a reagent bottle (labelled) is filled with HCL solution and the other (labelled as b) is filled with water. both liquids in the bottle are colourless. what method do you recommend to identify the acid and water?

There are several methods - look at the explanation.

Explanation:

So first you can use cobal chloride paper which turns from blue to pink when water is present or you can use anhydrous copper sulfate which turns from white to blue and the blue substance would be called hydrated copper sulfate which is formed when water and anhydrous copper sulfate mix.