Mixtures and Solutions

The word mixture can be defined as a heterogeneous association of substances that cannot be represented by a single chemical formula. This definition does not limit mixtures to solids mixed with liquids, nor is every mixture considered to be a solution. Two or more gases, solids, or liquids can be mixed, and two or more different phases of matter can be combined in a mixture. A biologist must be able to work with a variety of mixtures, many, but not all, of which are made by mixing a solid with water. Not all such mixtures are true solutions, either. Here are some examples of mixtures that a biologist might encounter in a laboratory.

• physiological saline solutions
• buffers
• cell suspensions
• soil suspensions
• staining solutions
• microbiological media
• chromatography slurries
• dishwater
• milk
• protein solutions
• DNA solutions
• density gradients

Wilhelm Ostwald, Nobel Laureate in Chemistry in 1909, was one of the founders of modern physical chemistry. He is reported to have said, "There are no sharp differences between mechanical suspensions, colloidal solutions, and molecular [true] solutions. There is a gradual and continuous transition from the first through the second to the third.” A mixture, regardless of type, is described as "uniformly dispersed." This means that one or more minor components are evenly distributed throughout a major component. The major component is the substance that is present in the greatest proportion. In the biology laboratory the major component is often a liquid, and minor components can be solids, other liquids, or even gases.

The "mechanical suspension" to which Ostwald referred is the easiest to describe. The minor component in a suspension is typically visible in an optical microscope and is often visible to the naked eye. A colloidal mixture is sometimes called a colloidal system, a colloidal suspension, or simply a "colloid." The smallest dimension of the minor component of a colloidal mixture can range from approximately one nanometer (1 billionth of a meter) to one micrometer (1 millionth of a meter). Examples of liquid colloidal mixtures are milk, paints, and muddy water. The medium can be a gas, in the cases of smog, smoke, or aerosol sprays. Some solids are considered to be colloidal mixtures, as in steel or foam rubber.

In a true solution, one or minor components interact at the molecular level or ionic level with the major component. The minor components are atoms or molecules, and are not distinguishable in any optical microscope. Learn to apply the correct term when describing a mixture. For example, mixing cells in buffer does not usually produce a solution. Solutions are completely homogeneous mixtures, a property that is often attributed to suspensions and colloids as well. The minor components of a true solution, however, remain dispersed due to interactions at the molecular level. A substance is considered to be soluble in a particular solvent if it is capable of interaction with the solvent so as to form a solution.