[xangelofxdeathx]

A colorless solution is known to contain one of these ions. Which ion is present if adding dilute HCl produces a white precipitate that dissolves when the solution is warmed?

A) Ag⁺
B) Cu²⁺
C) Hg₂²⁺
D) Pb²⁺

I have no idea how I can figure this problem out 

This is what I "think" I am supposed to do:
First, the ion will react with the Cl^- in HCl, therefore you get either AgCl, CuCl₂, Hg₂Cl₂, or PbCl₂. then I assume I am supposed to know which one is a white precipitate? Oh yeah also CuCl₂ is not a precipitate so that one can be eliminated. Hmmm is this the kind of problem where I either know it or don't know it?

Is this the right way to do this problem? Or is there a better way? (The answer is D by the way)

[Vixklen]

Hg2Cl2 is a yellowish white precipate.
The only white precipitate here is AgCl and PbCl2. From the data I get, AgCl slightly dissolves in 50deg Celcius. While PbCl2 slightly dissolves in 20deg C. That's why a PbCl2 precipitate in a solution could dissolve more than AgCl does when the solution is warmed.

peace
vixklen

[AWK]

CuCl2 is soluble in water (blue solution).
All other precipitates are colourless (white). Only one of them is soluble in warm water (PbCl2)