Demonstration Experiment for Amphoteric Oxide

Our lab has ZnO and Al2O3.

ZnO ( about 0.5g solid ) + HCl (1M), turns universal indicator from
red to green (pH7.0).

But things didn't go so well when I tried

ZnO + NaOH (I've tried 0.5M, 1M and 4M)

I set two conical flasks:

One contains (NaOH + universal indicator) as a control.
One contains the same stuff plus the amphoteric oxide.

Two things happened:
1. In the control flask, NaOH turned the purple universal indicator
'blue'! Anyone care to suggest what may have happened? This happened
for all different concentrations of NaOH.

2. No matter how much amphoteric oxide i added into the NaOH solution,
the universal indicator remained blue. It also turned red litmus paper
blue. Theoretically, amphoteric oxdie should be able to neutralise
both acids and bases. What could have gone wrong?

Please help. Thanks.

.

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