In this topic you will learn about how chemical reactions make new substances.

How can you recognize a change? In a physical change, matter changes in size, shape, or state without changing identity. Physical changes involving mixing or separating as long as no new substances are made. Examples of physical changes include evaporating, boiling, dissolving, cutting into smaller pieces, or changing the shape of a material.

Chemical changes occur when atoms link together in new ways. A chemical change produces a new substance with different properties from the original materials. The reaction between baking soda and vinegar is an example of a chemical change. When these two materials are mixed, gas bubbles form. The gas is carbon dioxide.

A chemical reaction is a chemical change of original substances into one or more new substances. A reactant is one of the original substances before a chemical reaction takes place. One of the new substances produced when a chemical reaction takes place is called a product. In a reaction between baking soda and vinegar, the baking soda and the vinegar are reactants. The carbon dioxide, water, and a chemical called sodium acetate are the products.

In general, chemical changes are difficult to erase. Imagine trying to "unburn" toast or "unspoil" milk. On the other hand, physical changes can sometimes be easily reversed. For example, melting an ice cube can be easily reversed by cooling it until it freezes again.

Chemical reactions often show one or more signs that a chemical change has occurred. These signs include a color change, formation, and a change in temperature.

Some chemical reactions are familiar. Rocket engines use chemical reactions that produce lots of heat. Rust forms when iron atoms in steel react with oxygen from the air to form a red powder. We fight rusting because this chemical reaction eats its way through iron-containing metals—weakening the metal. Painting or oiling steel surfaces can slow rusting.