Why do some substances burn, while others do not?
Let's first look into the process of burning and what it exactly is. Burning is the reaction of oxidation of a substance by oxygen. Oxygen takes away electrons from a substance, breaks the bonds between atoms, puts itself into molecules and turns a substance into a mixture of oxides. The difference between burning and other oxidation reactions (e.g. iron corrosion) is that during burning a large amount of heat is released. 2 conditions must be carried out for a substance to burn:
When it is oxidized, a large amount of heat must be released, enough to sustain burning
The oxidation reaction go quite fast
Why doesn't a piece of sugar burn?
The oxidation of sugar is capable of releasing enough heat. It's easy to remember, if you look at a pack of cookies and its calorie capacity: the amount of calories in food is the heat generated during its oxidation in the body. However, the oxidation of sugar is quite slow, so sugar does not burn.
Why does sugar start to burn when you add ashes?
In the burning of sugar ashes serve as an accelerator (catalyst) of the reaction. A catalyst is a substance that makes reactions, which theoretically should occur, but in practice are too slow, possible in the real world. Imagine that you are on a valley, and you need to go down to the sea, but a mountain range has grown on your way. To go straight through the mountains is one way, it is long and difficult. It is a whole other thing to find a guide who knows the path through a narrow pass and can take you directly to the sea through this narrow hidden pass without tedious and dangerous climbs. That is the role that a catalyst plays - it allows a chemical reaction to find a way to a more favorable state, spending minimal "strength" (energy) on the road. Usually in nature a more favorable state means a state with minimum energy.
What are ashes? What are they made of?
Ashes are substances that cannot continue to burn (be oxidized). This means that ashes may consist of salts or oxides. Ashes contain metal carbonates - salts of carbonic acid (H2CO3). These exact substances act as catalysts (accelerators) of the sugar burning reaction. The best catalysts are lithium salts which are found, for example, in cigarette ashes. However, plain paper ashes, which contain sodium and calcium salts, may also catalyze the combustion of sugar.
What is the result of burning sugar?
If sugar burns through to the end, the result of combustion should turn out to be carbon dioxide CO2. However, in reality, sugar does not fully down down and a black mass of products of incomplete oxidation of sugar - complex organic substances - aldehydes, ketones, carboxylic acids are left after it.