Class 9th Science Study Notes

NCERT Class 9 – Science Chapter 2: Is Matter Around Us Pure? Comprehensive Study Notes By Sandeep Sen

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NCERT Class 9 – Science, Chapter 2: Is Matter Around Us Pure? Comprehensive Study Notes By Sandeep Sen

These educational resources focus on the classification and properties of matter, specifically distinguishing between pure substances and mixtures. Pure substances, like elements and compounds, consist of a single type of atom or molecule and exhibit consistent physical properties. In contrast, mixtures are categorized as homogeneous, like true solutions, or heterogeneous, like suspensions and colloids, based on their particle size and uniformity. The text details various scientific techniques to isolate these components, including chromatography, sublimation, and distillation. Additionally, it provides practical formulas for calculating solution concentration and explains the Tyndall effect, which describes how certain mixtures scatter light. By exploring these concepts, the materials help students understand the chemical and physical behaviors of substances in the world around them.

STUDY NOTES: IS MATTER AROUND US PURE?

1. Classification of Matter

Matter is anything that has mass and occupies space. Scientific classification divides matter into two broad categories based on its chemical composition: Pure Substances and Impure Substances (Mixtures).

A. Pure Substances

A pure substance is a homogeneous material with a definite and invariable chemical composition and distinct properties. It consists of only one type of atom or molecule..

  • Characteristics (The “CID” Rule):
  • C – Consistent Properties: They have fixed boiling points, melting points, density, and color.
  • I – Insensperability: They cannot be separated into simpler components by any physical process.
  • D – Definite Composition: They have a uniform and fixed chemical arrangement throughout their mass.

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B. Impure Substances (Mixtures)

An impure substance or mixture contains two or more different types of particles (atoms, molecules, elements, or compounds) combined physically in any random proportion.

  • Properties of Mixtures:

  1. The constituents retain their original chemical identities and properties.

  2. They can be separated into their components using physical methods.

  3. They do not have fixed melting or boiling points.

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2. Types of Pure Substances

Pure substances are further classified into Elements and Compounds.

I. Elements

An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions.

  • Metals: These are typically lustrous (shiny), good conductors of heat and electricity, malleable (can be hammered into sheets), and ductile (can be drawn into wires). Examples include Iron (Fe), Copper (Cu), and Gold (Au).
  • Non-metals: These are generally poor conductors, non-lustrous, and exist in various states (solid, liquid, or gas). Examples include Carbon (C), Oxygen (O₂), and Iodine (I₂)..
  • Metalloids: Elements that show intermediate properties between metals and non-metals. Examples include Boron (B), Silicon (Si), and Germanium (Ge).
II. Compounds

A compound is a pure material made of two or more elements chemically combined in a fixed proportion by mass.

Properties:

  • A compound has entirely new properties that differ from its constituent elements.

  • Constituents can only be separated using chemical or electrochemical reactions.

  • Formation of a compound involves an energy change (absorption or evolution of heat/light).

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3. Types of Mixtures

Mixtures are classified based on the uniformity of their distribution.

Types of mixture
Types of mixture

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4. Solutions, Suspensions, and Colloids

A. Solutions

A solution is a homogeneous mixture of two or more non-reacting substances.

  • Components:

  1. Solvent : The component present in a larger quantity that dissolves the other substance.

  2. Solute: The component present in a smaller quantity that gets dissolved.

  • Concentration Formulas:

  1. Mass by Mass % = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100.

  2. Mass by Volume % = \frac{\text{Mass of Solute}}{\text{Volume of Solution}} \times 100.

  3. Saturation: A saturated solution is one in which no more solute can be dissolved at a specific temperature.

B. Suspensions

A suspension is a heterogeneous mixture in which solute particles do not dissolve but remain suspended throughout the bulk of the medium.

  • Properties: Particles are larger than 100 nm, visible to the naked eye, and settle down when left undisturbed

C. Colloids

A colloid is a heterogeneous mixture that appears homogeneous to the naked eye.

  • Tyndall Effect: This is the scattering of a beam of light by colloidal particles, making the path of light visible.

  • Brownian Motion: The continuous zig-zag movement of colloidal particles caused by collisions with dispersion medium molecules.

  • Components: The Dispersed Phase (solute-like) and the Dispersion Medium (solvent-like).

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5. Physical vs. Chemical Changes

  • Physical Change: A temporary change affecting only the physical properties (state, shape, size) without forming a new substance. Most are reversible. Example: Melting ice, dissolving salt in water.

  • Chemical Change: A permanent change where substances react to form new substances with entirely different chemical identities. These are typically irreversible. Example: Burning wood, rusting iron, digestion of food.

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6. Methods of Separation

Separation techniques rely on differences in the physical properties of components.

  1. Evaporation: To separate a non-volatile solute (dye) from a volatile solvent (water).

  2. Centrifugation: Used to separate denser particles from lighter ones by spinning rapidly (e.g., cream from milk).

  3. Separating Funnel: To separate two immiscible liquids based on their density differences (e.g., oil and water).

  4. Sublimation: To separate a sublimable component (ammonium chloride, camphor) from a non-sublimable one (salt).

  5. Chromatography: Used to separate different solutes that dissolve in the same solvent (e.g., dyes in ink).

  6. Distillation: Used for miscible liquids with a significant difference in boiling points (more than 25°C).

  7. Fractional Distillation: Used for miscible liquids with boiling points differing by less than 25°C (e.g., air gases, petroleum).

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Analogy for Understanding:

Imagine a mixture is like a salad; you can see the tomatoes and cucumbers separately, and you can pick them out with your hands. A compound is like a baked cake; once the flour, eggs, and sugar are chemically combined by heat, you can no longer see them as individual ingredients, and they have transformed into something with completely different properties.