A student mixes two solutions together. One is believed to be HI, the other is believed to be HNO3. A precipitate forms. Believing the results may be in error the student repeats the process twice in separate wells. After repeating the trial there was no observed precipitate. What is the most likely reason for the initial precipitate result?

The presence of silver nitrate impurities is the most probable cause for the initial precipitate result.

Explanation:

In test for halide ions, silver nitrate is used to test for the presence of soluble halide ions. The presence of in the reaction mixture (possibly the HNO3) gave the semblance of test for chlorides, allowing for formation of precipitation in the initial experiment.

This error could have been caused by handling of reagents and used laboratory apparatus during the initial reaction.