Is Matter Around Us Pure? – Complete Class 9 Science Notes (CBSE Chapter 2)
Are you a Class 9 student preparing for your CBSE Science exams and wondering, "Is Matter Around Us Pure?" This comprehensive guide provides detailed notes on CBSE Class 9 Science Chapter 2, covering everything from pure substances and mixtures to solutions, colloids, and separation techniques. Designed as a pillar page for topical authority, these notes are based on NCERT curriculum and include definitions, examples, key points, tables, and diagram descriptions to help you build a strong foundation in chemistry. Whether you're revising for exams or seeking a deeper understanding, this resource answers key questions like what makes a substance pure, the differences between elements and compounds, and how to separate mixtures.
Author: Semester Exam Point • Subject: Class 9 Science • Last updated:

Introduction to Matter and Purity
Matter is anything that occupies space and has mass. In our daily lives, we encounter various forms of matter, but is the matter around us pure? This chapter explores the concept of purity in substances. A pure substance consists of only one type of particle and cannot be separated into simpler components by physical means. In contrast, most matter we see is impure, made up of mixtures of different substances.
Pure substances have fixed properties, such as constant melting and boiling points, while mixtures do not. Understanding this distinction is crucial for topics like chemical reactions and material science. This chapter classifies matter into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous), and delves into solutions, suspensions, and colloids.
- What is a pure substance?
- How do mixtures differ from compounds?
- What are the types of solutions and their properties?
Pure Substances: Definition and Types
A pure substance is matter that contains only one kind of particle, has a uniform composition, and cannot be broken down into simpler substances by chemical or physical methods. Pure substances exhibit consistent physical and chemical properties, such as fixed melting and boiling points.
Types of Pure Substances
1) Elements
- Definition: The simplest form of pure substances, consisting of only one type of atom. Elements cannot be further broken down by chemical reactions.
- Properties: Represented by symbols (e.g., H for Hydrogen, C for Carbon). There are about 118 known elements.
- Classification:
- Metals: Hard, shiny, malleable, ductile, good conductors of heat and electricity. Examples: Iron (Fe), Copper (Cu), Aluminium (Al), Gold (Au).
- Non-Metals: Brittle, poor conductors, not malleable or ductile. Examples: Carbon (C), Sulphur (S), Oxygen (O), Hydrogen (H).
- Metalloids: Show properties of both metals and non-metals. Examples: Silicon (Si), Boron (B), Arsenic (As).
- Examples: Oxygen gas, diamond (pure carbon).
- Key Point: Most elements are solids at room temperature, except mercury (liquid) and bromine (liquid non-metal).
2) Compounds
- Definition: Pure substances formed by the chemical combination of two or more elements in a fixed ratio by mass.
- Properties: Have different properties from their constituent elements; cannot be separated by physical methods; fixed composition.
- Examples: Water (H₂O – 2 hydrogen atoms + 1 oxygen atom), Carbon Dioxide (CO₂), Ammonia (NH₃).
- Types:
- Inorganic: From non-living sources, e.g., NaCl (common salt), H₂SO₄ (sulphuric acid).
- Organic: Contain carbon, from living sources, e.g., glucose (C₆H₁₂O₆).
- Key Point: Formation involves energy exchange (endothermic or exothermic reactions).
Comparison: Elements vs. Compounds
Feature | Elements | Compounds |
---|---|---|
Composition | One type of atom | Two or more elements in fixed ratio |
Separation | Cannot be broken down | Can be decomposed chemically |
Examples | Hydrogen, Iron | Water, Sodium Chloride |
Properties | Unique to the element | Different from constituent elements |
Mixtures: Definition and Types
A mixture is formed when two or more substances are physically combined in any proportion, without chemical bonding. Mixtures retain the properties of their components and can be separated by physical methods.
Types of Mixtures
1) Homogeneous Mixtures
- Uniform composition throughout; components are not visible separately.
- Examples: Salt in water (saline solution), air (mixture of gases like N₂, O₂). Also known as solutions.
2) Heterogeneous Mixtures
- Non-uniform composition; components are visible and can settle.
- Examples: Sand in water, oil and water, mixture of iron filings and sulphur.
Key Differences: Homogeneous vs. Heterogeneous Mixtures
Aspect | Homogeneous Mixture | Heterogeneous Mixture |
---|---|---|
Composition | Uniform | Non-uniform |
Visibility | Components not visible | Components visible |
Separation | Difficult (e.g., distillation) | Easy (e.g., filtration) |
Examples | Sugar solution, alloys | Fruit salad, concrete |
Solutions, Suspensions, and Colloids
Solutions
- Definition: Homogeneous mixture where solute particles are very small (<1 nm) and dissolve completely in the solvent.
