One student observation was ‘as the candle
burns it gets shorter.’ This prompted the question and
problem ‘Where does the wax go to?’ from the tutor.
The students were unable to answer this definitively.
The tutor suggested some experiments. First the
students were instructed to blow their candles out,
‘What did you see?’ There were sometimes plumes
of white smoke, and the students concluded this was
wax steam leaving the candle, and that this was what
burnt in the candle flame.
Next the tutor suggested that students, with
the use of tongs, place a slide over the flame.
Students observed that black soot collected on the
slides in large patches. The tutor then prompted
students to place a slide actually into the center of the
flame. Soot collected in a ring shape, with the center
of the ring which had been held in the middle of the
flame being clear of soot. The students thus
concluded that smoke is only produced at the outside
of the flame, and not in the middle. One student
thought this was because the center of the flame was
the hottest and that the soot was simply burnt away
without collecting on the slide.