What is the Tyndall effect?

The scattering of light by particles in a colloid or a very fine suspension is called the Tyndall effect. When a beam of light passes through a colloidal solution, the particles in the solution scatter the light in different directions, making the path of the light visible.

What are elements and compounds?

An element is a substance made up of only one type of atom, while a compound is composed of two or more elements chemically bonded in a fixed ratio.

What is the difference between a solution, suspension, and colloid?

Solution: A homogeneous mixture with tiny particles that do not settle or scatter light. Suspension: A heterogeneous mixture with large particles that settle over time and scatter light. Colloid: A heterogeneous mixture with intermediate-sized particles that scatter light but do not settle.

Can elements exist in more than one form?

Yes, elements can exist in different forms, known as allotropes, such as carbon, in diamonds and graphites.

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Class 9 Science Is Matter Around Us Pure

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.

Class 9 Science Chapter 2 Revision Notes:

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.

Types	Solute	Solvent	Example
Solid solutions	solid	solid	Brass, bronze. steel,
	liquid	solid	dental amalgam,
	gas	solid	hydrogen in palladium
Liquid solutions	solid	liquid	sugar in water, seawater
	liquid	liquid	Vinegar, alcohol in water
	gas	liquid	soft drinks
Gaseous solutions	solid	gas	naphthalene in air
	liquid	gas	perfume in air
	gas	gas	carbon monoxide in the air

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.

$M a ss b y ma ss p erce n t a g e = \frac{M a ss o f so l u t e}{M a ss o f so l u t i o n} \times 100$

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

$M a ss b y v o l u m e p erce n t a g e = \frac{M a ss o f so l u t e}{V o l u m e o f so l u t i o n} \times 100$

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

$V o l u m e b y v o l u m e p erce n t a g e = \frac{V o l u m e o f so l u t e}{V o l u m e o f so l u t i o n} \times 100$

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.

Dispersed Phase	Dispersion Medium	Type of colloid	Example
Solid	Solid	Solid-sol	Ruby glass, alloys
Solid	Liquid	Sol	paint, ink
Solid	Gas	Aerosol	Smoke, haze
Liquid	Solid	Gel	Curd, cheese
Liquid	Liquid	Emulsion	Milk, Cream
Liquid	Gas	Liquid Aerosol	Cloud, mist
Gas	Solid	Solid foam	Cork, pumice stone
Gas	Liquid	Foam	Froths of air

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:

Property	True Solutions	Suspension	Colloidal Solution
Particle Size	Less than 10⁻⁹ m / 1 nm	Greater than 10⁻⁶ m / 1000 nm	Between 10⁻⁹ to 10⁻⁶ m / 1 to 1000 nm
Visibility of Particles	Invisible to naked eye and under a powerful microscope	Easily visible	Invisible to naked eye; visible under a powerful microscope
Sedimentation of Particles	Do not settle down	Settle down due to gravity	Settle down under high centrifugation
Filtration through Filter Paper	No residue is formed	Residue is formed	No residue is formed
Mixture Type	Homogeneous	Heterogeneous	Heterogeneous
Appearance	Clear and Transparent	Translucent or Opaque	Translucent

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

Mixtures	Compounds
Elements or compounds mix together without forming a new substance.	Elements react to form new substances.
A mixture has a variable composition.	The composition of a compound is always fixed.
A mixture retains the properties of its individual components.	The properties of a compound are different from those of its components.
Physical methods can separate components.	Components can only be separated by chemical or electrochemical reactions.

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