- Components: Solute (dissolved substance) and Solvent (does the dissolving; water is a common solvent).
- Concentration Types: Unsaturated (can dissolve more), Saturated (no more dissolves at that temperature), Supersaturated (more than saturated; unstable).
- Properties: Particles don't settle; no Tyndall effect; transparent.
- Examples: Tincture of iodine (iodine in alcohol), soda water (CO₂ in water).
- Solubility: Amount of solute that dissolves in 100 g of solvent at a given temperature; usually increases with temperature for solids but decreases for gases.
Suspensions
- Definition: Heterogeneous mixture where particles (>1000 nm) do not dissolve and settle down on standing.
- Properties: Opaque; particles visible; show Tyndall effect; can be separated by filtration.
- Examples: Muddy water, chalk in water.
Colloids
- Definition: Heterogeneous mixtures with particle size 1–1000 nm; particles dispersed but do not settle.
- Properties: Show Tyndall effect; appear homogeneous but are not; stable.
- Types by phase: Sol (solid in liquid, e.g., paint), Emulsion (liquid in liquid, e.g., milk), Foam (gas in liquid, e.g., whipped cream), Aerosol (liquid/solid in gas, e.g., fog), Gel (liquid in solid, e.g., jelly).
Comparison Table: Solution, Suspension, Colloid
Property | Solution | Suspension | Colloid |
---|---|---|---|
Particle Size | < 1 nm | > 1000 nm | 1–1000 nm |
Appearance | Transparent | Opaque | Translucent |
Tyndall Effect | No | Yes | Yes |
Settling | No | Yes | No |
Separation | Evaporation/Distillation | Filtration | Centrifugation |
Examples | Salt water | Sand in water | Milk |
Separation Techniques for Mixtures
To obtain pure substances from mixtures, various physical methods are used. These exploit differences in boiling point, solubility, particle size, density, or magnetic properties.
Method | Principle | Used For | Example |
---|---|---|---|
Evaporation | Solvent has lower boiling point and evaporates | Solute from solution | Salt from seawater |
Centrifugation | Spinning separates by density | Fine solids from liquids | Cream from milk |
Decantation | Settling (sedimentation) then pouring | Insoluble solid from liquid | Sand + water |
Sublimation | Solid → Gas directly | Sublimable solid from mixture | NH₄Cl from salt |
Chromatography | Different adsorption/solubility | Colored components | Ink pigments |
Distillation | Different boiling points | Liquid from liquid | Water + alcohol |
Fractional Distillation | Close boiling points + fractionating column | Mixture of liquids | Petroleum fractions, air gases |
Crystallization | Solubility changes with temperature | Purify solids | CuSO₄ crystals |
Magnetic Separation | Magnetic vs non-magnetic | Iron mixtures | Iron filings + sulphur |
Physical and Chemical Changes
- Physical Change: No new substance formed; reversible; change in state/shape (e.g., melting ice, dissolving sugar).
- Chemical Change: New substance formed; often irreversible; energy change (e.g., rusting iron, burning paper, souring milk).
Concentration of Solutions
- Mass by Mass % = (Mass of solute / Mass of solution) × 100
- Mass by Volume % = (Mass of solute / Volume of solution) × 100
- Volume by Volume % = (Volume of solute / Volume of solution) × 100
Practice: MCQs & Numericals
MCQs (Class 9 level)
- Which of the following is a pure substance?
- Air
- Milk
- Sodium chloride
- Soil
- Which mixture shows the Tyndall effect strongly?
- Salt solution
- Milk
- Alcohol-water solution
- Sugar solution
- Which method separates a dye mixture best?
- Filtration
- Chromatography
- Decantation
- Crystallization
Numerical (Solved)
Q: Find % (m/m) when 15 g sugar is dissolved to make 150 g solution.
Solution: % (m/m) = (15/150)×100 = 10%.
Frequently Asked Questions (FAQs)
What is the difference between a mixture and a compound?
Mixtures have variable composition and can be separated physically, while compounds have fixed ratios and require chemical separation.
Why is air considered a mixture?
Air has variable composition (N₂ 78%, O₂ 21%, etc.) and can be separated by fractional distillation.
What is the Tyndall effect?
Scattering of light by colloidal particles, making the beam visible (e.g., in fog).