There are two types of changes in matter: physical changes and chemical changes Figure 3. As the names suggest, physical changes affect physical properties, and chemical changes affect chemical properties of a substance. Many physical changes are reversible (such as heating or cooling), whereas a chemical change is often irreversible or only reversible accompanying an additional chemical change.
Physical changes are those changes that do not cause a substance to become a fundamentally different substance. Chemical changes, on the other hand, cause a substance to change into something chemically new. Blending a smoothie (Figure 1), for example, is a combination of several physical changes including a change in shape of each fruit component and the mixing of many different pieces of fruit. But because none of the chemicals in the smoothie components is changed when blending (the water and vitamins from the fruit are unchanged, for example), the blending produces changes that are purely physical and not at all chemical.
Cutting, tearing, shattering, and grinding are examples of physical changes because they change the form but not the composition of a material. Mixing substances also produces physical changes. For example, the mixing of salt and pepper changes each seasoning without affecting its chemical properties.
Phase changes are physical changes that occur when a substance is melted, frozen, boiled (for example, water: Figure 2), condensed, sublimated, or deposited. They do not change the nature of the substance and thus are physical, not chemical, changes.
Chemical changes are also known as chemical reactions. The "ingredients" of a reaction are the reactants, and the end results are called the products. The change from reactants to products can be signified by an arrow:
Reactants → Products
Reactions that cause gas bubbles to form often reflect a chemical change (except in the case of boiling, which is a physical change). The formation of a precipitate is also a chemical change: When dissolved substances are mixed, and a cloudy precipitate appears, there has been a chemical change.
Rotting, burning, cooking, and rusting are all examples of chemical changes: These changes produce substances that are entirely new chemical compounds. For instance, burned wood becomes ash, carbon dioxide and water; when exposed to water, iron becomes a mixture of several hydrated iron oxides and hydroxides; sugar ferments into alcohol. Changes of color or release of odors often indicate chemical changes. For example, the element chromium can take on many different colors depending on its oxidation state, but a single chromium compound will not change color on its own without some sort of reaction that causes it to be oxidized or reduced. The heat from cooking an egg changes the interactions and shape of the albumin proteins in the egg white, thereby changing its molecular structure and converting the egg white from translucent to opaque.
To be 100 percent certain of whether a change is physical or chemical, the best way to confirm is by performing chemical analysis such as mass spectroscopy to determine a substance's composition before and after the change.