oxygen and nitrogen in air sodium and chlorine in salt hydrogen and oxygen in water nitrogen and hydrogen in ammonia
I gave my students this multiple choice question: Which of the following combinations is a mixture rather than a compound?

Only one answer is correct among these 4 choices:
- oxygen and nitrogen in air
- sodium and chlorine in salt
- hydrogen and oxygen in water
- nitrogen and hydrogen in ammonia

Could you tell me which one is correct?

oxygen and nitrogen in air