This is true of all substances. For instance, hydrogen gas, as it is found on earth, is made up of molecules containing 2 hydrogen atoms each (H₂). Add a certain amount of energy and you pull the atoms apart; allow the atoms to come back together into paired molecules, and the added energy is given back again. The same is true for the oxygen molecule, which is made up of 2 oxygen atoms (O₂) and of the water molecule (H₂O). Always the amount of energy absorbed in one change is given off in the opposite change. The amount absorbed and the amount given off are always exactly equal. However, the amount of energy involved differs from molecule to molecule. It is quite hard to pull hydrogen molecules apart, and it is even harder to pull oxygen molecules apart. You have to supply about 12% more energy to pull an oxygen molecule apart than to pull a hydrogen molecule apart. Naturally, if you let 2 oxygen atoms come together to form an oxygen molecule, you get back 12% more energy than if you allow 2 hydrogen atoms to come together to form a hydrogen molecule. It takes a considerably larger amount of energy to pull apart a water molecule into separate atoms than to pull apart either hydrogen or oxygen molecules. Naturally, that greater energy is also returned once the hydrogen and oxygen atoms are allowed to come back together into water molecules. Next, imagine pulling apart hydrogen and oxygen molecules into hydrogen and oxygen atoms and then having those atoms come together to form _water_ molecules. A certain amount of energy is put into the system to break up the hydrogen and oxygen molecules, but then a much greater amount of energy is given off when the water molecules form. It is for that reason that a great deal of energy (mostly in the form of heat) is given off if a jet of hydrogen gas and a jet of oxygen gas are allowed to mix in such a way as to form water. Just mixing the hydrogen and oxygen isn’t enough. The molecules of hydrogen and oxygen must be separated and that takes a little energy. The energy in a match flame is enough to raise the temperature of the mixture and to make the hydrogen and oxygen molecules move about more rapidly and more energetically. This increases the chance that some molecules will be broken up into separate atoms (though the actual process is rather complicated). An oxygen atom might then strike a hydrogen molecule to form water (O + H₂ → H₂O) and more energy is given off than was absorbed from the match flame. The temperature goes up still higher so that further breakup among the oxygen and hydrogen molecules is encouraged. [Illustration: _The formation of a sodium chloride molecule._] This happens over and over again so that in very little time, the temperature is very high and the hydrogen and oxygen are combining to form water at an enormous rate. If a great deal of hydrogen and oxygen are well-mixed to begin with, the rate of reaction is so great that an explosion occurs.