When a chemical reaction happens, there are substances which go into the reaction called reactants and substances which are the result of the reaction called products. Sometimes the chemical reaction is easy to notice. One example is when there is a Space shuttle launch. (Drag your mouse over the link to open a new window to view a NASA video of a launch. Remember to close the window when you finish to return to this window.) In this chemical reaction, hydrogen combines with oxygen when the fuel is ignited causing the release of a great amount of energy in the form of heat. This energy is used to propel the rocket at speeds that exceed 22,000 miles an hour.

Perhaps one of the most interesting aspects of the reaction between hydrogen and oxygen is that these two ingredients combine to form water. During a space shuttle launch, you do not see the water because most is evaporated due to the large amounts of heat that is produced by this chemical reaction.

In every chemical reaction you have reactants and products. The reactants are those substances that are involved in the chemical reaction. The products are the result of the reaction, In the Chemical reaction for the formation of water the reactants are hydrogen (H₂) and oxygen (O₂) and the product is water (H₂O). A chemical equation for this would be:

In a different chemical reaction using the metal calcium and water you get different products. Check it out here: (Drag your mouse over the image at the right to open a video showing a QuickTime video of this reaction. Remember to close the window when you finish to return to this page.) During this video, you should be able to recognize evidence of a chemical reaction. You should observe the gas being given off. You may not notice the white precipitate falling to the bottom of the flask. In this reaction the reactants are calcium metal (Ca ) and water (H ₂O). The products of the chemical reaction are hydrogen gas (H₂) and a white precipitate called calcium hydroxide Ca(OH)₂. The chemical equation for this is written as follows:

In all chemical reactions the number and kind of each element cannot be lost . Elements are never lost, as with the statement that matter can not be created or destroyed, all of the atoms can and must be accounted for. They may be in the form of different compounds that look much different, or be given off as a gas but all the elements are still present somewhere.

Look at the following chemical equation. Notice the number of total atoms on the reactant side equal the total number of atoms on the product side.

Look closely at each side of the equation and observe that no atoms were lost in this chemical reaction.

It is a fact that the number of atoms which enter into a chemical reaction as reactants are always equal to the number of atoms that exit the reaction as products!