Physical and Chemical Changes

Matter can change in two main ways: physical changes and chemical changes. A physical change alters the size, shape, or state of a substance but does not produce a new substance. A chemical change produces one or more completely new substances with different properties.

Physical Changes

In a physical change, the substance stays the same — only its form is different. You can often reverse a physical change to get the original substance back.

• Ice melting into water — the molecules are the same; only the state changed.
• Tearing a piece of paper — the paper is still paper, just smaller pieces.
• Bending a metal wire — the metal is unchanged; only its shape is different.
• Dissolving sugar in water — the sugar is still present; it has only dispersed.
• Crushing a can — the aluminum is the same substance in a new shape.

Chemical Changes

In a chemical change, atoms rearrange to form new substances. Signs of a chemical change include color changes, gas production, heat or light being given off, and the formation of a precipitate.

• Wood burning into ash and smoke — completely new substances are made.
• Bread toasting — heat causes chemical reactions in the dough, making new compounds.
• Iron rusting — iron reacts with oxygen to form iron oxide (a new substance).
• Baking a cake — mixing and heating ingredients creates a substance that cannot be unbaked.
• Vinegar and baking soda reacting — produces carbon dioxide gas, a new substance.