Dissolving and melting are different changes
Melting turns a solid into a liquid form of the same substance, while dissolving disperses a substance among particles of a solvent. Sugar stirred into room-temperature water dissolves without becoming a pool of liquid sugar. The disappearance of visible grains does not tell you that melting occurred.
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A missing grain can have two explanations
When a solid melts, it changes phase. Ice becomes liquid water because its molecular arrangement changes while the substance remains water.
When a substance dissolves, its particles become distributed through a solvent. Sugar molecules can separate from their crystal and become surrounded by water molecules, even though the water is far below the temperature needed to melt dry sugar.
Visual disappearance is not enough to identify the process. A solid can stop being visible because its particles are dispersed too finely to see.
- Melting is a phase change.
- Dissolving forms a solution.
- Heating can affect both, but is not required for every case of dissolving.
The solvent matters in dissolution. A substance that dissolves readily in water may behave very differently in another liquid.