Why a silica powder is melting but not burnt when heated?
Silica (silicon dioxide) is "burnt" silicon. You cannot burn it for the same reason you cannot burn carbon dioxide.
What I meant as a burnt is such as the below image:
That is probably the result of a reaction between bread and oxygen.
Silica is the result of a reaction between silicon and oxygen. It does not look black, but it is similar to a burnt bread.
Actually burnt bread is more like effect of the carbohydrates decomposition - water leaves molecules, leaving mostly pure carbon. Silica is not similar to carbohydrates.
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