The gas that ammoniacal solution of CuCl\(_2\) is used to absorb from producer or water gas is

Producer gas is a mixture of carbon monoxide (CO) and nitrogen (N₂), while water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂). Both gases contain CO, which is toxic and must be removed for certain industrial applications.

Ammoniacal copper(I) chloride solution (CuCl dissolved in ammonia) is the reagent used to selectively absorb CO from these gas mixtures. The CO molecules form a coordination complex with the copper(I) ions in solution:

\[ \text{CuCl} + \text{CO} + 2\text{NH}_3 \rightarrow [\text{Cu(CO)(NH}_3\text{)}_2]\text{Cl} \]

This reaction is reversible: gentle heating releases the absorbed CO and regenerates the ammoniacal CuCl solution for reuse.

The other gases in these mixtures are not absorbed by this reagent. Hydrogen (H₂) does not form stable complexes with Cu⁺ under these conditions. Nitrogen (N₂) is chemically inert at room temperature. Carbon dioxide (CO₂) would be absorbed by alkaline solutions such as NaOH or KOH, not by ammoniacal CuCl.