A mixture is composed of two or more types of matter that can be present in varying amounts and can be separated by physical changes, such as evaporation (you will learn more about this later).
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A mixture with a composition that varies from point to point is called a heterogeneous mixture. Italian salad dressing is an example of a heterogeneous mixture (see image below). Its composition can vary because we can make it from varying amounts of oil, vinegar, and herbs. It is not the same from point to point throughout the mixture—one drop may be mostly vinegar, whereas a different drop may be mostly oil or herbs because the oil and vinegar separate and the herbs settle.
Other examples of heterogeneous mixtures are chocolate chip cookies (we can see the separate bits of chocolate, nuts, and cookie dough). Another example is granite (we can see the quartz, mica, feldspar, and more).
(a) Oil and vinegar salad dressing is a heterogeneous mixture because its composition is not uniform throughout. (b) A commercial sports drink is a homogeneous mixture because its composition is uniform throughout. Image credit:- (a) “left”: modification of work by John Mayer; (a) “right”: modification of work by Umberto Salvagnin; (b) “left”: modification of work by Jeff Bedford; (b) “right”: OpenStax, Chemistry
A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout. An example of a solution is a sports drink, consisting of water, sugar, coloring, flavoring, and electrolytes mixed together uniformly (see image above). Each drop of a sports drink tastes the same because each drop contains the same amounts of water, sugar, and other components.
Note that the composition of a sports drink can vary—it could be made with somewhat more or less sugar, flavoring, or other components, and still be a sports drink. Other examples of homogeneous mixtures include air, maple syrup, gasoline, and a solution of salt in water.
Although there are just over 100 elements, tens of millions of chemical compounds result from different combinations of these elements. Each compound has a specific composition and possesses definite chemical and physical properties by which we can distinguish it from all other compounds. And, of course, there are innumerable ways to combine elements and compounds to form different mixtures. A summary of how to distinguish between the various major classifications of matter is shown in the figure below.
Depending on its properties, a given substance can be classified as a homogeneous mixture, a heterogeneous mixture, a compound, or an element. Image credit: OpenStax, Chemistry
Earth’s Composition of Elements
Eleven elements make up about 99% of the earth’s crust and atmosphere (see table below). Oxygen constitutes nearly one-half and silicon about one-quarter of the total quantity of these elements. A majority of elements on earth are found in chemical combinations with other elements; about one-quarter of the elements are also found in the free state.
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In the past few lessons, we have been looking at matter and how we classify matter. Matter is everywhere around us. Make a list by name of fifteen different kinds of matter that you encounter every day. Your list should include (and label at least one example of each) the following: a solid, a liquid, a gas, an element, a compound, a homogeneous mixture, a heterogeneous mixture, and a pure substance. You are encouraged to leave your list as a comment below.
This article has been modified from "Phases and Classification of Matter," by OpenStax, Chemistry, CC BY 4.0. Download the article for free at http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6