What is a pure substance?

What is a pure substance?

• A. A substance with fixed composition and definite properties; either an element or a compound.
• B. A substance that contains two or more substances.
• C. A substance that can be separated by physical means.
• D. A substance with variable composition.

Answer: (A)

A pure substance has fixed composition and definite properties; it is either an element or a compound. This means every sample of a pure substance has the same ratio of parts and behaves consistently under the same conditions. Elements are substances made of one kind of atom, like oxygen or gold, while compounds are made from two or more elements bonded together in a definite ratio, like water (hydrogen and oxygen) or table salt (sodium and chlorine). Because the composition is fixed, pure substances show uniform properties such as melting and boiling points. If a substance contains two or more substances or can be separated into simpler parts by physical means, it is not pure but a mixture. Mixtures can vary in composition and can often be separated by physical processes like filtration or evaporation. A substance with variable composition is also not pure, since its makeup isn’t fixed. Therefore, the statement that best describes a pure substance is that it has fixed composition and definite properties, being either an element or a compound.

A pure substance has fixed composition and definite properties; it is either an element or a compound. This means every sample of a pure substance has the same ratio of parts and behaves consistently under the same conditions. Elements are substances made of one kind of atom, like oxygen or gold, while compounds are made from two or more elements bonded together in a definite ratio, like water (hydrogen and oxygen) or table salt (sodium and chlorine). Because the composition is fixed, pure substances show uniform properties such as melting and boiling points.

If a substance contains two or more substances or can be separated into simpler parts by physical means, it is not pure but a mixture. Mixtures can vary in composition and can often be separated by physical processes like filtration or evaporation. A substance with variable composition is also not pure, since its makeup isn’t fixed. Therefore, the statement that best describes a pure substance is that it has fixed composition and definite properties, being either an element or a compound.