Mixture (chemistry)
In chemistry, a mixture is defined as a physical combination of two or more substances that retain their individual chemical identities. Unlike compounds, where elements are chemically bonded and have a fixed composition, the components in a mixture can vary in proportion and can be separated by physical means. Mixtures can be categorized into three main types: suspensions, colloids, and solutions. Suspensions contain larger particles and are heterogeneous, allowing components to be visually distinguished, such as in muddy water. Colloids feature smaller particles that create a uniform appearance but can scatter light, seen in items like milk or fog. Solutions are homogeneous mixtures where all substances are in the same state of matter, such as sugar dissolved in water. Each type of mixture exhibits distinct properties based on particle size and distribution, reflecting the diversity of matter in our world.
Subject Terms
Mixture (chemistry)
Matter exists in many forms. It can take the form of a pure substance, such as an element, or it can be a mixture. Most things in the world are mixtures of some kind. A mixture is a physical combination of substances that do not chemically combine. That is, each substance making up a mixture retains its own chemical identity. Solutions, colloids, and suspensions are different types of mixtures.
Mixtures vs. Compounds
Although both compounds and mixtures can contain similar elements, these two types of substances differ in important ways. Iron (Fe) and sulfur (S) are elements that are both available in powder form. These powders can be combined to form an iron-sulfur mixture. When this mixture is heated, the two elements form a compound called iron sulfide.
There are several differences between the iron-sulfur mixture and the iron sulfide compound. Iron sulfide forms when a chemical reaction joins the elements iron and sulfur. Being an ionic compound, iron sulfide consists of an orderly arrangement of ions. In contrast, the iron and sulfur powders in the mixture are not chemically joined.
The ratio of elements in any compound is consistent. The type of iron sulfide in this example has the formula FeS. This means that for every atom of iron, there is one atom of sulfur. If the chemical reaction were repeated, the resulting iron sulfide would have the same formula and ratio of atoms. In contrast, an iron-sulfur mixture can contain any ratio of iron to sulfur.
Furthermore, the elements in the iron-sulfur mixture can be separated by physical means. For example, a magnet can be used to remove the iron powder from the mixture. However, the elements in the iron sulfide can only be separated by another chemical reaction. Note that the iron and sulfide in the mixture keep their unique physical properties (iron is magnetic, and sulfur is not), but the iron sulfide compound has new physical properties, distinct from those of either element.
Types of Mixtures
A mixture can be classified as a suspension, a colloid, or a solution depending on the sizes of its particles. A suspension, such as muddy water or a mixture of iron and sulfide powders, has the largest particles. In a suspension, at least two types of particles can be observed with the unaided eye. A colloid has smaller particles and appears uniform to the unaided eye. Fog, smoke, milk, and gelatin are examples of colloids. Finally, a solution is a special type of mixture in which all the substances are in the same state of matter (solid, gas, or liquid). Solutions are homogenous mixtures, and suspensions and colloids are heterogeneous mixtures.
Suspensions
Suspensions are heterogeneous mixtures. When viewed closely, the different components of a heterogeneous mixture can be seen. The substances making up a suspension may separate from each other. For example, the components of an oil and vinegar salad dressing will form separate layers until the mixture is shaken. Similarly, mud particles in muddy water will settle to the bottom of a container if the mixture is left alone. Because the particles in a suspension are large, they can be separated by passing the suspension through a filter.
Colloids
In terms of particle size, colloids are midway between suspensions and solutions. Colloids appear homogenous, or uniform. However, viewing them closely will reveal two or more types of substances. The tiny particles in a colloid reflect light. When a beam of light passes through a colloid, the light becomes visible. This is a good way to tell a colloid from a solution—a beam of light will not be visible when it passes through a true solution.
The components of a colloid may be in different states of matter, or they may be substances that do not normally remain mixed together. For example, milk is a mixture of water, protein, and fat. Because fat normally separates from water, milk is homogenized by breaking up the fat particles until they are very tiny. This prevents them from joining and separating out of the water solution. Homogenized milk is an example of an emulsion, which is a colloid of two or more liquids that would normally separate.
Other types of colloids have different names. Aerosols are colloids made up of liquid or solid particles suspended in a gas. Fog and smoke are aerosols. A foam is a colloid of gas particles in liquid, such as whipped cream or shaving cream. A gel, such as gelatin, is a colloid of liquid particles suspended in a solid, and a sol, such as blood, consists of solid particles suspended in a liquid.
Solutions
A solution is a special type of mixture in which every component is in the same state of matter. Solutions are homogenous. Sugar dissolved in water forms a solution. The sugar, a solid, takes on the state of the water when it dissolves. Sugar water appears the same throughout, and you cannot see the individual grains of sugar. There are no large particles in the solution to reflect light, so a beam of light will not be visible when passing through a solution.
A solution is made up of a solvent and at least one solute. The solute is the substance that dissolves, such as the sugar in the water. The solvent is the substance in which the dissolving occurs. Because many substances dissolve in water, it is known as the "universal solvent."
When the components of a solution exist in the same state, the substance with the greatest quantity is considered the solvent. For example, air is made up of 21 percent oxygen gas, 78 percent nitrogen gas, and a small percentage of other substances. Because nitrogen makes up the largest fraction of the gas, it is considered the solvent for oxygen.
Solutions can also be solids. Solutions of metals—such as those that make coins, jewelry, and most other metal objects—are called alloys.
Bibliography
Helmenstine, Anne Marie. "Solutions, Suspensions, Colloids, and Dispersions." About Education: Chemistry. About.com. Web. 30 Dec. 2014. http://chemistry.about.com/od/lecturenotesl3/a/colloids.htm
"Mixtures." Geeks for Geeks, 1 Apr. 2024, www.geeksforgeeks.org/what-is-a-mixture/#. Accessed 27 Nov. 2024.
Tro, Nivaldo. Chemistry, a Molecular Approach. 2nd ed. New York: Pearson Prentice Hall, 2010. Print.