A solute is the substance to be dissolved (sugar). The solvent is the one doing the dissolving (water). The amount of solute that can be dissolved by the solvent is defined as solubility. Water can form a gas f hotnot heated and if you put something cold in the water that has that was formed can be dissolved by that cold substance.
The chemical formula for magnesium sulfate is MgSO4, so it shows that there are four atoms of oxygen but not sulfur. There is only one atom of sulfur in the formula