Answer:
Incomplete precipitation of barium sulfate
Step-by-step explanation:
The student has precipitated and digested the barium sulfate on his/her side. But on the addition of
in the solution, the solution become cloudy. This happened because incomplete precipitation of barium sulfate by the student. When
is added, there are still sulfate ions present in the solution with combines with
and forms
and the formation of this precipitate makes the solution cloudy.