Overview
Mixtures and compounds are important combinations of elements. They are essential to the chemical processes that support living things, as well as those that occur every day in nonliving objects. The basic difference is that elements and molecules in mixtures remain the same substances, and compounds result from chemical reactions, forming chemical bonds.
Solutions
One common type of mixture is a solution, in which molecules of one substance are dissolved in another substance, but neither substance changes what it is. For example, a simple solution of sugar water that is safe to use in hummingbird feeders contains 4 parts water to every part of sugar. The mixture is stirred and boiled to remove impurities, but neither the sugar or water are changed to form the solution. In the same way, the oxygen dissolved in water is available for fish to use.
Suspensions
Suspensions are another common mixture found in living things. Unlike solutions, particles that are mixed with liquid are larger than individual molecules. For example, pond water that is full of silt looks cloudy, and the silt will eventually fall to the bottom of the pond. Red and white blood cells are suspended in blood plasma.
Colloids
Colloids are a special type of suspension that combine some of the features of a solution with some of the more typical features of a suspension. That is because the particles suspended in the solvent are too small to sink to the bottom of the suspension but too large to dissolve and make a solution. The best example of the colloid in biology is the cell interior. The structures within the cell remain suspended, as do food particles within the cell. This allows cells to use food and their interior parts to remain in the same relationship.
Compounds
Compounds differ from mixtures in that a new substance is formed by a chemical reaction. For example, water is a compound of hydrogen and oxygen. Two colorless gases combine to form a substance that is necessary to life . Even though both hydrogen and oxygen are highly flammable in their pure state as separate elements, their compound water is used to put out fires. Compounds are formed from elements when their electrons are bonded in chemical reactions, and those chemical reactions release the energy needed to support life.
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