(BCR) Chemical Reactions Lesson

Chemical Reactions

A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right).

Reactants → Products

Consider the simple example in the picture below. When carbon (C) reacts with oxygen (O2), it produces carbon dioxide (CO2). The chemical equation for this reaction is:

Chemical Equation Example c + O2 -> CO2 Carbon reacts with oxygen to produce carbon dioxide

The reactants are one atom of carbon and one molecule of oxygen. When there is more than one reactant, they are separated by plus signs (+). The product is one molecule of carbon dioxide. If more than one product were produced, plus signs would be used between them as well.

Chemical reactions may occur quickly or slowly. Look at the two pictures below.  Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust.

Chemical Reaction Examples of water and rusty metal

Same Atoms, New Bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products.

Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The picture below represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products, there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water.

Water Reaction of hydrogen and oxygen molecules

Evidence of Chemical Reactions

Not all changes in matter involve chemical reactions. For example, there are no chemical reactions involved in changes of state. When liquid water freezes or evaporates, it is still water. No bonds are broken and no new products are formed.

How can you tell whether a change in matter involves a chemical reaction? Often, there is evidence. Four common signs that a chemical reaction has occurred are:

  • Change in color: the products are a different color than the reactants.
  • Change in temperature: heat is released or absorbed during the reaction.
  • Production of a gas: gas bubbles are released during the reaction.
  • Production of a solid: a solid settles out of a liquid solution. The solid is called a precipitate.

You can see examples of each type of evidence in the picture below.

Evidence: See description

Evidence description Links to an external site.

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