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Multiple Choice
Which of the following is a pure substance?
A
NaCl (table salt)
B
Seawater
C
Brass
D
Air
Verified step by step guidance
1
Understand the definition of a pure substance: A pure substance has a fixed composition and distinct chemical properties. It can be either an element or a compound, but not a mixture.
Analyze each option to determine if it is a pure substance or a mixture:
Seawater is a mixture because it contains water, salts, and other dissolved substances in varying amounts.
Brass is a mixture (an alloy) composed mainly of copper and zinc, so it does not have a fixed composition.
Air is a mixture of gases such as nitrogen, oxygen, carbon dioxide, and others, with variable composition.
NaCl (table salt) is a pure substance because it is a chemical compound with a fixed ratio of sodium and chlorine atoms.