Final answer:
The yellow compound formed in the reaction between ammonium chloride, concentrated sulphuric acid, and copper turnings is likely to be a form of copper sulphate, potentially colored by impurities.
Step-by-step explanation:
In the described dry test for acid radical, a mixture is created containing ammonium chloride and concentrated sulphuric acid, which upon heating releases sulfur dioxide gas, recognizable by its distinctive odor. The addition of copper turnings to the mixture, which then turns yellow, suggests the formation of a copper compound. This yellow compound is likely to be copper sulphate, as copper reacts with sulfate ions from the sulfuric acid. In reactions similar to this, copper turnings generally form blue copper(II) compounds when reacted with sulfate or nitrate ions. The yellow color, in this case, could be due to impurities or possibly due to the formation of a complex between the copper ions and chloride or other ions present in the mixture.
The reaction that occurred when the student mixed ammonium chloride with concentrated sulfuric acid and heated the mixture is a redox reaction. The concentrated sulfuric acid serves as an oxidizing agent, while the ammonium chloride acts as a reducing agent. The yellow compound formed is likely to be ammonium tetrathiomolybdate(IV), (NH4)2MoS4.