Leakage in domestic LPG (liquefied petroleum gas) cylinders is easily detected because a small amount of which strongly smelling substance is added to the otherwise odourless gas?

Introduction / Context:
Liquefied petroleum gas, commonly called LPG, is widely used as domestic cooking gas. In its pure form LPG is almost odourless, which would make gas leaks very dangerous because people could not detect them by smell. To improve safety, gas companies deliberately add a substance with a strong, unpleasant odour so that even a small leak can be noticed quickly. This question tests whether you know the type of compound that is added to LPG and the reason behind it, which is an important part of everyday chemistry awareness and safety education.

Given Data / Assumptions:

• LPG consists mainly of hydrocarbons such as propane and butane that have very little smell in pure form.
• The additive must have a strong and characteristic odour detectable at very low concentrations.
• The options include mercaptans, propylene, carbon disulphide, phosgene, and chloroform.
• We assume typical domestic gas cylinders used for cooking and heating.

Concept / Approach:
The compounds most commonly added to LPG to give it a warning smell are called mercaptans, also known as thiols. A typical example is ethyl mercaptan, chemically written as C2H5SH. Thiols contain a sulphur hydrogen group that gives them a very strong, unpleasant odour even at extremely low concentrations. This makes them ideal as odorants in fuel gases. Other substances listed in the options either do not have a suitable smell profile, are too dangerous, or are not used for this specific purpose. Therefore, the correct answer is mercaptans such as ethyl mercaptan.

Step-by-Step Solution:
Step 1: Recognise that LPG itself is nearly odourless, so a special additive is required for safety. Step 2: Recall that gas companies add a substance with a characteristic rotten cabbage or rotten egg like smell so that leaks can be detected quickly. Step 3: Identify mercaptans, especially ethyl mercaptan, as classic odorants added to LPG and natural gas supplies. Step 4: Compare this with propylene, which is another hydrocarbon gas without a strong warning odour, so it is not used as an odorant. Step 5: Note that carbon disulphide, phosgene, and chloroform are toxic industrial chemicals and solvents, not standard safety additives in household LPG.

Verification / Alternative check:
Information from gas utilities and safety leaflets usually states that LPG is odorised with ethyl mercaptan or similar thiol compounds. Safety posters often mention that if you smell a strong and unfamiliar gas odour in the kitchen, you should suspect an LPG leak and open doors and windows. Technical descriptions of gas distribution systems also list mercaptans as the class of compounds used for odorisation. There is no mention of propylene, carbon disulphide, phosgene, or chloroform being used as odorants in domestic LPG. This evidence supports mercaptans as the correct choice.

Why Other Options Are Wrong:
Propylene is a hydrocarbon similar to propane and would not add a strong identifying odour. Carbon disulphide has a smell but is highly toxic and flammable, and it is not the standard choice for LPG odorisation. Phosgene is a very toxic choking gas once used as a chemical weapon, so it would never be added deliberately to domestic gas. Chloroform is a volatile liquid with its own hazards and is not suitable or used as an LPG odorant. Thus, these alternatives do not match the safe, standard practice adopted worldwide.

Common Pitfalls:
Some learners may choose carbon disulphide or phosgene simply because they recognise them as smelly or toxic and assume that any strong smelling chemical could be used. Others might pick propylene because it looks chemically similar to gases already present in LPG. To avoid such mistakes, remember that the odorant must be effective at very low concentrations, reasonably stable, and compatible with fuel systems. Mercaptans meet these requirements and are specifically mentioned in textbooks and safety training materials.

Final Answer:
Leakage in LPG cylinders is easily detected because a small amount of mercaptans such as ethyl mercaptan is added to the otherwise odourless gas.