In a solution, the quantity of a solute and the quantity of a solvent is measured in terms of concentration.
Molarity (M):
The most common unit of concentration used by chemists
is molarity. It is defined as the number of moles of solute
dissolved in 1 liter of solution and is abbreviated as the
uppercase letter M.
A solution with molarity units is prepared by measuring the solute and quantitatively transferring it into a volumetric flask of the desired volume. The solute is first dissolved in the enough solvent and then the flask is filled with the solvent up to the mark of the volumetric flask and the mixture is shaken. Because the volume is affected by temperature, so is molarity i.e., a unit volume of hot solution contains slightly less solute than a unit volume of cold solution.