Chapter 2: IS MATTER AROUND US PURE?

1. Introduction to Purity and Mixtures

• Scientific Definition of Pure: To a common person, "pure" means no adulteration. To a scientist, it means all constituent particles of a substance are the same in their chemical nature. A pure substance consists of a single type of particle.
• Mixtures: Most matter around us (like sea water, minerals, soil, milk) exists as mixtures. Mixtures are constituted by more than one kind of pure form of matter (substance).
• Separation: A substance cannot be separated into other kinds of matter by any physical process (e.g., dissolved sodium chloride can be separated from water by evaporation, but sodium chloride itself is a pure substance).

2. Types of Mixtures

Depending on the nature of the components, mixtures are classified into two main types:

• Homogeneous Mixtures (Solutions): Mixtures with a uniform composition throughout.
  Examples: Salt dissolved in water, sugar in water.
• Heterogeneous Mixtures: Mixtures containing physically distinct parts and non-uniform composition.
  Examples: Sodium chloride and iron filings, oil and water, salt and sulphur.

3. What is a Solution?

A solution is a homogeneous mixture of two or more substances. Homogeneity exists at the particle level.

Components of a Solution

• Solvent: The component that dissolves the other component (usually present in larger amount).
• Solute: The component dissolved in the solvent (usually present in lesser quantity).
• Examples: Tincture of iodine (iodine in alcohol), aerated drinks (CO₂ in water), air (gas in gas).

Properties of a Solution

• It is a homogeneous mixture.
• Particles are extremely small (less than 1 nm in diameter) and cannot be seen by naked eyes.
• They do not scatter a beam of light (path of light is not visible).
• The solution is stable; solute particles do not settle when undisturbed.
• Filtration cannot separate the solute particles.

Concentration of a Solution

The concentration is the amount of solute present in a given amount of solution.

• Saturated Solution: A solution in which no more solute can be dissolved at a given temperature.
• Unsaturated Solution: If the amount of solute is less than the saturation level.
• Solubility: The amount of solute present in a saturated solution at a specific temperature.

Formulas for Concentration:

4. Suspensions

A suspension is a heterogeneous mixture containing solids dispersed in liquids where the solute particles do not dissolve but remain suspended.

Properties of a Suspension

• It is a heterogeneous mixture.
• Particles are visible to the naked eye.
• Particles scatter a beam of light making the path visible (until they settle).
• It is unstable; particles settle down when left undisturbed.
• Components can be separated by filtration.

5. Colloidal Solutions

A colloid appears homogeneous but is actually heterogeneous (e.g., milk). The particle size is between that of a true solution and a suspension.

Properties of a Colloid

• It is a heterogeneous mixture.
• Particles are too small to be seen individually by the naked eye.
• Tyndall Effect: Colloids are big enough to scatter a beam of light passing through them, making the path visible (e.g., sunlight through a canopy of a dense forest).
• Colloids are stable (particles do not settle).
• Cannot be separated by filtration but can be separated by centrifugation.

Components

• Dispersed Phase: The solute-like component.
• Dispersion Medium: The component in which the dispersed phase is suspended.
• Examples: Fog (liquid in gas), Smoke (solid in gas), Shaving cream (gas in liquid), Milk (liquid in liquid).

6. Physical and Chemical Changes

• Physical Change: Changes that occur without a change in composition or chemical nature.
  Characteristics: Interconversion of states (solid to liquid to gas). Although ice, water, and steam look different, they are chemically the same.
• Chemical Change: One substance reacts with another to undergo a change in chemical composition.
  Characteristics: Brings change in chemical properties and results in new substances. Also called a chemical reaction.
  Example: Burning (change in odour and inflammability).

7. Types of Pure Substances

Pure substances are classified into elements and compounds based on chemical composition.

Elements

An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions (Definition by Lavoisier). They are divided into:

• Metals: Lustrous, conduct heat/electricity, ductile, malleable, sonorous (e.g., Gold, Iron, Copper). Mercury is the only liquid metal at room temperature.
• Non-metals: Non-lustrous, poor conductors, not malleable/sonorous, variety of colours (e.g., Oxygen, Carbon, Iodine).
• Metalloids: Intermediate properties between metals and non-metals (e.g., Boron, Silicon).

Compounds

A compound is a substance composed of two or more elements chemically combined in a fixed proportion.

Key distinction from mixtures: When elements are just mixed (e.g., iron filings and sulphur powder), they retain their individual properties (magnetism). When heated strongly to react, they form a compound with entirely new properties (non-magnetic).

8. Comparison: Mixtures vs. Compounds