Why does stirring your spoon in a cup dissolve salt or sugar?
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Just like mixing anything will make it more homogenous – you’re adding kinetic energy to the system by stirring – this causes those bonds holding the crystals together to break.
Dissolving is essentially making new bonds between water molecules and sugar/salt molecules. You need energy for that to happen. Stirring provides that energy.
The previous two answers are wrong. You don’t stir to heat the drink.
I’ll use sugar here, it’s the same for salt: If you just put sugar into the drink then it’ll sit in a pile at the bottom and start dissolving into the surrounding water. You quickly get a region with a lot of dissolved sugar molecules in the water around the solid sugar, some of these molecules stick to the sugar grains again: The process slows down while most of the drink doesn’t have any sugar yet. Stirring mixes the sugar-loaded water with the rest and also distributes the sugar grains better in the drink.
Sugar and table salt are highly soluble in water but there comes a limit where the water physically can’t accept any more of them (saturation point).
If you don’t stir the liquid, you will end up with some water that is saturated with them and other parts of the liquid not having any, but being unable to get to the solid at any great speed (water molecules can move around the bulk – diffusion – but this is a random process so it will take a long time for molecules to get from the top to the bottom of the container).
By agitating the container you are moving the liquid (and also the solid) around very quickly so they come into contact a lot more frequently, allowing more of the water molecules to get involved in dissolving the solute.
Water is a liquid, lots of molecules bouncing around in a sort of soupy wiggly blob. Salt and sugar are solid, lots of atoms stacked up like Lego bricks in a big crystal. Water molecules are like those Lego brick remover tools that can yank that stubborn thin 1×2 block off the corner of your castle. When this happens, the water molecule gets full, just like the Lego remover tool. Imagine if you had a Lego castle in your bedroom and then you completely filled it with living Lego remover tools. The outer layers of the castle would be removed immediately, but now most of the remover tools are full. Imagine they wiggle around a bit but don’t exactly swim like fish. So now the process stalls and you have partially disassembled castle. Now imagine someone comes in with a giant shovel and scoops the full remover tools away from the castle and let’s fresh ones get near. Even better, they bash the castle with the shovel and spread a bunch of castle chunks around the room. Now it will quickly end up with every single piece stuck in a remover tool.
Think of it as a stack of papers. Everyone in the room wants a piece of paper. But because they’re in a pile, they have to line up one by one to grab them. The process can speed up if we spread the papers around so more people can access them at once. The same sorta happens with water and sugar. We stir the water to spread the sugar around, so it can be absorbed by the water more efficiently rather than absorbing from the outside of the pile to the inside.
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