An explosion is a terrible thing. It is terrible because the explosion occurs instantly, in a split second. 

    But what is an explosion? It is just an ordinary chemical reaction accompanied by the evolution of a large amount of gases. It exemplifies a chemical process which takes place instantaneously, such as the combustion of gunpowder in a bullet shell or the explosion of dynamite. 

    But an explosion is a sort of extreme. Most reactions take some length of time to occur. 

     There are many reactions which proceed so slowly that they can hardly be detected. 

    Imagine a mixture of two gases, hydrogen and oxygen, the components of water, in a glass container. They may stand in it for a very long time: a month, a year, a hundred years, without a single drop of moisture being detected on the surface of the glass. One might think the hydrogen is not combining with the oxygen at all, but actually it is, though very slowly. It would take thousands of years for a hardly perceptible quantity of water to form at the bottom of the container. 
 

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