Final answer:
The heating of the metal powder in air results in an oxidation reaction forming metal oxide, and the subsequent passing of hydrogen gas causes a reduction reaction, reverting the metal oxide back to the metal.
Step-by-step explanation:
When a powder of a common metal is heated and turns black in color, and then the color is regained by passing hydrogen gas over this substance, two types of chemical reactions are occurring. In the first step, the metal powder is reacting with oxygen in the air to form a metal oxide, which is known as an oxidation reaction. When the black substance, which is the metal oxide, regains its original color upon being exposed to hydrogen gas, a reduction reaction is taking place. The metal oxide is being reduced by the hydrogen to form the metal again and water as a by-product.