The properties of solutions, suspensions and colloids depend to large extent on their concentrations. A dilute or weak solution has a relatively small amount of solute dissolved in the solvent. It has a characteristic different from those concentrated or strong solution of the same substances, in which a relatively large amount of solute is present. Since concentrations need to be expressed quantitatively, instead of qualitatively terms like dilute or strong, concentration are usually expressed in terms of mass per unit volume, part per million or billion, or percent.
MASS PER UNIT VOLUME: One of the common types of concentration is milligram per liter (mg/L). For example, if a mass of 10 mg of oxygen is dissolved in a volume of 1 L of water, the concentration of that solution is expressed simply as 10mg/L. If 0.3g of salt is dissolved in 1500mL of water, then the concentration is expressed as 300mg/1.5L=200mg/L, where 0.3g = 300mg and 1500mL = 1.5L (1g=1000mg/L; 1L=1000mL).
Very dilute solutions are more conveniently expressed in term of micrograms per liter (g/L). For example, a concentration of 0.004mg/L is preferably written as its equivalent 4g/L. Since 1000g=1mg, e.g., a concentration of 1250g/L is equivalent to 1.25mg/L.
In air, concentrations of particulate matter of gases are commonly expressed in terms of micrograms per cubic meter (g/m3).