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How does chlorine react with hydrogen in light?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

Chlorine possesses a very high affinity for hydrogen, and when these two elements are mixed in the presence of light, the reaction proceeds with explosive violence to produce hydrogen chloride. The chemical equation for this reaction is:12

$H_{2} + C l_{2} \to 2 H C l$

In this context, light acts as an initiator for the reaction. While this combination is explosive under light, it can occur rapidly without an explosion in the dark if charcoal is present as a catalyst.1