Chemical reactions occur everyday; you might not even notice the reactions. Cooking breakfast in the morning to fuel your body for the day and heating your home are examples of chemical reactions. Chemical reactions occur when elements or compounds form new substances.
The law of conservation of mass states that matter cannot be created or destroyed. Matter can however, be rearranged in different forms and types of particles. This means that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.
Example
The Law of Conservation of Mass is obeyed in the following word equation that represents the chemical reaction that takes place during the burning of natural gas.
Methane (natural gas) + oxygen gas $\rightarrow$ water + carbon dioxide
28 grams + 72 grams $\rightarrow$ 45 grams + 55 grams
Total mass of the reactants is 100 grams.
Total mass of the products is 100 grams.
Notice the total mass of the reactants, methane and oxygen gas, totals 100 grams. The total mass of the products, water and carbon dioxide, is also 100 grams. The mass of the matter at the beginning of the chemical reaction is the same as the mass of the matter at the end of the chemical reaction. The only difference is the matter has rearranged into different compounds.
Many chemical reactions you encounter in everyday life do not seem to obey this law. Driving a car burns gasoline to propel the vehicle down the road. Where did the products of this reaction go? Since the tailpipe of an automobile is open to the atmosphere, the water vapour and gases are released into the environment and not destroyed when you burn the gas.