Summary
Classifying Matter: Pure Substances and Mixtures
Highlights
Hierarchical Classification of Matter
Matter is organized into a hierarchy ranging from complex to fundamental. It is broadly categorized into two types: a pure substance, which maintains constant composition and properties throughout, and a mixture, which consists of two or more substances combined physically.
Elements and Compounds
Pure substances are further divided into two groups. An element is a substance that cannot be broken down into simpler chemical components, such as aluminum. A compound is a substance composed of two or more elements that can be chemically separated, such as water.
Understanding Mixtures
A mixture is a physical blend where components retain their individual identities. Unlike pure substances like sodium chloride or water, the composition of a mixture, such as salt water, can vary depending on the proportions of the ingredients added.
Homogeneous Mixtures
A homogeneous mixture features a uniform composition throughout. While these solutions may appear identical to pure substances, they differ because the ratio of their components can vary between samples.
Heterogeneous Mixtures
A heterogeneous mixture is one where the composition is not uniform. A common example is vegetable soup, where the distribution of components varies significantly from one portion to another.
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Original text
One useful way of organizing our understanding of matter is to think of a hierarchy that extends down from the most general and complex to the simplest and most fundamental (Figure ). Matter can be classified into two broad categories: pure substances and mixtures. A pure substance is a form of matter that has a constant composition (meaning that it is the same everywhere) and properties that are constant throughout the sample (meaning that there is only one set of properties such as melting point, color, boiling point, etc. throughout the matter). A material composed of two or more substances is a mixture. Elements and compounds are both examples of pure substances. A substance that cannot be broken down into chemically simpler components is an element. Aluminum, which is used in soda cans, is an element. A substance that can be broken down into chemically simpler components (because it has more than one element) is a compound. For example, water is a compound composed of the elements hydrogen and oxygen. Today, there are about 118 elements in the known universe. In contrast, scientists have identified tens of millions of different compounds to date.
Figure : Relationships between the Types of Matter and the Methods Used to Separate Mixtures
Ordinary table salt is called sodium chloride. It is considered a substance because it has a uniform and definite composition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt easily dissolves in water, but salt water cannot be classified as a substance because its composition can vary. You may dissolve a small amount of salt or a large amount into a given amount of water. A mixture is a physical blend of two or more components, each of which retains its own identity and properties in the mixture. Only the form of the salt is changed when it is dissolved into water. It retains its composition and properties.
A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample. Often it is easy to confuse a homogeneous mixture with a pure substance because they are both uniform. The difference is that the composition of the substance is always the same. The amount of salt in the salt water can vary from one sample to another. All solutions are considered homogeneous because the dissolved material is present in the same amount throughout the solution.
A heterogeneous mixture is a mixture in which the composition is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Any given spoonful of soup will contain varying amounts of the different vegetables and other components of the soup.