Reduction reactions are chemical processes where a substance gains electrons, leading to a decrease in its oxidation state. This often occurs alongside oxidation reactions, where another substance loses electrons. Together, these reactions are part of redox (reduction-oxidation) reactions, which are essential in various chemical and biological processes. In many cases, reduction reactions involve the addition of hydrogen or the removal of oxygen from a compound. For example, in the formation of water, hydrogen gas reacts with oxygen gas, resulting in the reduction of oxygen as it gains electrons from hydrogen. These reactions are crucial in energy production, such as in cellular respiration.