Chemical reactions are processes where substances, called reactants, transform into new substances, known as products. This transformation involves breaking and forming chemical bonds, which changes the arrangement of atoms. For example, when hydrogen gas reacts with oxygen gas, they form water, a completely different substance. During a chemical reaction, energy is often released or absorbed, which can cause changes in temperature or light. Reactions can be classified into different types, such as combination, decomposition, and displacement reactions. Understanding these reactions helps scientists develop new materials, medicines, and solutions to various problems in our world.