So why the explosion? When chlorine and hydrogen combine to form hydrogen chloride, a great deal of energy is released. Looked at another way, if you have a volume of hydrogen chloride gas, you must expend energy to separate them and get back to chlorine gas and hydrogen gas. Your old Quantum Butcher could spend a lot of time explaining the essentials of thermochemistry, but for our purposes it’s sufficient to simply say that a chemical system tends to go from a state of relatively high energy and instability to a state of low energy and stability. The energy contained in the covalent bonds between hydrogen and chloride in the two HCl molecules is less than the total energy contained in the covalent bonds that hold together the H₂ and Cl₂ molecules. The difference in energy is given off as heat and light when the bonds are rearranged to give the more stable molecule. It’s important to realize that chemical bonds can store energy when formed and release energy when broken or rearranged, because chemical energy is at the heart of living systems.