Reactant molecules are any substance that goes into a reaction. When forming product molecules, the reactant molecules combine and create a new molecule (i.e. the combination of methane and oxygen can make carbon dioxide and water).
Reactant molecules collide and break their bonds to rearrange into product molecules during a chemical reaction. This process involves several steps: collision, bond breaking, rearrangement, and formation of new bonds. The end result is the creation of new substances with different properties from the original reactants.
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