Why is water considered a pure substance but orange juice a compound?

A mixture is when two or more substances combine physically together. However, in water, two hydrogen atoms combine with one oxygen atom chemically, forming a new substance that has properties different from hydrogen alone or oxygen alone. Therefore, water is not a mixture; it is a compound and it is pure. A Orange juice contains particles of solid (pulp) as well as liquid; it is not chemically pure. B Because its composition is not uniform throughout, orange juice is a heterogeneous mixture. A Aluminum oxide is a single, chemically pure compound. A Selenium is one of the known elements.

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answer; i believe that the correct answer is (a); a large rock being weathered by wind and rain;