A coagulant is a substance that helps to cause coagulation, which is the process of blood or other liquids thickening and forming a solid or semi-solid mass. In water treatment, coagulants are used to remove impurities by causing small particles to clump together, making them easier to filter out. Common coagulants include alum and iron salts. In the medical field, coagulants can refer to agents that promote blood clotting, which is essential for stopping bleeding. These agents are often used in treatments for patients with bleeding disorders or during surgical procedures. Examples include vitamin K and thrombin.