有關melting point 既問題

the melting point of a substance is related to its structure.

when the force between particles(atom, molecule or ion) are weak,

less energy is needed to convert them from solid state to liquid state.

so the melting point is low.

on the contrary, when the force between particles are strong, more

energy is needed to convert them from solid state to liquid state.

so the melting point is high.

for sugar and starch , the force between molecules are weak, so their

melting point are weak.

but in the salt , the force between ions are strong, so their melting

point is high.

Sugar is a collective name and I assume you are talking about the 昔table sugar- sucrose. The melting point of sucrose is 186C. However, during the heating, some of the sugar may dehydrate and give the brownish color. We call it caramelization (焦糖化).
Salt (NaCl) has a melting point of 801C which is much higher than sugar because the NaCl form a giant ionic lattice. In this structure, the ions are held together by strong ionic bond. In contrast, the sugar is held together by relatively weak Van de Waals Force. So, its melting point is lower.
In case of starch, the starch is a polymer of glucose. It has no defined melting point. Upon heating, instead of turning to liquid, this sugar chains may burn, dehydrate to yield carbon and water. In fact, not all substances have a melting point.