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atom the smallest particle of an element that has the chemical properties of that element. Atoms of an element are identical to each other, and different from the atoms of all other elements.

chemical property a characteristic of a substance that describes a type of chemical change it can undergo. A substance's flammability is one kind of chemical property.

chemical reaction a change in which two or more substances combine to form a new substance or substances. Reactions can produce or absorb energy. Some common chemical reactions include seeing, burning, breathing, and rusting.

chromatography a process by which components of a mixture are separated.

compound a substance made up of atoms of two or more elements. The different atoms are bound together in a molecule and can only be separated by a chemical reaction. Water is a compound made up of hydrogen and oxygen.

crystal solids in which atoms or molecules link together in geometric shapes.

density the ratio of mass to volume of a substance.

element a substance that is composed of one type of atom. An element cannot be chemically separated.

emulsion a dispersion of tiny particles of one liquid in another liquid. Example: oil and vinegar salad dressing.

endothermic a chemical reaction in which heat is absorbed.

exothermic a chemical reaction in which heat is released.

gas a physical state in which a substance has no definite volume or shape. A gas will fill a container of any shape.

liquid a physical state in which a substance has definite volume, but no definite shape. A liquid will flow into a container and can be changed into a solid by cooling and into a gas by heating.

matter what substances are made of.

mixture a combination of substances in which each substance retains its own chemical and physical properties.

molecule the smallest unit of a compound that retains the chemical properties of the compound. Molecules are a combination of atoms chemically bonded together.

periodic table an arrangement of chemical elements based on their atomic numbers and similarity of properties.

physical property a characteristic of a substance that can be observed, such as color, taste, texture, and density.

product in a chemical reaction, a substance that is formed as a result of the reaction.

reactants the substances that combine in a chemical reaction.

solid a physical state in which a substance has both volume and shape, with molecules held tightly together. A solid can be changed into a liquid by heating.

solution a mixture in which the atoms or molecules dissolve, or become evenly dispersed and the same size.

volume the amount of space taken up by matter.