Let's analyze the question about burning hydrocarbon fuels and what a yellow flame with lots of black smoke tells us about the fuel.

Understanding Hydrocarbon Combustion and Flame Colour

When hydrocarbon fuels burn, they react with oxygen in the air. This process is called combustion. The colour of the flame and the presence of smoke provide clues about how complete the combustion is.

• Complete Combustion: Occurs when there is enough oxygen available for the fuel to burn completely. The products are typically carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)). This type of combustion usually produces a blue flame and very little or no smoke. A general equation for complete combustion of a hydrocarbon (\(\text{C}_x\text{H}_y\)) is: \(\text{C}_x\text{H}_y + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}\).
• Incomplete Combustion: Occurs when there is not enough oxygen available. In addition to \(\text{CO}_2\) and \(\text{H}_2\text{O}\), incomplete combustion can produce carbon monoxide (\(\text{CO}\)) and unburned carbon particles (soot). These soot particles glow brightly when hot, causing the flame to appear yellow. The release of these soot particles is seen as black smoke. A general equation for incomplete combustion is: \(\text{C}_x\text{H}_y + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{CO} + \text{C}\) (soot).

Why Some Hydrocarbons Cause Sooty Flames

Hydrocarbons are compounds made only of hydrogen and carbon atoms. They can be classified based on their structure:

• Saturated Hydrocarbons: These have only single bonds between carbon atoms (e.g., alkanes like methane, ethane, propane). They have a higher proportion of hydrogen atoms relative to carbon atoms. When they burn, they tend to undergo more complete combustion if sufficient oxygen is present, producing a clean blue flame.
• Unsaturated Hydrocarbons: These have double or triple bonds between carbon atoms (e.g., alkenes like ethene, alkynes like ethyne). They have a lower proportion of hydrogen atoms relative to carbon atoms compared to saturated hydrocarbons. Because they have more carbon atoms per molecule compared to saturated hydrocarbons of similar molecular weight, they require more oxygen for complete combustion. If the oxygen supply is limited (which is often the case when burning in air), they are more likely to undergo incomplete combustion, producing a yellow, sooty flame and black smoke due to the formation of unburned carbon particles (soot).

Analyzing the Options Based on Flame and Smoke

The observation is a yellow flame with lots of black smoke. This indicates incomplete combustion, which results from a less favourable carbon-to-hydrogen ratio for complete burning in limited oxygen.

• Option 1: made of saturated hydrocarbons. Saturated hydrocarbons typically burn cleanly with a blue flame under sufficient oxygen. This option is unlikely to produce a yellow, sooty flame and black smoke.
• Option 2: made of unsaturated hydrocarbons. Unsaturated hydrocarbons have a higher carbon content relative to hydrogen content compared to saturated ones. This makes them more prone to incomplete combustion, leading to the formation of soot which causes a yellow flame and black smoke. This option aligns with the observation.
• Option 3: burning completely. Complete burning produces a blue flame and no black smoke. The observation of a yellow flame and black smoke indicates incomplete combustion, not complete combustion.
• Option 4: wet. While water in fuel can affect combustion efficiency and potentially lead to some incomplete burning, it is not the primary characteristic of the fuel's structure that directly causes a yellow, sooty flame and significant black smoke in the way that the type of hydrocarbon (saturated vs. unsaturated) does. The yellow flame and black smoke are characteristic of the carbon content and combustion completeness, which is strongly influenced by the hydrocarbon's structure.

Based on the relationship between hydrocarbon structure, combustion completeness, and flame characteristics, a yellow flame with lots of black smoke is a strong indicator that the fuel is made of unsaturated hydrocarbons, which are more prone to producing soot during combustion.

Revision Table: Combustion Types

Additional Information on Soot Formation and Flame Luminosity

The yellow colour of a flame during incomplete combustion is due to the incandescence (glowing due to heat) of tiny solid carbon particles (soot) formed in the oxygen-deficient environment. These particles are heated to high temperatures in the flame and emit light across the visible spectrum, with the peak intensity often in the yellow range, resulting in a bright, luminous yellow flame. The black smoke seen is simply the collection of these unburned soot particles escaping the flame.

The tendency of unsaturated hydrocarbons to produce more soot is linked to their higher carbon-to-hydrogen ratio. More carbon atoms are present per molecule relative to hydrogen atoms, making it harder for all the carbon to be fully oxidized to \(\text{CO}_2\) when oxygen is limited, thus leading to more leftover carbon forming soot.