What is the difference between contaminants and impurities?

Answer 1

The difference between "impurities" and "contaminants" is really semantic. Either term denotes the presence (usually in small concentrations) of undesirable chemical or physical substances dissolved into or mixed with some other desirable substance.

However, the word "contaminant" has the connotation of unnatural addition of undesirable substances while the word "impurities" connotes substances that are present, but which were probably there to begin with and need to be removed to make the desirable part "pure". There is also a connotation with the word "contaminant" which suggests substances that can cause harm, either to people, anumals, or the environment.

Discussion
Impurities usually refers to small amounts of chemical elements in a finite quantity of a "host" substance or material, such as in liquids, metals, or inorganic crystals. They do not "harm" anything, but may by their presence cause the physical or chemical properties of the host substance to change (e.g., boiling point, vapor pressure, solubility, reactivity, etc.).

Contaminants are (usually) small amounts of chemical elements also found in a host substance, such as water, air, soil, or food, which can cause upset or harm to the system or to organisms exposed to or consuming them. The broader term for this is the "pollution", which describes the presence of contamination (i.e., contaminants) in an environmental medium. In the context of "pollution", the definition of the word "contaminant" can be extended beyond chemical substances to include such things as heat, noise, light, or anything introduced into a natural environment in excess of its normal quantity, and which can cause harm to either the "system" or to its inhabitants (i.e., birds, fish, insects, people, microbes, etc.).

Examples
A good example of each can be demonstrated by looking at liquid water (as opposed to vapor/gas). Water found in the natural environment usually contains many other substances either dissolved in it (known as "solutes") or mixed with it as tiny particles (known as "particulate matter" or "colloids" depending on their size). In fact, finding "pure" liquid water, or water molecules all by themselves, in the natural environment is virtually impossible. So, the substances that are in the natural water would be considered "impurities", because they make the liquid water "impure", but not necessarily dangerous to consume (although in some cases this could be true).

Water with many natural impurities is sometimes called "hard water" and is the type that typically will leave stains or scale behind in sinks and showers. The natural impurities precipitate out of the water when something changes, like the temperature, or mixing with air. These natural impurities include elements such as manganese, calcium, iron, and other things that can be found in the natural environment. This is the reason for using distillation and deionization processes on water that is used in things like clothes irons, or your car radiator, or other places where precipitation of impurities is undesirable or would damage the equipment. But drinking water with so-called impurities is usually not a health problem.

OK, so what's the difference then with "contamination"? So-called "contaminants" are also substances other than water molecules, that are either dissolved into or mixed with the water. However, "contamination" is a process whereby an originally "uncontaminated" substance (i.e., the water in this case) becomes mixed with contaminants, or undesirable and possible harmful substances. Staying with the water analogy, consider the use of pesticides and fertilizers on farm fields.

Farmers will typically apply more fertilizers than the plants can actually absorb, and spray pesticides from the air, which after killing the desired pests, remain chemically unchanged and blanket the farm field and its soil. When it rains, these substances are washed off the plants and soil surfaces and become dissolved in or mixed with the rainwater. The rainwater eventually flows into a river. In the river, the pesticides and fertilizers become "contaminants", because they were 1) introduced to the river by an unnatural process (i.e., human industrial activity), 2) are not found in the natural environment, and 3) can or will cause harm at the concentrations found in the water.

Pesticides remain toxic not only to insects, but to mammals, birds, and amphibians for a long time (known as "persistence"). Fertilizers flowing from farm fields into lakes, streams, and rivers upset the normal balance or nutrients available to microbes, and in warm weather will cause unnatural algal blooms (i.e., explosions of the algae population) in surface water, which in turn causes depletion of dissolved oxygen and can lead to massive and sudden fish mortality (so-called "fish-kills").

Let's say also that a nearby community uses the river as a source of drinking water. Water from a river may have many naturally-occurring impurities, which if consumed cause no harm, but the pesticides are contaminants, because consuming them has a measurable negative biological effect.