# Finding ions in water samples

So for a school chemistry assignment I was given 5 water samples and a bunch of different reagents to help me find out what was in the solutions. I'm stuck on two.

The first one had a pH of 7 and only reacted with silver nitrate and formed a white precipitate. the second had a pH of 14 and turned brown when silver nitrate was added to it. Does anyone know of any chemicals that would do this?

• The white precipitate is pretty obvious what it could be, for the brown one, see my answer below. Also welcome to SE. – Pritt Balagopal Jun 5 '17 at 9:36

@Pete's answers correctly determined what is the salt in the water sample which react with silver nitrate to form the white and brown precipitate respectively. My answer just describe the reactions.

The first one is very much likely to be a chloride salt as it would react with silver nitrate to form silver chloride. Silver chloride has a prominent white color and it is not at all soluble in water.

$$\ce{AgNO3 + Cl- → AgCl + NO3-}$$

It could have been a sulfate salt as silver sulfate is white but is somewhat soluble in water.

The second one is a hydroxide salt.

$$\ce{2AgNO3 + 2OH- → Ag2O + 2NO3^{-} + H2O}$$

Silver oxide is brown in color. It is somewhat soluble in water and actually form dihydroxoargentate ion instead of silver hydroxide.

It is slightly soluble in water due to the formation of the ion $\ce{[Ag(OH)2]^{−}}$ and possibly related hydrolysis products.

Note that silver hydroxide is not an actual compound. It is just like a solution of silver oxide in water just like ammonium hydroxide(solution of ammonia in water).

It's Silver(I) Hydroxide

The solution was very alkaline, as evident by it's pH value of 14. Although $\ce{AgOH}$ is sparingly soluble, with a $K_{\text{sp}}$ of $\pu{6.8×10^{-9}}$, under alkaline conditions it precipitates as a brown solid.

This is a picture of precipitating silver hydroxide. Is it what you saw as well?

The first one contains chloride, likely. AgCl is a white solid.

The second one contains some sort of hydroxide. Likely sodium hydroxide. If you expect there is only 1 salt in each sample, then you can stop there.