Answer:
physical change because even though gas formation was observed the water was undergoing a state change which means that it's original properties are preserved
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Explanation:
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oxidation reaction
2 Na2O → 4 Na + O2
This is an oxidation-reduction (redox) reaction:
4 NaI + 4 e- → 4 Na0(reduction)
2 O-II - 4 e- → 2 O0(oxidation)
Na2O is an oxidizing agent, Na2O is a reducing agent.
Na2S
Na is a metal, S is a non-metal, so Na2S is an ionic compound.
, where 
(in some books 6.022).