When a compound is heated with copper(II) oxide, it is a test for carbon. This is known as the carbon test or the test for carbon dioxide gas.

Explanation:
- The test involves heating a compound with copper(II) oxide (CuO) in a test tube.
- If carbon is present in the compound, it will react with the copper(II) oxide to produce carbon dioxide gas (CO2).
- The carbon dioxide gas produced can be identified by passing it through limewater (calcium hydroxide solution). If carbon dioxide gas is present, it will react with the limewater to form a white precipitate of calcium carbonate (CaCO3).
- The presence of a white precipitate confirms the presence of carbon in the original compound.

Key Points:
- The test with copper(II) oxide is specific for carbon only and does not detect other elements such as nitrogen, halogens, or phosphorus.
- Nitrogen does not react with copper(II) oxide to produce any significant reaction. It requires specific tests such as the sodium fusion test or the Dumas method for nitrogen determination.
- Halogens (fluorine, chlorine, bromine, iodine) do not react with copper(II) oxide to produce carbon dioxide gas. They have their own specific tests, such as the silver nitrate test for halide ions.
- Phosphorus does not react with copper(II) oxide to produce carbon dioxide gas. It requires specific tests such as the ammonium molybdate test or the yellow phosphorus test for phosphorus detection.

In conclusion, the test with copper(II) oxide is used to determine the presence of carbon in a compound. Other elements such as nitrogen, halogens, and phosphorus require different tests for their detection.