The element oxygen, represented by the symbol O, is classified as
A. a compound.
B. a mixture.
c. a pure substance.
D.
a solution.

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3. how much energy in joules is required to evaporate .0005 kg of liquid ammonia to vapor at the same temperature? 4. how much energy ( in megajoules ) is given up by .75 kg of water at 0c when it freezes to form ice at 0c? 5. explain how heat works between and at critical temperatures?