrol to increase octane number. When TEL burned, it left lead oxide deposits inside the engine. Ethylene dibromide reacted with this lead oxide to form volatile lead bromide, which was expelled through the exhaust. This kept the engine clean of lead deposits.

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Q6. CDS 2014-II
The main constituent of Gobar gas is:
(a) Ethane (b) Methane (c) Propane (d) Acetylene
Answer: (b) Methane
Gobar gas is biogas made from cow dung. Bacteria anaerobically decompose the organic matter in cow dung to produce a gas that is mainly methane (CH₄). Methane makes up 55-65% of biogas. It is the combustible component that makes biogas useful as a fuel.

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Q7. CDS 2015-I
Which one among the following fuels is used in gas welding?
(a) LPG (b) Ethylene (c) Methane (d) Acetylene
Answer: (d) Acetylene
Acetylene (ethyne, C₂H₂) burns with oxygen to produce a flame of about 3500°C. This is hot enough to melt any metal. LPG and methane burn at lower temperatures and are used for cooking, not welding. Ethylene is not used as a welding fuel.

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Q8. CDS 2010-I
Which among the following elements is abundant on the lunar surface and holds the potential to put an end to the energy crisis of the earth?
(a) Helium-I (b) Helium-II (c) Helium-III (d) Helium-IV
Answer: (c) Helium-III (Helium-3)
Helium-3 is a rare isotope of helium on Earth but is deposited on the Moon’s surface by solar winds. Helium-3 is a potential fuel for nuclear fusion reactors. If fusion technology using He-3 is developed, it could provide virtually limitless clean energy.

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Q9. NDA 2011-I
The foul-smelling substance added to domestic LPG for easy detection of leakage is:
(a) Thioethanol (b) Ethanol (c) Methanol (d) Chloroform
Answer: (a) Thioethanol
Thioethanol (ethanethiol) is added to LPG at very low concentrations. It has a powerful rotten smell that is detectable even at one part per billion. This allows people to immediately notice even small gas leaks and take safety action.