Precipitate (Chemistry)

Question

Given a "substance A" that reacts
with a "substance B" to form a
precipitate. If you have an "unknown
solution" and add some substance B
in it, and get a precipitate, does it
mean that your "unknown" is
automatically made of substance A ?

Explain this for me please

Answer 1

Since this is posted in Chemistry, one assumes you are taking a course in Chemistry.  Randall's answer is quite correct, but I thought I'd just add an example to demonstrate his point.  If you have learned the solubility rules, you will know that nitrates are soluble and that silver chloride is insoluble.  So, let's say (B) is AgNO3 and (A) is NaCl, or KCl or even MgCl2.

NaCl(aq) + AgNO3(aq) ==> NaNO3(aq) + AgCl(s)

KCl(aq) + AgNO3(aq) ==> KNO3(aq) + AgCl(s)

MgCl2(aq) + 2AgNO3(aq) ==> Mg(NO3)2(aq) + 2AgCl(s)

In all three cases a precipitate is formed.

Answer 2

Hey Mesh, if A and B react to yield a precipitate, and adding B to an unknown solution yields a precipitate, that does not necessarily mean that that the unknown solution contains A.  Any unknown solution to which B is added may yield a precipitate, be it A, B, C, D, E,...etc.

 

Hope this helps.

 

Rand