 Endothermic reactions are sometimes not as obvious as exothermic ones because the energy is being absorbed and that's harder to see. Perhaps the most well-known kind of endothermic reaction is the kind that makes cold packs cold. Instant cold packs have two or more chemicals in separate pouches inside a larger pouch. When the inside pouches are broken and the reactants mix, an endothermic reaction occurs and the chemicals absorb heat from the surroundings, which could include, for example, your sprained ankle. Another example of endothermic reactions is the decomposition of copper carbonate. In fact, this is the case for a lot of carbonate chemicals. When green copper carbonate powder is heated, it decomposes to give black copper oxide and carbon dioxide gas. This reaction only happens if it's heated up and it's that heating energy that gets absorbed as the copper carbonate is transformed into the products. You may at this point wonder why the copper carbonate has to be heated and yet the cold pack reaction just happens as soon as the chemicals are mixed. Well, that has to do with the activation energy of the reaction and we'll deal with that in more detail later on. Another vitally important endothermic reaction is photosynthesis where carbon dioxide gas and water are combined to give glucose and oxygen and this is done by plants. This is exactly the opposite reaction to respiration, so whereas respiration produced energy as the glucose and oxygen were turned into carbon dioxide and water. In photosynthesis, energy has to be put in to turn the carbon dioxide and water into glucose and oxygen and that energy comes from the sun.