Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: daveshorts on 30/04/2008 17:53:35

Title: Why does adding ash enable sugar to burn?
Post by: daveshorts on 30/04/2008 17:53:35

I have a query about a possible kitchen science. The original experiment was that you can't set light to a sugar lump with a  match (or even easily with a blowtorch) normally because it just caramelises, but if you add a little ash to the surface of the sugar, it will burn leaving a black charcoal like residue (very like the standard school, conc sulphuric demo), so it seems to be just burning the water off the sugar.

I have done some more experiments and found that it works in exactly the same way with sodium of potassium carbonate (which should be in the woodash) - I got the idea after seeing a yellow powder explosion in Pete Wother's talk and thought it might be something special about potassium carbonate, but sodium carbonate and hydrogen carbonate work too.

All the references I can find on the web just talk about something woolly involving metal ions acting as catalysts - I tried it with NaCl and unsurprisingly it has no effect...

I have thought of a mechanism, but I'm a physicist so in my experience it is probably wrong...

I thought that if you heat Na2CO3 it will break down to form Na2O and CO2 The sodium oxide will then rip water out of the sugar to form NaOH, making the sugar molecule unstable so the rest can break up and burn, releasing enough energy to dry out the NaOH and form Na2O again, in order to restart
the process.

If you have any better ideas I would love to know

Title: Why does adding ash enable sugar to burn?
Post by: lightarrow on 01/05/2008 10:13:58

Quote from: daveshorts on 30/04/2008 17:53:35

I have a query about a possible kitchen science. The original experiment was that you can't set light to a sugar lump with a  match (or even easily with a blowtorch) normally because it just caramelises, but if you add a little ash to the surface of the sugar, it will burn leaving a black charcoal like residue (very like the standard school, conc sulphuric demo), so it seems to be just burning the water off the sugar.

I have done some more experiments and found that it works in exactly the same way with sodium of potassium carbonate (which should be in the woodash) - I got the idea after seeing your yellow powder explosion and thought it might be something special about potassium carbonate, but sodium carbonate and hydrogen carbonate work too.

All the references I can find on the web just talk about something woolly involving metal ions acting as catalysts - I tried it with NaCl and unsurprisingly it has no effect...

I have thought of a mechanism, but I'm a physicist so in my experience it is probably wrong...

I thought that if you heat Na2CO3 it will break down to form Na2O and CO2 The sodium oxide will then rip water out of the sugar to form NaOH, making the sugar molecule unstable so the rest can break up and burn, releasing enough energy to dry out the NaOH and form Na2O again, in order to restart
the process.

If you have any better ideas I would love to know

Interesting, I didn't know of this effect.
NaOH or KOH however cannot be dried to their oxides with heat, even if, however, they are strong water absorbers; I would also think about a cathalytic effect of the high basicity of those compounds, but I really don't know.
P.S.
Which yellow powder explosion were you referring to?

Title: Why does adding ash enable sugar to burn?
Post by: daveshorts on 01/05/2008 10:24:25

Yes I am getting less convinced of it... the yellow powder is a mixture of sulphur potassium carbonate and pottasium nitrate, which actually goes off with a bang even if it isn't confined.