Dissolving.

Step 1 of 4

Where it went

A glass holds 100 g of water, and 10 g of table salt, sodium chloride, $\mathrm{NaCl}$⁠, has gone in. The crystals shrink, and after a minute the water is perfectly clear. The salt has dissolved.

A solution has two parts: the solvent is what you dissolve in, here the water, and the solute is what dissolves, here the salt. The mixture they make is a solution. Change Salt added and watch the crystals.

Step 2 of 4

Inside the water

A salt crystal is a lattice of ions: positive sodium ions, $\mathrm{Na^+}$⁠, and negative chloride ions, $\mathrm{Cl^-}$⁠, in turn. A water molecule is polar: its oxygen atom carries a partial negative charge, and its two hydrogens a partial positive charge. Look inside will show what the eye cannot; before you switch it, guess what is in the water.

Step 3 of 4

Mass is conserved

When salt dissolves, no ion is made and none disappears: they only move from the crystal to between the water molecules. So the mass of a solution is always the mass of the solvent plus the mass of the solute. The glass now holds 20 g of salt.

Step 4 of 4

Solution or mixture

Every solution is a mixture: two or more substances mingled without reacting, each keeping its own properties. But not every mixture is a solution. In a solution the solute breaks down to single molecules or ions, so it is uniform: clear, every drop the same as the rest, and it does not separate even after a week. Look inside is set to Particles.