When copper oxide (CuO) is heated in the presence of hydrogen gas (H₂), the following reaction takes place:

CuO (black) + H₂ (colorless) → Cu (reddish-brown) + H₂O (colorless)

Here's the breakdown of the color change:

* Copper oxide (CuO): Starts as a black solid.

* Hydrogen (H₂): Is a colorless gas.

* Copper (Cu): Forms as a reddish-brown solid.

* Water (H₂O): Is a colorless gas (steam) at the reaction temperature.

Therefore, the color change you'll observe is from black (copper oxide) to reddish-brown (copper).