Substances that are physically combined without being chemically bonded are known as mixtures. These combinations are fundamental to chemistry and everyday life. A mixture’s properties depend heavily on how uniformly its components are spread out. Classifying a mixture based on uniformity helps predict its physical behavior and determine how its components might be separated.
Defining Homogeneous Mixtures
A homogeneous mixture is defined by its uniform composition throughout the sample. The ratio of the components is identical regardless of where the sample is taken. The individual components are not visually distinguishable, even when using a powerful microscope. For instance, filtered coffee or brass appear to be a single substance, even though they contain multiple ingredients. The particles are typically so small that they do not settle out over time and cannot be separated by simple physical means like filtration.
The Term “Solution”
The most common alternative name for a homogeneous mixture is a solution. A solution is composed of at least two parts: the solute and the solvent. The solute is the substance that is dissolved, typically present in the lesser amount. The solvent is the substance that does the dissolving, usually present in the greatest quantity. When table salt dissolves in water, the salt is the solute and the water is the solvent, forming a saltwater solution.
Solutions are not limited to solids dissolving in liquids; they can form across all states of matter. For example, air is a gaseous solution where oxygen and other gases are solutes dissolved in a nitrogen solvent. Alloys, such as steel, are solid solutions where carbon is dissolved within iron.
Distinguishing Homogeneous from Heterogeneous
Homogeneous mixtures are contrasted with heterogeneous mixtures. A heterogeneous mixture is non-uniform, meaning its components are not evenly distributed, and the composition varies across the sample. The different substances in a heterogeneous mixture are visibly distinct, such as sand mixed with water or a bowl of cereal. This difference in uniformity dictates the method of separation.
Components in a heterogeneous mixture, like a suspension of sand in water, can often be separated using simple techniques like filtration or decantation because the particles are large. Conversely, the small particle size and uniform distribution in a homogeneous mixture prevent separation by filtration. Instead, methods like distillation, which exploits the difference in boiling points, or evaporation are necessary to isolate the solute from the solvent.