A number of clues show that a chemical reaction has taken place, for example:
Something new is visibly produced
A gas is created
Heat is given off or taken in
Reactants→ProductsFor example, take a look at the reaction that occurs when you light your natural gas range. Methane (natural gas) reacts with the oxygen in the atmosphere to produce carbon dioxide and water vapor. The chemical equation that represents this reaction is written like this:
You can read the equation like this: One molecule of methane gas reacts with two molecules of oxygen gas to form one molecule of carbon dioxide gas and two molecules of water vapor. The 2 in front of the oxygen gas and the 2 in front of the water vapor are called the reaction coefficients. They indicate the number of each chemical species that reacts or is formed.
Methane and oxygen (oxygen is a diatomic — two-atom — element) are the reactants, while carbon dioxide and water are the products. All the reactants and products are gases (indicated by the g’s in parentheses).
In this reaction, all reactants and products are invisible. The heat being evolved is the clue that tells you a reaction is taking place. This is a good example of an exothermic reaction, a reaction in which heat is given off. A lot of reactions are exothermic. Some reactions, however, absorb energy rather than release it. These reactions are called endothermic reactions.
You have to supply a little energy to get a reaction going. The energy you have to supply to get a reaction going is called the activation energy of the reaction.