What is meant by pure substance?

A pure substance consists of only one kind of particles with a fixed composition throughout.

What are impurities in matter?

Impurities are unwanted substances mixed with a pure substance that alter its composition and properties.

What are the two categories of pure substances?

Elements and compounds are the two categories of pure substances.

An element is a substance made up of only one type of atom, for example, hydrogen or oxygen.

A compound is a pure substance formed by the chemical combination of two or more elements in fixed proportions.

Give an example of a compound.

Water (H2O) is a compound made from hydrogen and oxygen.

A mixture is a combination of two or more substances physically mixed but not chemically combined.

What are the types of mixtures?

Mixtures are of two types: homogeneous and heterogeneous.

Define a homogeneous mixture.

A homogeneous mixture has uniform composition and appearance throughout, like salt dissolved in water.

Define a heterogeneous mixture.

A heterogeneous mixture has visibly different components and non-uniform distribution, like sand in water.

Give two examples of homogeneous mixtures.

Sugar solution and air are homogeneous mixtures.

Give two examples of heterogeneous mixtures.

Oil and water, and soil are heterogeneous mixtures.

A solution is a homogeneous mixture of two or more substances, including a solute and solvent.

The substance present in smaller amount that dissolves in a solvent is called a solute.

The substance in which the solute dissolves to form a solution is called the solvent.

NCERT Science Class 9 Chapter 2: Is Matter Around Us Pure

9 CBSE Marks

★★★★★ Difficulty

10 Topics

Very High Board Weight

Topics Covered

10 key topics in this chapter

Pure Substances vs Mixtures

Types of Mixtures

Solutions: Solute & Solvent

Concentration of a Solution

Solubility

Suspension

Colloid (Tyndall Effect)

Separation Techniques

Physical & Chemical Changes

Elements, Compounds & Mixtures

Study Resources

📖 NCERT Notes ✏️ MCQ Practice 📝 Exercise Solutions ✅ True / False Quiz

Key Concepts

01. Pure Substance vs Mixture

A pure substance has a fixed composition and definite properties (e.g., iron, water). A mixture contains two or more substances combined physically and can vary in composition.

02. Types of Mixtures

Homogeneous mixtures (solutions) have uniform composition throughout — e.g., salt water, air. Heterogeneous mixtures (suspensions, colloids) have non-uniform composition — e.g., muddy water, milk.

03. Solution & Concentration

A solution consists of a solute dissolved in a solvent. Concentration = (mass of solute / mass of solution) × 100%. A saturated solution holds the maximum dissolved solute at a given temperature.

04. Colloid & Tyndall Effect

Colloids have particle size 1–100 nm and scatter light (Tyndall Effect), making a beam visible. Examples: milk, fog, smoke. Suspensions have particle size > 100 nm and settle on standing.

05. Separation Techniques

Evaporation (salt from water), distillation (volatile mixtures), chromatography (dyes), centrifugation (cream from milk), magnetic separation (iron from sand), separating funnel (immiscible liquids).

06. Elements & Compounds

Elements are pure substances that cannot be split into simpler substances (e.g., H, O, Fe). Compounds are pure substances formed from two or more elements in fixed mass ratio by chemical combination (e.g., H₂O, NaCl).

Formulas at a Glance

#	Name	Expression	Notes
01	Concentration (mass/mass)	`C = (m_solute / m_solution) × 100`	Mass percentage of solute
02	Concentration (mass/volume)	`C = (m_solute / V_solution) × 100`	Mass-volume percentage
03	Tyndall Particle Size	`1 nm < d < 100 nm (colloid)`	Colloidal particle size range
04	Suspension Particle Size	`d > 100 nm`	Suspension particle size
05	True Solution Particle Size	`d < 1 nm`	Solution particle size

Important Notes

A compound's properties are entirely different from its constituent elements — NaCl is safe to eat, but Na is reactive and Cl is toxic.

Distillation separates liquids with different boiling points; fractional distillation is used when boiling points are close.

Chromatography works because different components travel at different speeds on a stationary phase — used to separate ink dyes and plant pigments.

Air is a homogeneous mixture, not a compound — its composition can vary (CO₂ levels change, humidity varies).

Miscible liquids (alcohol + water) are separated by distillation; immiscible liquids (oil + water) by a separating funnel.

Exam Tips & Common Mistakes

Draw and label a separating funnel diagram for 3-mark questions — examiners award marks for the valve detail.

The Tyndall effect is the key difference between a colloid and a true solution — use it in one-line definitions.

"Why is air not a compound?" is a common 2-mark — properties of mixture vs compound is the answer.

State the component that is present in larger amount as "solvent" and the one in smaller amount as "solute".

Memorise the particle size hierarchy: Solution < Colloid < Suspension.

All Class IX Science Chapters

Ch 01 Ch 02 Ch 03 Ch 04 Ch 05 Ch 06 Ch 07 Ch 08 Ch 09 Ch 10 Ch 11 Ch 12

Is Matter Around Us Pure?