Question

which of the following statements about compounds is true?
a pure substance that has variable composition throughout.
a substance composed of two or more elements in fixed, definite proportions.
a substance that is not as common as pure elements.
a substance that cannot be broken down into simpler substances.
none of the above

Explanation:

• A compound is a pure substance with a fixed composition. So, "A pure substance that has variable composition throughout" is false.
• Compounds are made of two or more elements in fixed, definite proportions. For example, water (\(H_2O\)) always has 2 hydrogen atoms and 1 oxygen atom.
• Compounds are very common. For instance, water, carbon dioxide (\(CO_2\)) are common compounds. So, "A substance that is not as common as pure elements" is false.
• Elements (not compounds) cannot be broken down into simpler substances. Compounds can be broken down (e.g., water can be decomposed into hydrogen and oxygen). So, "A substance that cannot be broken down into simpler substances" is false.

Answer:

A substance composed of two or more elements in fixed, definite proportions.