How does the reaction between formaldehyde and copper hydroxide take place?
Here's what I think.
They may be using Fehling's test or Benedict's test for the presence of an aldehyde.
Both tests use a solution of
Formaldehyde is such a powerful reducing agent that the complexed copper(II) ions are reduced to metallic copper.
The blue solution forms a copper mirror inside the test tube.
The formaldehyde is oxidized to formic acid, which exists as formate ion in the basic solution.
Combining the two half-reactions, we get