Salt and sugar dissolve in water at very high temperatures, roughly 100° C. Stirring achieves the same effect at room temperature or lower. How is this possible?
In: Physics
Salt and sugar do not dissolve in water only at 100°, they also do at lower temperatures.
Their solubility, which is how much sugar/salt you can dissolve in water before it becomes “full” and cannot accept anything else, changes with temperature, but the difference between room temperature and 100° C is very small: you can basically put in the same amount in water and it’ll be dissolved no problem.
Stirring moves the liquid around, helping it take more solute — when you drop a spoon of salt it falls to the bottom, and if the water is perfectly still the bottom of the glass will saturate because there’s too much salt in a single point. Now of course water is never perfectly still, and it will naturally slush around and bring more “clean” water to where the solute is, eventually absorbing everything.
You probably notice that boiling water absorbs salt or sugar easier because the boiling causes a strong “stirring” effect, making it easier to expose water to the solute.
Sugar and salt can dissolve at room temperature or cooler if it’s just sitting there, it just takes time. The reason a higher temperature helps salt or sugar dissolve is twofold. One reason is that hot water can hold more dissolved salt or sugar. The other is the movement of the molecules. in still water, the salt will dissolve, and the water molecules that have picked up salt or sugar won’t immediately be pushed away from the salt crystals. Eventually the salt water will diffuse through the whole thing, and more salt will dissolve, but that takes time. Stirring causes more water molecules to brush past the salt or sugar crystals and pick molecules off of them.
Salt and sugar dissolve at low temperatures as well. Adding heat to the solution increases the solubility, which means the water can dissolve MORE salt/sugar than room temperature water. As for the stirring, it speeds up the dissolution of the salt/sugar. But dissolution occurs at high AND low temperatures. The factors you mentioned are just ways to either dissolve more, or speed up the dissolution.
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