Chemistry provides a framework for understanding the physical world by examining the structure, properties, and reactions of matter. Among the most common organizational structures of matter are mixtures, which represent a fundamental way substances combine in our environment.
What Makes a Mixture
A mixture is formed when two or more distinct pure substances are physically combined without undergoing a chemical reaction. Unlike a compound, where atoms bond to create an entirely new substance with unique properties, the components of a mixture retain their individual chemical identities. For example, when sugar is mixed with water, the resulting solution still tastes sweet and remains a liquid, indicating that the sugar and water molecules have not chemically changed.
The defining characteristic of a mixture is its variable composition, meaning the ratio of its components can be changed. You can create saltwater that is slightly salty or extremely salty, unlike a compound such as water (H2O), which always maintains a fixed ratio of two hydrogen atoms to one oxygen atom. Because no chemical bonds form between the components, mixtures can also be separated relatively easily using physical methods, such as filtration, distillation, or evaporation.
Uniform Blends
One major type of mixture is the homogeneous mixture, often referred to as a solution, which exhibits a composition that is uniform throughout. In these blends, the components are so thoroughly intermingled that they appear to be a single substance, even under close inspection.
A common example of a homogeneous mixture is salt dissolved in water, where the salt molecules are dispersed evenly throughout the solvent. Even gases can form solutions; the air we breathe is a gaseous solution composed primarily of nitrogen and oxygen, along with trace amounts of other gases. Solid solutions, known as alloys, are also widespread, such as brass, which is a uniform blend of copper and zinc atoms. In all these cases, you cannot visually distinguish the separate components because they are mixed at the molecular level.
Nonuniform Combinations
In contrast to uniform blends, heterogeneous mixtures are those that do not have a consistent composition throughout. The individual components remain physically separate and are typically visible or easily distinguishable.
One subcategory is a suspension, which contains solid particles large enough to be seen and that will eventually settle out over time due to gravity. Muddy water is a classic example, as the suspended dirt particles will slowly fall to the bottom of the container upon standing. Another common heterogeneous mixture is oil and vinegar salad dressing, where the two liquids are immiscible and form distinct, visible layers until they are shaken.
Another type of nonuniform combination is a colloid, which features particles intermediate in size between those of a solution and a suspension. Although the particles are too small to settle out, they are large enough to scatter light, which is known as the Tyndall effect. Familiar examples of colloids include milk, which is an emulsion of tiny fat globules dispersed in a liquid, and fog, which consists of microscopic water droplets suspended in air.