Mixed, not changed
Drop a spoon of salt into water and stir. The grains shrink, then vanish, and the water looks exactly as it did. Taste it and the salt is plainly still there. Nothing has been created or destroyed; the salt has simply been taken apart into pieces too small to see and spread evenly through the water. That is a solution: a mixture so thoroughly blended that it looks like a single substance.
The thing that dissolves is the solute, here the salt. The thing it dissolves in is the solvent, here the water. Neither has become something new. Boil the water away and the salt is back, grain for grain.
How it happens
Water molecules are constantly moving and constantly bumping into the surface of the salt. Salt is built from charged particles, and water molecules, with their slightly charged ends, tug at them. Now and then a tug wins, a particle leaves the crystal, and the water molecules crowd round it and carry it off. Switch to Particles to watch that happening one particle at a time.
Only the surface of the pile can be attacked, so a fine powder dissolves faster than a lump: more surface, more places for the water to get a grip.
When the water is full
There is a limit. At any temperature, a given amount of water can hold only so much of a solute. Once it holds that much, the solution is saturated, and any more you add just sits at the bottom no matter how long you stir. The water is not lazy; it is busy. Particles are still leaving the pile, but just as many are landing back on it.
Try it: choose Heap and wait. Then warm the water and watch the pile shrink.
Hot water holds more
Warm the water and two things happen. The molecules move faster, so they knock particles free more quickly and the pile disappears sooner. And the water can hold more before saturating, so a pile that would not dissolve in cold water dissolves in hot. That is why sugar vanishes into tea and sits stubbornly in iced coffee, and why a saturated hot solution, left to cool, grows crystals as the water gives back what it can no longer hold.
Gases work the other way round: cold water holds more dissolved gas than warm, which is why a warm fizzy drink goes flat quickly and why warming rivers hold less oxygen for fish.
Not everything dissolves
Sand in water never disappears; oil floats in a separate layer. They are still mixtures, but not solutions. Whether a solute dissolves depends on whether the solvent’s molecules can pull it apart and hold onto the pieces. Water is good at charged and polar things, like salt and sugar, and bad at oily ones. Oily solvents are the reverse, which is why grease comes off in soap but not in plain water, and why paint thinner does what water cannot.