Which one of the following is not a solution?

Q: What is the answer to NDA (I) 2022 GAT Q57?

D. Correct — D, Sugar. A solution is a homogeneous mixture of two or more substances. An alloy is a solid solution of metals and air is a homogeneous mixture (solution) of gases, so both are solutions. Sugar, however, is a single pure chemical compound (sucrose) — not a mixture at all — so it cannot be a solution. It is the odd one out.

(a)Alloy
(b)Milk
(c)Air
(d)Sugar

Answer

Why

Correct — D, Sugar. A solution is a homogeneous mixture of two or more substances. An alloy is a solid solution of metals and air is a homogeneous mixture (solution) of gases, so both are solutions. Sugar, however, is a single pure chemical compound (sucrose) — not a mixture at all — so it cannot be a solution. It is the odd one out.

Why the others are wrong

(a)
Alloy — An alloy such as brass or bronze is a solid solution of two or more metals, so it is a solution — not the answer.
(b)
Milk — Milk is a mixture of water, fats and proteins; it is technically a colloid rather than a true solution, but it is still a mixture — whereas sugar is a single pure compound, making it the clearer 'not a solution'.
(c)
Air — Clean, dry air is a homogeneous mixture of nitrogen, oxygen and other gases — a gaseous solution — so it is not the answer.

Concept

A solution is a homogeneous mixture of two or more substances, such as a solid solution (alloy), a gaseous solution (air) or a liquid solution (salt in water). A pure substance made of only one kind of particle — an element or a single compound like sugar — is not a solution because there is nothing mixed into it.

Three of the four options are mixtures: an alloy is a solid solution, air is a gaseous solution, and milk is a colloid. Only sugar is a single pure compound and so not a solution of anything. Milk is the tempting trap because a colloid is not a true solution either, but sugar is the definitive answer since it is not a mixture at all.

Key facts

A solution is a homogeneous mixture; an alloy is a solid solution and air is a gaseous solution.
Sugar (sucrose) is a single pure compound, not a mixture, so it is not a solution.
Milk is a colloid — an emulsion of fat droplets in water — which is a mixture but not a true solution.
True solutions are transparent and do not scatter a beam of light, unlike colloids.

Polished golden brass vessels and objects, brass being an alloy of copper and zinc — Brass is a solid solution of copper and zinc — an everyday example of an alloy counting as a 'solution', while pure sugar is not a mixture at all.Image: user:shakko · Wikimedia Commons (CC BY-SA 3.0)

Study next

Pure substances versus mixturesTrue solutions, colloids and suspensionsAlloys as solid solutionsHomogeneous versus heterogeneous mixturesThe Tyndall effect

Common traps

Picking milk because it is not a 'true' solution — but milk is still a mixture, whereas sugar is a pure compound.
Forgetting that alloys and clean air both count as solutions.

Asks which option is NOT a solution; separate the pure substance (sugar) from the mixtures (alloy, air, milk).

Related PYQs

NDA_GAT_2021_I_Q602021Open & attempt →

Solder is an alloy of

(a) Cu and Sn.
(b) Fe and Zn.
(c) Pb and Sn.
(d) Ag and Zn.

Answer(c) Pb and Sn.

Same area — alloys as solid solutions. That NDA item identifies solder as an alloy of lead and tin; this one uses the fact that an alloy is a solution to rule it out as the 'not a solution' answer.

Practice

practice — not a real PYQ
Which one of the following is a colloid?
- (a)sugar solution
- (b)milk
- (c)brass
- (d)common salt
Answer(b) milk — a colloidal emulsion of fat droplets in water.
practice — not a real PYQ
An alloy is best described as a
- (a)compound
- (b)solid solution
- (c)pure element
- (d)suspension
Answer(b) solid solution — a homogeneous mixture of metals.