Mixtures are different from pure substances like elements and compounds because mixtures contain different substances that are combined physically but not chemically. The individual components in a mixture retain their identity.
Mixtures are of two types: homogeneous and heterogeneous. A homogeneous mixture has a uniform composition and appearance. Individual substances that constitute a homogeneous mixture cannot be visually differentiated. On the other hand, a heterogeneous mixture comprises two or more substances that can be distinctly observed, and even separated relatively easily.
|You can see the parts||Yes||No|
|Can be separated physically||Yes||No|
|Examples||Salad, trail mix||Olive oil, steel, salt in water|
All mixtures comprise two or more pure substances (elements or compounds). The difference between a mixture and a compound is how the elements or substances combine to form them. Compounds are pure substances because they only contain one type of molecule. Molecules are made of atoms that have bonded together. But in a mixture, elements and compounds are both found intermingled physically but not chemically—no atomic bonds form between the pure substances that constitute the mixture.
But regardless of atomic bonds, mixtures can become quite cohesive. Commonly called solutions, homogeneous mixtures are those where the substances mix so well that they cannot be individually seen in a differentiated, distinct form. Their composition is uniform i.e., same throughout the mixture. This uniformity is because the constituents of a homogeneous mixture occur in the same proportion in every part of the mixture.
Conversely a heterogeneous mixture is one where the constituent substances are not uniformly distributed. They can often be visually told apart and even separated relatively easily, although many methods exist to separate homogeneous solutions as well.
Examples of Homogeneous and Heterogeneous Mixtures
Examples of heterogeneous mixtures would be ice cubes (before they melt) in soda, cereal in milk, various toppings on a pizza, toppings in frozen yogurt, a box of assorted nuts. Even a mixture of oil and water is heterogeneous because the density of water and oil is different, which prevents uniform distribution in the mixture.
Examples of homogeneous mixtures are milkshakes, blended vegetable juice, sugar dissolved in coffee, alcohol in water, and alloys like steel. Even the air that's in our atmosphere is a homogeneous mixture of various gases and—depending upon the city you live in—pollutants. Many substances, such as salt and sugar, dissolve in water to form homogeneous mixtures.
Types of Mixtures
There are three families of mixtures: solutions, suspensions and colloids. Solutions are homogeneous while suspensions and colloids are heterogeneous.
Solutions are homogeneous mixtures that contain a solute dissolved in a solvent, e.g. salt dissolved in water. When the solvent is water, it is called an aqueous solution. The ratio of mass of the solute to the solvent is called the concentration of the solution.
Solutions can be liquid, gaseous or even solid. Not only that, the individual components of the solution can be different states of matter. The solute assumes the phase (solid, liquid or gaseous) of the solvent when the solvent is the larger fraction of the mixture.
- Gaseous solutions: When the solvent is a gas, it is only possible to dissolve gaseous solutes in it. The most common example of a gaseous solution is the air in our atmosphere, which is nitrogen (the solvent) and has solutes like oxygen and other gases.
- Liquid solutions: Liquid solvents are capable of dissolving any type of solutes.
- Gas in liquid: Examples include oxygen in water, or carbon dioxide in water.
- Liquid in liquid: Example include alcoholic beverages; they are solutions of ethanol in water.
- Solid in liquid: Sugar or salt solutions in water are examples of such mixtures. Many solid in liquid mixtures are not homogeneous so they aren't solutions. They could be colloids or suspensions.
- Solid solutions: Solid solvents can also dissolve solutes of any state of matter.
- Gas in solid: An example of this is hydrogen dissolved in palladium
- Liquid in solid: Examples of this include mercury in gold, forming an amalgam, and water (moisture) in salt
- Solid in solid: Alloys like steel, brass or bronze are an example of such mixtures.
A suspension is a heterogeneous mixture that contains solid particles that are large enough for sedimentation. The solid particles do not dissolve in the solvent but are suspended and freely floating. They are bigger than 1 micrometer and are usually large enough to be visible to the naked eye. An example is sand in water. A key feature of suspensions is that the suspended particles settle over time if left undisturbed.
Colloids are heterogeneous like suspensions but visually appear to be homogeneous because the particles in the mixture are very small—1 nanometer to 1 micrometer. The difference between colloids and suspensions is that the particles in colloids are smaller and that the particles will not settle over time.
|Homogeneity||Homogeneous||Heterogeneous at the microscopic level but visually homogeneous||Heterogeneous|
|Particle size||< 1 nanometer (nm)||1 nm – 1 micrometer (μm)||> 1 μm|
|Physically stable||Yes||Yes||Needs stabilizing agents|
|Exhibits Tyndall effect||No||Yes||Yes|
|Separates by centrifuge||No||Yes||Yes|
|Separates by decantation||No||No||Yes|
To a certain extent, you could say (if you were being pedantic) that the question of whether a mixture is homogeneous or heterogeneous depends on the scale at which the mixture is being sampled.
If the scale of sampling is fine (small), it could be as small as a single molecule. In that case, any sample would become heterogeneous because it can be clearly delineated at that scale. Similarly, if the sample is the entire mixture, you could consider it to be homogenous enough.
So to remain practical, we use this rule of thumb to decide if a mixture is homogeneous: if the property of interest of the mixture is the same regardless of which sample of it is taken for the examination used, the mixture is homogeneous.