Re: why do things dissolve faster in a liquid when the liquid is stirred?

Certain solids will dissolve in certain liquids because the molecules in the liquid are able to “surround” the molecules or ions that make up a solid. The liquid is called the solvent and the solid is called the solute. We say that the molecules in the solvent are able to solvate the solute.

When you dissolve a solid like sugar is a liquid like water, the water molecules start to surround the sugar molecules on the surface of the solid. Eventually, sugar molecules start to break off from the solid and are surrounded by water molecules (the sugar molecules are being solvated by the water). Since the water molecules are constantly in motion, they keep “breaking off” more and more sugar molecules. Now, just above the solid sugar there are sugar lots of molecules dissolved in water. At this point, something called “diffusion” takes over. Diffusion means that molecules dissolved in a liquid like to go from high concentration to low concentration. The sugar molecules that are dissolved in water want to get as far away as possible from each other.

Diffusion usually happens very slowly. Eventually, the dissolved sugar molecules start moving farther and farther away from the solid sugar, and so more molecules from the solid sugar start to dissolve to take the place of those that are moving away. You can help speed up the process by stirring.

When you stir a glass of water with solid sugar in it, you are helping the water and sugar molecules to move faster. This helps diffusion happen faster. You can think of a glass of water with sand sitting on the bottom. When you stir it, the sand starts to move and eventually is evenly distributed in the water. This happens with the sugar, except that when you stop stirring the sugar molecules stay “suspended” in the water instead of falling to the bottom of the glass like the sand.