 In this video, we'll look at a change that accompanies all chemical reactions, a change in energy. One of the signs that a chemical reaction has taken place is that there is some kind of temperature change caused by the reaction. When a chemical reaction occurs, there's always a transfer of energy. Either the reaction produces energy or it absorbs energy from the surroundings. This is a result of chemical bonds forming and breaking, and we'll go into the detail of that later in the course. When we look at the energy changes in reactions, it helps if we divide the universe into the reaction that we're studying and everything else. And we can then look at whether the energy is flowing from the reaction to the rest of the universe, which we can just call the surroundings, or the other way round. If a chemical reaction releases energy, then as the reactants are turning into the products, energy flows from the reacting chemicals out into the surroundings. And this energy is now free to move through the rest of the universe. This energy usually takes the form of heat, so we detect the energy change by the surroundings getting hotter. Such a reaction is called exothermic heat to the outside. If a chemical reaction absorbs energy, where is it going to get the energy from? It has to come from the surroundings. That is from somewhere in the rest of the universe. Energy flows from the surroundings into the reacting chemicals and is incorporated or stored as chemical energy in the molecules of the products of that reaction. Because energy is being removed from the surroundings to the reaction, you observe this as the surroundings cooling down. You may also observe it as the reaction only being able to go if you provide a continual supply of extra heat from the outside. Such a reaction is called endothermic heat to the inside, where the inside is the newly formed molecules.