A chemical reaction is a process where substances, called reactants, transform into new substances, known as products. This transformation involves breaking and forming bonds between atoms, leading to changes in the chemical structure. For example, when hydrogen gas reacts with oxygen gas, they form water, showcasing how different substances can combine to create something new. During a chemical reaction, energy is often released or absorbed, which can cause changes in temperature or light. Reactions can be simple, like rust forming on iron, or complex, like those occurring in our bodies to digest food. Understanding these reactions helps us in fields like chemistry and medicine.