Is air a mixture or compound?

Air is a homogeneous mixture of gases: ~78% N₂, ~21% O₂, ~1% Ar, and trace amounts of CO₂, water vapor, etc.

Is air a mixture or a compound?

Air is a homogeneous mixture, not a compound. It is roughly 78% N₂, 21% O₂, ~1% Ar plus traces of CO₂ and water vapour — but those proportions vary with place and altitude, there is no fixed formula, and the gases retain their own properties and can be separated by fractional distillation of liquid air.

Why does a mixture have a variable composition but a compound does not?

In a compound, atoms are chemically bonded in a fixed whole-number ratio set by the chemical formula (Law of Definite Proportions) — water is always 2 H : 1 O by atoms. In a mixture, substances are only physically mingled with no bonding, so you can add more or less of any component, making the ratio variable.

What is the difference between a homogeneous and a heterogeneous mixture?

A homogeneous mixture has a uniform composition throughout with no visible boundaries (salt water, air, brass) — also called a solution. A heterogeneous mixture has a non-uniform composition with visible different parts (sand in water, oil and water, a salad). Colloids like milk are technically heterogeneous at the microscopic level even though they look uniform.

Can a compound be separated by physical methods?

No. A compound can only be broken into its elements by a chemical change (e.g. electrolysis of water into H₂ and O₂, or heating to decompose it). Physical methods — filtration, evaporation, distillation, magnetism — separate the components of a MIXTURE, because nothing is chemically bonded there.

Last Updated: 19 June 2026Verified for JEE 2027

ChemistryJEE & NEET

Difference Between Mixture and Compound

Mixtures and compounds are fundamentally different. Mixtures are physical combinations of substances, while compounds are chemical combinations with fixed ratios and new properties.

Mixture vs Compound — Comparison Table

Aspect	Mixture	Compound
Composition	Variable proportions	Fixed proportions (by mass)
Separation	Physical methods (filtration, distillation)	Chemical reactions only
Properties	Retain individual properties	New properties emerge
Energy	No energy change on mixing	Energy released or absorbed on formation
Formula	No chemical formula	Has a definite chemical formula
Example	Air, salt water, alloys	H₂O, NaCl, CO₂

Key Points to Remember

Mixtures can be homogeneous (solutions) or heterogeneous (suspensions)

Compounds follow the Law of Definite Proportions

Alloys are homogeneous mixtures of metals

Solutions are homogeneous mixtures with uniform composition

How much this is worth in the exam

Classification of matter (element / compound / mixture, and homogeneous vs heterogeneous) is foundation Class 9 / early Class 11 material. It rarely appears as a direct question in JEE/NEET, but "classify the following" and pure-vs-impure-substance statements show up in NEET assertion-reason items and in the Some Basic Concepts of Chemistry unit. The real value is conceptual: getting this wrong corrupts mole-concept and solution problems later.

Three questions that classify any sample

Ask in order: (1) Is the composition fixed by a formula? Fixed → compound; variable → mixture. (2) Did mixing release/absorb heat and create NEW properties? Yes → compound; no → mixture. (3) Can you separate it by a physical method (filter, distil, magnet, evaporate)? Yes → mixture; only chemical reaction can split it → compound. Air passes the "mixture" test on all three; water fails all three (it is a compound).

Classify these — the samples examiners actually use

The plain mixture-vs-compound table is easy; marks are lost on borderline samples. Work through these and the reasoning sticks.

Sample	Element / Compound / Mixture	Why
Air	Homogeneous mixture	Variable ratio of N₂, O₂, Ar, CO₂; no fixed formula; gases keep their properties
Sea water	Homogeneous mixture	Water + dissolved salts in variable amount; salt separable by evaporation
Brass / Bronze (alloys)	Homogeneous mixture	Cu+Zn (brass) in variable ratio — alloys are solid solutions, NOT compounds
Common salt (NaCl)	Compound	Na:Cl fixed 1:1; new properties; split only by electrolysis (chemical)
Distilled water (H₂O)	Compound	Fixed 2:1 H:O ratio, single boiling point of 100°C at 1 atm
Milk	Heterogeneous mixture (colloid)	Fat droplets dispersed in water — looks uniform but is a colloid, not a true solution
Diamond / Graphite	Element (allotropes of carbon)	Only one kind of atom (C); not a compound despite different appearances

Common mistakes students make

"Alloys like brass and steel are compounds because they are uniform."

Alloys are homogeneous MIXTURES (solid solutions). Their composition varies (brass can be 60–70% Cu), they have no fixed formula, and the metals can in principle be separated. Uniform appearance ≠ compound.

"A mixture has a sharp, fixed melting/boiling point like a pure substance."

No — a pure compound melts/boils at one fixed temperature; a mixture melts/boils over a RANGE. This is exactly how chemists check purity: an impurity lowers and broadens the melting point.

"All mixtures are cloudy/heterogeneous; anything that looks uniform is a compound."

Homogeneous mixtures (solutions like salt water, air, brass) look perfectly uniform but are still mixtures. Appearance does not decide it — fixed composition and new properties do.

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