Copper turns green with age as its surface slowly reacts with air and moisture.

The longer version

A copper surface does not stay bright for long. Oxygen, water vapour and carbon dioxide in the air slowly react with the metal, building up a thin layer of compounds on top. That layer starts dull brown and gradually turns the blue-green shade familiar on old roofs and statues.

The coating matters because it clings tightly and blocks further attack. Unlike the flaky oxide that forms on iron, this layer seals the metal beneath and slows the reaction to a crawl. The colour therefore signals a protected surface rather than a failing one, and it develops faster where air is damp or salty.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.