Q.Explain the difference between complete and incomplete combustion of an alkane fuel, and state why this matters for an appliance burning LPG indoors.

When an alkane burns in a PLENTIFUL supply of oxygen, every carbon atom is fully oxidised to carbon dioxide and every hydrogen to water -- complete combustion, releasing the maximum possible heat, and burning with a clean blue flame. When the oxygen supply is LIMITED (insufficient air mixing with the fuel), combustion is incomplete: carbon is only partially oxidised to carbon monoxide, $\text{CO}$, or, if oxygen is even scarcer, not oxidised at all and released as fine soot (unburnt carbon particles) -- visible as a smoky, yellow flame. This matters critically for an indoor LPG appliance because carbon monoxide is a colourless, odourless but highly toxic gas that binds far more strongly to haemoglobin than oxygen does, starving the body of oxygen and causing poisoning or death at high enough concentrations in a poorly ventilated room -- which is exactly why LPG heaters/stoves us …

Compare the oxygen supply in each case and track what each carbon atom is oxidised to as a result; connect the incomplete-com …

Thinking soot and carbon monoxide are the same hazard (soot is a visible nuisance/pollutant; CO is a colourless, far more acutely toxic gas); assuming a yellow flame is …