Question

what does the black substance remaining in each test tube indicate ?​

Answer 1

Answer:

where is the picture

I think it's will be soot or carbon

Answer 2

1)The general formula for combustion of Methanol is but this is only for a complete combustion, now if you limit the oxygen supply, indicative by the red/yellow glow(which is due to the incandescence of the tiny soot particle), Carbon monoxide and soot is produced, which is the black substance at the bottom of your test ...

2)The black coloured substance is copper oxide. It has formed due to reaction of copper with oxygen.A test tube is a clear glass or plastic container that is much longer than it is wide, commonly has a U-shaped bottom, and has an open top. Test tubes are used to hold, mix, and heat chemical experiments. They are used as homes for microorganisms when people want to culture (grow) them.

i hope it helpfull to you