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During the process of a chemical change (also referred to as a chemical reaction), the starting substance(s) are changed into new and different ending substance(s).

In the video, you saw a couple of chemical reactions being represented by their chemical equations. A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as chemical formulas.

This image illustrates the chemical reaction that occurs when atoms of hydrogen and oxygen chemically combine to produce water. The reaction is represented by the chemical equation:

2H₂ + O₂ \( \rightarrow \) 2H₂O

The parts of the equation are described in the table.

Reactants	The reactants are the starting materials in a chemical reaction. They appear on the left side of the arrow in the chemical equation. If there are multiple reactants, they are separated by a plus sign (+).
Products	The products are the ending materials in a chemical reaction. They appear on the right side of the arrow in the chemical equation. If there are multiple products, they are separated by a plus sign (+).
Plus sign (+)	When there are two or more reactants or two or more products, the individual reactants or products are separated from one another by a plus sign.
Arrow (\( \rightarrow \))	The arrow separates the reactants from the products. When reading a chemical equation out loud, you will often hear the arrow described as "yields," "produces," or "forms."

In this reaction, the reactants are H₂ and O₂. The product is H₂O. So, you can read this equation as:

"Hydrogen and oxygen react to produce water."

What do the numbers in front mean?

The numbers that appear in front of the chemical symbols or chemical formulas in the chemical equation are called coefficients. A coefficient is the number written in front of a reactant or product. The coefficients show the relative proportions of each reactant or product of the reaction. For example, "2H₂O" means that there are 2 water molecules produced for every set of reactants. In a later lesson, you will learn more about coefficients and how they are used.

Let's Practice

Answer each of the following questions, then click the question to check your answer.

What are the reactants of photosynthesis?

The reactants are CO₂ and H₂O.

The reactants appear on the left side of the arrow in the chemical equation.

What are the products of photosynthesis?

The products are C₆H₁₂O₆ and O₂.

The products appear on the right side of the arrow in the chemical equation.

What part of the chemical equation can be read as "yields" or "react to produce?"

The arrow (\( \rightarrow \))

Question

How does looking at a chemical equation tell you that a chemical change has taken place?

Answer

During chemical changes, the chemical make-up of the substance is altered and at least one new substance is created. Thus, if a chemical change has occurred, there will be at least one new product that was not a reactant.