Chemical reactions change starting materials into new substances. Here’s a simple breakdown of how this works:

1. Reactants: These are the materials you start with in a reaction. For example, when methane burns ($CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$), the reactants are methane and oxygen.

2. Products: These are what you get after the reaction takes place. In the burning methane example, the products are carbon dioxide and water.

3. Chemical Bonds: In a reaction, the links between atoms in the reactants break, and new links form to create the products. For instance, when methane burns, the bonds in methane and oxygen break apart. Then, new bonds form in carbon dioxide and water.

4. Conservation of Mass: This important rule says that matter can’t be created or destroyed in a chemical reaction. So, if you add up the mass of the reactants, it will always equal the mass of the products. For example, if you have 16 grams of methane and 64 grams of oxygen, the total mass will still be 80 grams in the products (carbon dioxide and water).

By understanding these changes and the conservation of mass, you can learn a lot about chemical reactions!