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C6 - Chemical Reactions

I can distinguish between the products and reactants in a chemical reaction

Chemical Reactions and Equations

Lesson Objectives

The student will:

  • explain what happens during a chemical reaction.
  • identify the reactants and products in any chemical reaction.


  • chemical reaction
  • products
  • reactants


In a chemical change, new substances are formed. In order for this to occur, the chemical bonds of the substances break, and the atoms that make up the substances separate and re-arrange themselves into new substances with new chemical bonds. When this process occurs, we call it a chemical reaction. A chemical reaction is the process in which one or more substances are changed into one or more new substances.

In order to describe a chemical reaction, we need to indicate what substances are present at the beginning and what substances are present at the end. The substances that are present at the beginning are called reactants, and the substances present at the end are called products.

Writing Chemical Equations

When sulfur dioxide is added to oxygen, sulfur trioxide is produced. In the chemical equation shown below, sulfur dioxide and oxygen (\text{SO}_2 and \text{O}_2) are reactants, and sulfur trioxide (\text{SO}_3) is the product.

The general equation for a reaction is:

Reactants  \rightarrow  Products

There are a few special symbols that we need to know in order to communicate in chemical shorthand. In Table below is a summary of the major symbols used in chemical equations. There are other symbols, but these are the main ones that we need to know.

Common Symbols in Chemical Reactions
 \rightarrow separates reactants from products; can be read as “to produce” or “to yield”2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O}
+separate reactants from each other or products from each other; can be read as “is added to”\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3

Chemists have a choice of methods for describing a chemical reaction. They could draw a picture of the chemical reaction, like in the image shown below.

Alternatively, they could describe the reaction in words. The image above can be described as two molecules of hydrogen gas reacting with one molecule of oxygen gas to produce two molecules of water vapor.

Chemists could also write the equation in chemical shorthand.

                            2H2 + O2 --> 2H2O