A student burned three alcohols in separate spirit burners and held a cold glass beaker just above each flame. Fig. 8.1 shows the flames produced. The stude...

This question is about the trend in combustion as alcohol molecules get larger, and about safe practical technique.

(a) For butan-1-ol, the largest molecule of the three: the flame is yellow / orange (luminous) [1] and the soot on the beaker is heavy / a lot [1]. Following the trend of methanol (clean blue, no soot) then ethanol (mostly blue, slight soot), the biggest molecule is the sootiest.

(b) As the alcohol molecule gets larger the flame becomes more yellow / sootier [1], because combustion becomes less complete - there is a higher proportion of carbon in the molecule so less oxygen reaches each carbon atom [1]. Unburnt carbon glows and deposits as soot.

(c) The black solid is carbon (soot) [1]. Its presence shows the combustion is incomplete, meaning there was not enough oxygen to convert all the carbon to carbon dioxide [1]. Complete combustion of an alcohol gives only carbon dioxide and water, for example \( \text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \); a shortage of oxygen instead leaves carbon and carbon monoxide.

(d) To burn the same amount of each alcohol: measure out the same volume of each with a measuring cylinder, or weigh each burner before and after and burn until the same mass has been used on a balance [1]; and keep the burner the same distance from the beaker and burn for the same time [1].

(e) Any two safety precautions [2]: tie back long hair and loose clothing; wear eye protection; keep the bottles of flammable alcohol well away from the flames; and never refill a burner while it is lit or near a flame.