Which of the following are pure substances?
(A) Air
(B) Sea Water
(C) Gold
(D) Milk
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) only
A pure substance is made of only one kind of particles, so it has a fixed composition throughout: an element or a compound. A mixture contains two or more substances that are not chemically combined and can be present in any proportion.
(A) Air Incorrect. Air is a mixture of gases, mainly nitrogen and oxygen with argon, carbon dioxide and water vapour, in proportions that vary.
(B) Sea Water Incorrect. Sea water is a mixture of water and dissolved salts; the salts are left behind when the water evaporates.
(C) Gold Correct. Gold is an element, made of gold atoms only, so it is a pure substance.
(D) Milk Incorrect. Milk is a mixture (a colloid) of water, fat, proteins, sugar and minerals.