When different types of organic compounds burn, they often produce flames of different colours. This colour difference can give us clues about the type of compound and how completely it is burning.

Combustion of Unsaturated Organic Compounds

Unsaturated compounds are organic molecules that contain carbon-carbon double bonds ($\text{C=C}$) or triple bonds ($\text{C}\equiv\text{C}$). Examples include alkenes (like ethene) and alkynes (like ethyne). These compounds have a higher percentage of carbon compared to saturated compounds with the same number of carbon atoms.

During combustion, organic compounds react with oxygen. Complete combustion occurs when there is enough oxygen, producing carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$). Incomplete combustion happens when the oxygen supply is limited, leading to the formation of carbon monoxide ($\text{CO}$), water, and often unburnt carbon particles (soot).

Unsaturated compounds, due to their higher carbon content, require more oxygen for complete combustion. If the oxygen supply is not sufficient (which is common during burning in air), incomplete combustion occurs. This incomplete combustion releases tiny solid particles of carbon, known as soot.

These hot soot particles in the flame glow intensely, emitting yellow light. This is why the combustion of unsaturated compounds typically produces a characteristic yellow flame.

Comparing Flames: Saturated vs. Unsaturated

In contrast, saturated compounds (like alkanes) contain only carbon-carbon single bonds ($\text{C-C}$). They have a lower carbon percentage than their unsaturated counterparts with the same number of carbons. Saturated compounds generally undergo complete combustion more easily in air because they require less oxygen per carbon atom.

Complete combustion results in a clean flame with very little or no soot production. Such flames are often blue or pale blue, as the light is emitted primarily by excited molecules and radicals formed during the combustion process, rather than glowing soot particles.

Analyzing the Options

• red flame: A red flame is not typically associated with the standard combustion of hydrocarbons like unsaturated compounds.
• yellow flame: As explained, the incomplete combustion of unsaturated compounds, due to their high carbon content and soot production, results in a yellow, sooty flame. This matches the observation during the burning of substances like cooking gas (which contains some higher hydrocarbons) or candles (wax is a hydrocarbon).
• clean flame: A clean flame is characteristic of complete combustion, which is more common with saturated hydrocarbons burning in sufficient oxygen, producing a blue flame.
• blue flame: A blue flame indicates relatively complete combustion and is typical for saturated hydrocarbons burning efficiently or for unsaturated hydrocarbons burning with a very good supply of oxygen (like in a Bunsen burner with the air hole open).

Therefore, during the combustion of unsaturated compounds in typical conditions (like burning in air), a yellow flame is usually observed due to incomplete combustion and soot formation.

Revision Table: Combustion Flames

Additional Information on Hydrocarbon Combustion

The colour of a flame depends on several factors, including the fuel being burned and the amount of oxygen available. For hydrocarbons:

• Complete Combustion: Sufficient oxygen leads to carbon dioxide and water. The flame is typically blue or pale blue and clean. $\text{Hydrocarbon} + \text{Excess }\text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Energy}$
• Incomplete Combustion: Limited oxygen leads to carbon monoxide, water, and carbon (soot). The flame is often yellow and sooty. $\text{Hydrocarbon} + \text{Limited }\text{O}_2 \rightarrow \text{CO} + \text{H}_2\text{O} + \text{C} + \text{Energy}$

Unsaturated hydrocarbons' tendency to produce a yellow flame is primarily due to their higher carbon-to-hydrogen ratio compared to saturated hydrocarbons. This higher ratio means that for the same amount of carbon, there's less hydrogen to aid in complete combustion with available oxygen, leading to more unburnt carbon (soot) forming.