Which one of the following is a "substance" in the sense of the word as used in your textbook?

Question

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In the context of chemistry, a substance is a pure form of matter that has a fixed composition and distinct properties. Water is a substance because it is a pure compound made up of hydrogen and oxygen in a fixed ratio, and it has distinct physical and chemical properties. Air, tap water, and sea water are all mixtures and therefore not considered pure substances. Toothpaste is also a mixture of various substances and not a pure substance.

Which One of the Following Is a "Substance" in the Sense