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

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:


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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.

Key questions addressed:
  • 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

FeatureElementsCompounds
CompositionOne type of atomTwo or more elements in fixed ratio
SeparationCannot be broken downCan be decomposed chemically
ExamplesHydrogen, IronWater, Sodium Chloride
PropertiesUnique to the elementDifferent 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

AspectHomogeneous MixtureHeterogeneous Mixture
CompositionUniformNon-uniform
VisibilityComponents not visibleComponents visible
SeparationDifficult (e.g., distillation)Easy (e.g., filtration)
ExamplesSugar solution, alloysFruit 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

PropertySolutionSuspensionColloid
Particle Size< 1 nm> 1000 nm1–1000 nm
AppearanceTransparentOpaqueTranslucent
Tyndall EffectNoYesYes
SettlingNoYesNo
SeparationEvaporation/DistillationFiltrationCentrifugation
ExamplesSalt waterSand in waterMilk
Diagram tip (Tyndall effect): Draw two beakers—one with milk (beam visible) and one with salt solution (beam not visible). Label the incident light and scattered light.

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.

MethodPrincipleUsed ForExample
EvaporationSolvent has lower boiling point and evaporatesSolute from solutionSalt from seawater
CentrifugationSpinning separates by densityFine solids from liquidsCream from milk
DecantationSettling (sedimentation) then pouringInsoluble solid from liquidSand + water
SublimationSolid → Gas directlySublimable solid from mixtureNH₄Cl from salt
ChromatographyDifferent adsorption/solubilityColored componentsInk pigments
DistillationDifferent boiling pointsLiquid from liquidWater + alcohol
Fractional DistillationClose boiling points + fractionating columnMixture of liquidsPetroleum fractions, air gases
CrystallizationSolubility changes with temperaturePurify solidsCuSO₄ crystals
Magnetic SeparationMagnetic vs non-magneticIron mixturesIron 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).
Exam tip: If properties change and a new substance forms (new color/gas/precipitate/temperature change), it’s likely a chemical change.

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
Example: 20 g salt in 200 g solution ⇒ (20/200) × 100 = 10% (m/m).

Practice: MCQs & Numericals

MCQs (Class 9 level)

  1. Which of the following is a pure substance?
    1. Air
    2. Milk
    3. Sodium chloride
    4. Soil
  2. Which mixture shows the Tyndall effect strongly?
    1. Salt solution
    2. Milk
    3. Alcohol-water solution
    4. Sugar solution
  3. Which method separates a dye mixture best?
    1. Filtration
    2. Chromatography
    3. Decantation
    4. 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).