If you set fire to paper, it burns to ash. Why when you set fire to a plastic bag, does it melt into a solid lump?
It melts and then burn, if you keep heating it with a flame. If a combustible material undergoes a phase transition (solid-->liquid or liquid-->vapour) then its temperature cannot increase until the transition is completed; so the plastic cannot burn when it's still liquid, if the amount of combustible vapour present is not enough to produce a flame. After melting, it starts decomposing becoming a material with higher melting point so its temperature increases and at the same time the decomposition produces volatile combustibles; now it starts burning.
With wood, for example, the first part of the process doesn't happen because it doesn't melt;
so,
for this reason when you heat it with a flame its temperature can increase to a much higher value, then it starts decomposing forming volatile combustibles as methyl alcohol and acetone (if you need to clean your nails and shops are closed, you can distil wood!).
With a combustible like petrol, you already have combustible vapours present (if the temperature is not very low!) so it can burn even if it's still liquid, because what
actually burns are the vapours, not the liquid.
With wax, for example, you have to melt it with a flame and then keep increasing the temperature a little till the amount of vapours present are enough to generate a flame with air.
The same to burn a metal like magnesium, for example: your lighter gives heat to the metal; it absorbs heat to the point it melts; then, keeping the flame, the liquid tiny drop formed increases its temperature to the boiling point (or almost) which is ~ 1100°C and so the vapours formed can burn with air forming that Very brilliant white light.
This is the reason you can burn magnesium but not aluminum (it's very very difficult), even if they have similar melting points (~ 650 °C): they have Very different boiling point: ~ 1100°C for magnesium (you can reach that temperature with a lighter) and ~ 2500°C for aluminum! (you certainly cannot reach it with a lighter or with a burner).
The reason paper and wood forms ash is because they contains mineral salts, especially the potassium salt K
2CO
3. Infact they used ash to make soap, ~ 100 years ago (boiling ash with grease).
