CBSE Class 9 Science Notes Chapter 2 - Matter Around Us Pure

Matter is composed of two or more components known as substances. In science, a substance is a form of matter that cannot be separated into other forms by physical means. A "pure substance" refers to only one component, with no other mixed substances. When different substances are combined, the result is called a mixture.

1.0Chemical Classification of Matter

Matter around us can be classified into pure substances and mixtures based on its chemical nature

Pure Substances

A pure substance consists of only one type of particle and is divided into:

• Elements: Substances that cannot be broken down into simpler substances. Examples include oxygen, gold, and hydrogen.
• Compounds: Substances formed by combining two or more elements in fixed ratios. Examples include water (H₂O) and salt (NaCl).

Mixtures

Mixtures contain two or more pure substances physically combined, retaining their individual properties:

• Homogeneous Mixtures: Have a uniform composition throughout, like saltwater or air.
• Heterogeneous Mixtures: Have a non-uniform composition, like a salad or sand mixed with iron filings.

2.0Solutions

A solution is a homogeneous mixture of a solvent (major component) and a solute (minor component). Examples include lemonade and soda water.

Properties of a Solution:

• Homogeneous mixture.
• Particles are smaller than 1 nanometer and invisible to the naked eye.
• Does not scatter light (Tyndall effect).
• Stable and cannot be separated by filtration.

Concentration of a Solution

It is the amount of solute in a given solution. Based on solute quantity, solutions can be dilute, concentrated, or saturated (when no more solute can dissolve at a given temperature). Concentration is commonly expressed in three ways:

Mass by Mass Percentage: Mass of solute per mass of solution.

$Massbymasspercentage=\frac{Massofsolute}{Massofsolution}\times 100$

Mass by Volume Percentage: This measures the solute's mass relative to the solution's volume.

$Massbyvolumepercentage=\frac{Massofsolute}{Volumeofsolution}\times 100$

Volume by Volume Percentage: Volume of solute per volume of solution.

$Volumebyvolumepercentage=\frac{Volumeofsolute}{Volumeofsolution}\times 100$

3.0Suspensions

A suspension is a heterogeneous mixture in which solid particles are dispersed in a liquid but remain undissolved.

Properties of a Suspension:

• Heterogeneous mixture.
• Visible particles.
• It scatters light, showing the Tyndall effect.
• Particles settle over time.
• It can be separated by filtration.

4.0Colloidal Solutions

A colloid is a heterogeneous mixture in which particles are uniformly distributed but not visible to the naked eye.

Properties of Colloidal Solutions:

• Heterogeneous mixture.
• Scatter light, making the Tyndall effect visible.
• Particles do not settle out.
• It cannot be separated by filtration but can be separated through centrifugation.

Components of a Colloid:

• Dispersed Phase: The particles that are distributed within a mixture.
• Dispersion Medium: The substance in which these particles are suspended.

The following table clearly compares the properties of true solutions, suspensions, and colloidal solutions:

5.0Physical and Chemical Changes

• Physical Changes: Changes that affect the physical properties of matter without changing its chemical composition. Examples include changes in state (melting, boiling).
• Chemical Changes  Result in the formation of new substances with different properties. Examples include iron rusting and combustion.

6.0Pure Substances: Elements and Compounds

Elements: Form of matter that cannot be broken down into simpler substances by chemical reactions. First defined by Antoine Lavoisier, elements are classified as:

• Metals: Shiny, malleable, good conductors (e.g., gold, silver, copper).
• Non-metals: Dull, brittle, poor conductors (e.g., oxygen, iodine).
• Metalloids: Have properties of both metals and nonmetals (e.g., silicon, boron).

Compounds: Formed when two or more elements chemically combine in fixed proportions. The properties of compounds differ from their constituent elements, and their composition is uniform throughout.

7.0Differences Between Mixtures and Compounds