by Ron Kurtus (revised 13 January 2006)
A mixture is the blending of two or more dissimilar substances.
A major characteristic of mixtures is that the materials do not chemically combine. Mixtures can be divided into those that are evenly distributed (homogeneous) and those that aren't (heterogeneous). The types of mixtures are a suspension, colloid or solution.
Examples of mixtures include various combinations of solids, liquids and gases. Separation of mixtures can be by mechanical means, such as by weight, evaporation or other methods.
Questions you may have include:
- What are characteristics of mixtures?
- What are examples of simple mixtures?
- How do mixtures separate?
This lesson will answer those questions.
A mixture is a combination of two or more materials, compounds or elements where there is no chemical combination or reaction. There are two ways material is distributed throughout a mixture. There are also three types of mixtures.
Comparing mixtures with compounds
Mixtures are quite different than chemical compounds.
Mixtures combine physically in no definite proportions. They just mix. On the other hand, in a compound the substances combine chemically, forming molecules. The elements in a compound unite in definite proportions. For example, in the water molecule (H2O), there are always two parts Hydrogen and one part Oxygen.
No new substances
When you create a mixture, there are no new substances formed. Each part of a mixture retains its own properties. When a compound is formed, it is a new substance with new properties.
You could mix various proportions of Hydrogen and Oxygen gas. As long as you did not ignite the mixture with a match so that it would explode in a chemical reaction, the combination would form a mixture that could be separated by the different weights of the gases. Each gas would retain its own properties.
The parts of a compound can be separated only by chemical means, while a mixture can be separated by physical means and not chemical means.
Distribution of material
The distribution of the materials in a mixture can be heterogeneous or homogeneous.
Heterogeneous mixtures are those where the substances are not distributed evenly. They usually involve a mixture of a solid in a solid. A mixture of stones in soil is an example of a heterogeneous mixture.
Homogeneous mixtures are those where the materials are evenly distributed throughout.
Types of mixtures
Mixtures can be classified into three types: suspension, colloidal and solution. Some fluid mixtures are solutions.
(For more information, see: Types of Mixtures.)
Suspension mixtures have larger particles and are heterogeneous. Most mixtures are suspension mixtures.
Colloidal mixtures fall between suspension and solution mixtures. The ingredients in colloidal mixtures are smaller and usually homogeneous.
Homogenized milk is a colloidal mixture of cream and butterfat particles in skim milk. From its name, you can assume the particles are homogeneously distributed.
Solutions are homogeneous mixtures that consist of microscopic particles and even molecules. The solute and solvent in a solution are either both polar or non-polar molecules, under normal conditions.
Vinegar is a homogeneous mixture or solution of water and acetic acid. Salt water is another example of a solution.
Simple mixtures can involve various combinations of solids, liquids and gases.
Solid in solid
Sand is an example of a suspension mixture of solid particles. By sifting the sand, you can separate particles according to size.
Solid in liquid
Muddy water is an example of solid particles mixed in a liquid. Dirt is added to the water and made into a mixture by stirring the ingredients. After a while, gravity will cause the particles to settle to the bottom.
Blood is another example of solid particles in a liquid. The blood cells can be separated with a centrifuge.
Solid in gas
Smoke is an example of solid particles mixed in a gas. The solute smoke particles are added to the solvent air and mixed by convection currents.
After a while, the particles will settle to the ground. Solid particles in the air are a major part of air pollution.
Liquid in liquid
If you thoroughly mix the solute oil and the solvent water, breaking the liquids into small globules, the mixture will soon separate. Oil and water do not mix on a permanent basis.
Note that you could also mix the water in some oil. In that case, the water would be considered the solute and the oil the solvent.
Standard milk will soon separate into skim milk with cream at the top. By extreme mixing of the combination, they do not readily separate. This is called homogenized milk. Although it is not supposed to separate, it is not a real solution, because after a very long time, the cream will rise to the top. But by then, the milk has most likely gone bad.
Liquid in gas
Liquid particles can mix in a gas but will soon separate out. An example is a fine mist spray of water particles in air.
Gas in liquid
Bubbles of air or a gas can be seen in a liquid. Being lighter, they soon rise to the top.
Gas in gas
Gases mix at a molecule level. Air is a homogeneous mixture of Oxygen molecules, Nitrogen, Carbon Dioxide and some other gases. By the very nature of gases being in constant motion, so the heavier molecules seldom settle.
There have been cases where a large amount of Carbon Dioxide gas was naturally discharged and did not immediately mix with the air, but instead settled in a low area for a while. This happened some years ago to a village in Africa, suffocating all the people and animals. By the time authorities came to the village, the CO2 had been absorbed into the atmosphere. It took scientists to figure out how the people died.
You can separate a simple mixture by physical or mechanical means.
In many cases, the difference in weight of the substances will allow the effect of gravity to separate them.
A centrifuge will accelerate the effect of gravity by using centrifugal force to separate the materials. It is possible to separate the milk and cream particles (or cream globules) by spinning the liquid in a centrifuge. Hospitals use the centrifuge to separate blood cells from the plasma, which can be preserved longer.
Changing a liquid into a gas can often separate liquid mixtures. This can be done by natural evaporation or by boiling the liquid mixture. This is often done in separating salt-water solutions.
There are other miscellaneous methods to separate simple mixtures.
Sifting materials of different sizes can separate some mixtures.
If you had a mixture of iron filings and some non-magnetic material, you could use a magnet to separate the mixture.
A simple mixture consists of substances that do not react chemically and can be separated by mechanical means, while in a compound the substances react and combine chemically.
Mixtures can be heterogeneous or homogeneous. The types of mixtures are a suspension, colloid or solution. Combinations of solid, liquid or gas can be involved in a mixture. Gravity, boiling and sifting are some methods to separate mixtures.
Resources and references
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