What is the Law of Conservation of Mass? The law of conservation of mass states that during a chemical reaction, matter is neither created nor destroyed. In any chemical reaction, the total mass of the reactants must equal the total mass of the products. According to the law of conservation of mass, atoms are not created or destroyed in a chemical reaction. However, they may be rearranged, forming new substances. 1. Define In your own words, write a definition of the law of conservation of mass.

What Does a Balanced Chemical Equation Tell You? A chemical equation uses formulas and other symbols to represent a chemical reaction. For example, the following equation tells you that hydrogen (H) reacts with oxygen (O), forming water (H2O). H2 + O2  H2O Look at the diagram below that shows the molecules in this reaction. 2. Interpret What does the number 2 in the formula H2 tell you? (contd.)

Atoms help explain the law of conservation of mass Atoms help explain the law of conservation of mass. The total number of atoms of each element in the reactants is equal to the total number of atoms of each element in the products. A balanced chemical equation uses coefficients—numbers written in front of a chemical formula—to make the number of atoms of each element equal on both sides of the equation. Look again at the equation for the reaction of hydrogen and oxygen, forming water. This equation is balanced because it has four atoms of hydrogen and two atoms of oxygen on both sides of the equation. 2 H2 + O2  2 H2O 3. Recognize Tell whether or not the following chemical equation containing coefficients is balanced. Explain your answer. 2 Fe2O3 + 3 C  4 Fe + 3 CO2 (contd.)

The diagram below shows the molecules represented by the balanced equation of the reaction of hydrogen and oxygen that forms water. Notice that the number of hydrogen atoms is the same on both sides of the arrow. The same is true for the oxygen atoms. 2 H2 + O2  2 H2O