Marina tested a solution containing one of 0.1 M of NH4+, Ca2+, Fe3+, Cr, or PO43+. Several drops of 6. M NaOH solution is add to 2 mL of this solution in the test tube and the solution was gently heated in a water bath.When a piece of moist litmus was held at the mouth of the test tube, it turned blue. Which ion was in this solution?

Fig: 1