Overview
All matter can undergo physical and chemical changes. While physical changes do not affect the composition of matter, chemical changes transform one type of matter to another type of matter through chemical reactions. Without chemical reactions, many processes would not be possible, including life itself.
What Are Physical Changes?
If a substance changes physically, it does not change from one substance to another. For example, water has the same chemical composition, H2O, whether it is liquid water, ice, or water vapor. A block of gold is the same chemically whether it is in a gold bar or hammered into thin sheets.
What Evidence Shows Physical Changes Have Occurred?
Since physical changes do not form new substances, the properties of the substance do not change. If a wooden board is cut into several smaller pieces, each piece of wood will have the same chemical composition as it did when it was a larger board. Each piece will have changed size and shape from the original piece.
What Are Chemical Changes?
If a substance changes chemically, new substances are formed that do not have the same properties as the original elements or compounds that formed them. For example, common table salt (NaCl) does not resemble either sodium, a metal, or chlorine, a poisonous gas. Likewise, water (H2O) does not resemble either of the gases hydrogen or oxygen.
What Evidence Shows Chemical Changes Have Occurred?
When gas bubbles form after two substances are mixed, such as when vinegar (acetic acid) is poured on baking soda (sodium bicarbonate), it may show that there has been a chemical reaction. When an electric current is passed through water, gas bubbles of hydrogen and oxygen are formed. In some chemical reactions, two liquid solutions can be mixed, and a solid precipitate can be formed.
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