Suppose we have a mixture of two gases, chlorine and hydrogen, in a glass flask. Under ordinary conditions, they react very slowly.

But try lighting a strip of magnesium near the flask. An explosion occurs immediately (if anybody wants to try this experiment, be sure to shield the flask with a hood made of thick wire).

Now why does the mixture of chlorine and hydrogen explode under the action of bright light?

The answer is that a chain reaction is involved. If we heated the flask to about 700 degrees, it would also explode: the chlorine and the hydrogen would combine instantly, in a split second.

This would not surprise us, because we know that heat increases the activation energy of the molecule manifold. But in the experiment just mentioned the temperature did not change. This reaction was caused by light.

Quanta, these tiniest portions of light, carry a large amount of energy. Much more than that needed to activate molecules.

Now when a chlorine molecule happens to get into the path of a light quantum, the quantum tears it apart into atoms and passes its energy over to them.

The chlorine atoms are now in an excited, energy-rich state.

These atoms, in their turn, bear down upon the hydrogen molecules and tear them apart into atoms too. One of the latter combines with a chlorine atom and the other remains free. But it is excited. It craves to give away part of its energy. To whom? Why, to a chlorine molecule. And when it collides with one, that is the end of the phlegmatic chlorine molecule.

And now again there is an active chlorine ion at large, but it does not take long for this atom to find an outlet for its energy.

Thus we get a long consecutive chain of reactions.

As soon as the reaction starts, more and more molecules are activated by the energy liberated as a result of the reaction. The rate of the reaction increases like an avalanche of snow rolling down a mountain.

When the avalanche reaches the valley it dies down. The chain reaction dies out when all the molecules have been caught up by it, when all the hydrogen and chlorine molecules have reacted.

Chemists know multitudes of chain reactions. Our prominent scientist Nikolai Semyonov has studied how these reactions occur in great detail.

Chain reactions are known to physicists too. The fission of uranium atoms is an example of a physical chain reaction.

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