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The concept of dissolving is when a solid (called a solute) completely mixes with a liquid (called a solvent) at the molecular level to become a single visible phase and become a homogeneous mixture. At the molecular level, the individual solid molecules become separated and interact less with each other and more with surrounding solvent molecules. So a single solute molecule becomes surrounded and makes interactions with diffferent solvent molecules. Random molecular motion plays into this. As the solute molecules become more separated from each other, random molecular motion will continue to separate them among the solvent molecules, thus speeding up the dissolving process. This is why solutes dissolve better in hot solvents versus cold solvents. Hot solvents have more kinetic energy to impart to the molecules to increase the random molecular motions. An example is the dissolving of sugar in water. The individual sugar molecules separate from one another and become surrounded by the water molecules. Solutes like sugar with lots of surface area dissolve better because there is more exposed surface to the solvent.
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