A solute is the substance that dissolves to make a solution. In salt solution, salt is the solute. A solvent is the substance that does the dissolving – it dissolves the solute. In salt solution, water is the solvent.
During dissolving, particles of solvent collide with particles of solute. They surround the particles of solute, gradually moving them away until the particles are evenly spread through the solvent.
For each solute and solvent, there is a limit to the mass of solute that will dissolve in a particular volume of the solvent. When no more solute will dissolve, we say that the solution is a saturated solution.
The solute does not cease to exist when it dissolves. If the water in the solution is evaporated, the solute is left behind. The total mass stays the same during dissolving. For example, if 1 g of salt is dissolved in 100 g of water, the mass of salt solution formed is 101 g (1 + 100). This is called conservation of mass.