Yellow precipitate in the presence of ammonium molybdate? We had analytical chemistry experiments to determine acidic radicals from a mixture of two salts. After the chloroform-chlorine water test for distinction between halides $(\ce{salt\ solution + H2SO4 + CHCl3 + Cl2_{(aq)}})$ when I emptied the test-tube in the sink, all of it suddenly turned yellow.

What reaction could have occurred to cause this?
The sink mostly contained diluted $\ce{H2SO4}$ and $\ce{HNO3}$ along with small quantities of $\ce{AgNO3, BaCl2, NH4OH}$, lead acetate and ammonium molybdate.

All of these would mostly be in small quantities since I had rinsed the sink with water before emptying the test-tube.
[...] diluted $\ce{H2SO4}$ and $\ce{HNO3}$ [...] $\ce{NH4OH}$ [...] and ammonium molybdate.


The yellow precipitate indicates that phosphate was present too. Under these conditions a yellow ammonium molybdatophosphate is formed:

$\ce{H2PO4^- + 22 H+ + 12 MoO4^2- +3 NH4+ -&gt; \underset{yellow}{(NH4)3[P(Mo3O10)4]} +12H2O}$