A solvent is any chemical substance that dissolves other chemical substances, while a solute refers to any chemical substance that dissolves in other chemical substances. The best way to know when a chemical substance dissolves in another chemical substance is when a solid or gas dissolves in water. The solid or the gas can now be referred to as the solute and it will be shown to be in the the aqueous state, while the solvent is usually shown to be in the liquid state in any chemical equation.
Let us use the particular example of the dissolution of oxygen gas in water as shown below;
O2(g) + H2O(l)⇄O2(aq) + H2O(l)
The aqueous oxygen is the solute while the liquid water is the solvent.
Also, the substance having a smaller mass must be the solute and the substance having the larger mass must be the solvent.
