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Introduction to Chemistry

Elements, Compounds, and Mixtures

Matter can be classified as pure substances or mixtures. Substances are evenly distributed in homogeneous mixtures but unevenly distributed in heterogeneous mixtures.

Chemistry is the scientific discipline that is concerned with describing matter and how matter behaves. Matter, which is anything that has mass and occupies space, can be divided into two types: pure substances and mixtures. Elements and compounds are pure substances, and mixtures are combinations of substances.

An element is a substance composed of atoms that have the same number of protons. Elements cannot be broken down into simpler substances by chemical reactions. An element that exists as a single atom is called monatomic. Elements can combine to form a molecule, which is a group of atoms that are held together by covalent bonds. Each element has specific characteristics. For example, neon (Ne) is a monatomic gas, while hydrogen gas (H2) is a diatomic molecule composed of two atoms of hydrogen. All elements are listed in the periodic table and are arranged by the configuration of their electrons and their general properties. Some elements are found naturally, while other elements, in particular heavier ones, are produced in a laboratory.
Neon exists as a monatomic gas. Hydrogen, nitrogen, and oxygen commonly exist as diatomic molecules in which the atoms are held together by chemical bonds.
A compound is a substance in which atoms of two or more elements are bonded together. Elements can be joined by bonds in which electrons are shared by atoms to form molecular compounds, or they can be joined by bonds in which forces between oppositely charged particles form ionic compounds. Every compound has a unique, specific ratio of elements or ions. A compound can be broken down by chemical processes into its respective elements. The making and breaking of bonds in a compound requires energy. The properties of a compound differ from the properties of its constituent elements, and the properties of no two compounds are exactly alike. For example, water is a compound composed of hydrogen and oxygen atoms in a 2:1 ratio. The chemical formula for water is H2O. Note that both hydrogen and oxygen are diatomic gases in their natural state, whereas water is a liquid at room temperature with entirely different properties than those of molecular hydrogen and oxygen. Molecules can vary widely in size. Chlorophyll, fructose, and cellulose are examples of compounds with large molecules composed of many atoms, while water, carbon dioxide, and methane are composed of molecules with only a few atoms. Some molecules, such as plastics, are so large they contain more than a thousand atoms of various elements.

Fructose and Water Molecules

A fructose molecule contains carbon (C), hydrogen (H), and oxygen (O) atoms held together by chemical bonds. A water molecule is much smaller than a fructose molecule and consists of bonded hydrogen and oxygen atoms.
A mixture is a substance composed of two or more substances that are not chemically bonded. Mixtures can be separated into their original components, such as salts, molecules, or gases, using physical processes. For example, evaporation can be used to separate salt dissolved in water by evaporating the water, leaving solid salt. They can vary in ratios of components, and the chemical properties of the constituents are retained in a mixture.
Particles in a homogeneous mixture are evenly distributed, whereas in a heterogeneous mixture the substances remain distinct. Substances in mixtures are not chemically bonded to each other.
Mixtures are classified as either homogeneous or heterogeneous. A homogeneous mixture is a combination of two or more substances that are evenly distributed. A solution is a specific type of homogeneous mixture in which one substance, the solute, is dissolved in another substance, the solvent. Salt water is an example of a solution in which salt is the solute and water is the solvent. An alloy is a solid mixture of two or more metals that is a solution if the mixture is homogeneous. Solid solution alloys, such as ornamental gold, which is a mixture of pure gold and copper, are homogeneous alloys. Homogeneous mixtures can be separated into components. For example, a mixture of two liquids can often be separated by their boiling points using distillation.


The solute potassium permanganate dissolves in the solvent water to make a solution.
A heterogeneous mixture is a combination of two or more substances that are unevenly distributed. An example is a mixture of oil and water. Suspensions are a type of heterogeneous mixture in which large particles, such as sand, are mixed with a solvent, such as water. Usually if suspensions are not stirred, the particles settle as sediment at the bottom of the container. Alloys can be heterogeneous as well. For example, pearlite is a layered iron-carbon alloy in which layers have different compositions of iron and carbon. Heterogeneous mixtures can be separated into the individual substances. For example, sand and water can be separated by decantation, in which the liquid is poured off, leaving the solid sand behind.
In a suspension of sand and water, the sand will settle to the bottom of the container if the solution is undisturbed.
Colloids are another type of mixture. In colloids, microscopic particles are mixed with another substance, such as a liquid. Unlike a suspension, the particles of a colloid are so small that they do not settle. Fog and milk are examples of colloids.

Classification of Matter

Matter is classified as a pure substance or a mixture. A pure substance can have one type of atom or more than one type. A mixture can have a uniform or nonuniform composition.