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Which of the following statements correctly explains why saturated hydrocarbons burn with a blue flame, while unsaturated hydrocarbons tend to burn with a yellow, sooty flame?
Saturated hydrocarbons react with nitrogen in the air to prevent soot formation.
Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio, leading to incomplete combustion and soot formation.
Unsaturated hydrocarbons contain more oxygen, which leads to incomplete combustion.
Saturated hydrocarbons have weaker bonds, so they burn more completely. Question ID : 441009323748 Option 1 ID : 4410091261191 Option 2 ID : 4410091261190 Option 3 ID : 4410091261188 Option 4 ID : 4410091261189 Status : Answered Chosen Option : B
Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio, leading to incomplete combustion and soot formation.
Saturated hydrocarbons (CnтАЛH2n+2тАЛ) have a higher hydrogen-to-carbon ratio, allowing them to burn completely with a sufficient oxygen supply, resulting in a clean blue flame. Unsaturated hydrocarbons (CnтАЛH2nтАЛ or CnтАЛH2nтИТ2тАЛ) possess a higher proportion of carbon, which requires more oxygen for complete combustion, leading to incomplete combustion and the release of unburnt carbon particles (soot) that glow yellow.
Saturated hydrocarbons (CnтАЛH2n+2тАЛ) have a higher hydrogen-to-carbon ratio, allowing them to burn completely with a sufficient oxygen supply, resulting in a clean blue flame. Unsaturated hydrocarbons (CnтАЛH2nтАЛ or CnтАЛH2nтИТ2тАЛ) possess a higher proportion of carbon, which requires more oxygen for complete combustion, leading to incomplete combustion and the release of unburnt carbon particles (soot) that glow yellow.
Think of a gas stove burner: a clear blue flame indicates a healthy, efficient mix of gas and air (perfect combustion), while a flickering, yellow flame suggests the burner is dirty or lacks enough air, resulting in soot accumulating on the bottom of your cookware.
CnтАЛH2n+2тАЛ+(23n+1тАЛ)O2тАЛтЖТnCO2тАЛ+(n+1)H2тАЛO тАФ Complete combustion of an alkane
High┬аHCтАЛ┬аratioтЯ╣Incomplete┬аCombustionтЯ╣Soot тАФ General principle
In saturated alkanes, the lower carbon percentage ensures that oxygen molecules (O2тАЛ) can readily oxidize all carbon atoms to carbon dioxide (CO2тАЛ). In unsaturated alkenes or alkynes, the high carbon content makes complete oxidation difficult in atmospheric air. These unburnt carbon particles become incandescent in the heat of the flame, emitting a characteristic yellow-orange light, a phenomenon known as soot formation.
Blue flames indicate complete combustion due to high oxygen availability.
Yellow flames indicate incomplete combustion and the presence of glowing carbon particles.
Unsaturated hydrocarbons have multiple bonds (double or triple), resulting in fewer hydrogen atoms per carbon atom compared to alkanes.
Saturated fuels like LPG provide high heat efficiency without blackening utensils.
Incomplete combustion releases carbon monoxide (CO), a toxic and hazardous gas.
LPG and Natural Gas (methane) use in domestic cooking stoves.
Welding (oxy-acetylene flame) requires controlled oxygen ratios to prevent excess soot.
| Feature | Saturated Hydrocarbons | Unsaturated Hydrocarbons |
|---|---|---|
Combustion Property | Blue, clean flame | Yellow, sooty flame |
Saturated hydrocarbons (CтИТC single bonds) burn completely in air.
Unsaturated hydrocarbons (C=C or CтЙбC bonds) have a higher carbon density, leading to oxygen deficiency.
Option A is incorrect because nitrogen is chemically inert and does not prevent soot formation.
Option C is incorrect because unsaturated hydrocarbons do not contain oxygen in their molecular structure.
Option D is incorrect because the type of bond (sigma vs pi) does not determine completeness of combustion as much as the carbon-to-hydrogen ratio does.
B is correct тАФ Unsaturated hydrocarbons have a higher carbon-to-hydrogen ratio, leading to incomplete combustion and soot formation.
Always remember: higher carbon percentage in a fuel requires more oxygen for combustion. If oxygen is limited, that excess carbon becomes soot.
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