Which of the following gases is released during the decomposition of organic matter?

Step 1: Analyze the question The question asks which gas is released when organic matter (dead plants, animals, waste) decomposes. This is mostly anaerobic decay carried out by bacteria and fungi.

Step 2: Explain the concept Decomposers break down proteins, carbohydrates and other compounds in organic matter. The gases produced depend on the compounds being broken down:

• Carbohydrates give carbon(IV) oxide (CO₂), and methane (CH₄) in the absence of oxygen.
• Proteins contain sulphur (in amino acids such as cysteine and methionine). When they are broken down by bacteria, especially without oxygen, hydrogen sulphide (H₂S) is released. This is the "rotten egg" smell of decaying matter.
• Nitrogen compounds give ammonia (NH₃).

Step 3: Evaluate the options

• A. Carbon(II) oxide (CO): This comes from incomplete combustion of fuels, not from decay. Wrong.
• B. Sulphur(IV) oxide (SO₂): This comes from burning sulphur-containing fuels and from volcanoes. Wrong.
• C. Hydrogen sulphide (H₂S): This is produced by bacterial decomposition of sulphur-containing proteins. Correct.
• D. Nitrous oxide (N₂O): It is produced in small amounts by denitrifying bacteria in soil, but it is not the typical gas associated with decomposition of organic matter. Wrong here.

Step 4: Answer The correct option is **C.

Exam tip: Remember "decay = rotten egg smell = H₂S". Also, CO and SO₂ are mostly linked to burning (combustion), not decay. If you see "decomposition" with these options, think of the smell of rotting matter.