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Describing Solutions

Author(s): David R. Caprette, PhD

Volume, Amount, and Concentration

The common language of scientists uses units of measurement that are recognized in labs all over the world, appropriately called the International System of Units. The seven SI (for Systeme Internationale) base units are defined in terms of well established physical quantities or standards. Other units of measure were derived from the SI base units and are called SI-derived units. In solution making, we work with SI or SI-derived units for volumes, amounts, and concentrations.

Volume describes the space taken up by something. Since space is three-dimensional, a unit of volume is the cube of a unit of length, such as centimeters, feet, inches, or meters. An SI-derived unit for volume is the liter, defined as 1,000 cubic centimeters. One cubic centimeter, or one thousandth of a liter, is a milliliter. We use prefixes to simplify quantitative expressions of volume, amount, and concentration. For example, we might find it convenient to describe a volume in microliters (one millionth liter) or deciliters (1 tenth liter).

The SI base unit for an amount of substance is the mole. The mole and its use in defining solutions will be discussed later. A less specific unit with which to describe an amount of substance is a unit of mass. The SI base unit for mass is the kilogram, which is 1,000 grams. We may describe quantities using milligrams (one thousandth of a gram), micrograms (one millionth of a gram), or perhaps even nanograms (one billionth of a gram).

Concentration refers to the amount of substance within a specified volume. Solutions are defined by the type of solute or solutes, the type of solvent, the concentration of each solute, and often (for aqueous solutions), pH.  Suspensions also can be defined as the amount of a minor component per unit total volume (volume of minor components plus volume of solvent). The concentration of a suspension can be described as the number of particles per unit volume, as often is the case for cell suspensions.