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Chapter 5: Water and Solution

Form 2 Science Bab 5: Water and Solution

5.1 Physical Characteristics of Water

Pure water is a neutral compound composed of hydrogen and oxygen molecules in a 2:1 ratio (H2O).

Physical Properties of Pure Water

  • Freezing point: 0°C
  • Boiling point: 100°C (at standard atmospheric pressure)
  • Density: 1 g/cm³ (or 1000 kg/m³) at 4°C
  • Color & Odor: Colorless, odorless, and tasteless liquid at room temperature
  • Surface Tension: High cohesive force between water molecules allows light insects (e.g., pond skaters) to walk on water.
  • Capillary Action: Combined action of cohesive forces (attraction between water molecules) and adhesive forces (attraction between water molecules and tube walls) allowing water to travel up narrow vessels in plants.

Effects of Impurities on Water

  • Impurities (e.g., salt, sugar) raise the boiling point of water above 100°C.
  • Impurities lower the freezing point of water below 0°C.
  • Impurities increase the density and electrical conductivity of pure water.

Composition of Water (Electrolysis)

Water can be broken down into its constituent elements using electrolysis:

  • Anode (Positive pole): Collects Oxygen gas (Ratio = 1 volume). Tests positive with a glowing wooden splint (relights).
  • Cathode (Negative pole): Collects Hydrogen gas (Ratio = 2 volumes). Tests positive with a lighted wooden splint ('pop' sound).

Evaporation of Water

Evaporation is the process where liquid water turns into water vapor at any temperature below its boiling point, occurring only at the surface.

Factors affecting rate of evaporation:

  1. Humidity: Higher humidity decreases evaporation rate.
  2. Surrounding Temperature: Higher temperature increases evaporation rate.
  3. Exposed Surface Area: Larger surface area increases evaporation rate.
  4. Movement of Air (Wind): Faster air movement increases evaporation rate.

5.2 Solution and Rate of Solubility

Key Terms

  • Solute: The substance that dissolves in a liquid (e.g., salt, sugar).
  • Solvent: The liquid that dissolves the solute (e.g., water, alcohol).
  • Solution: The homogeneous mixture formed when a solute dissolves in a solvent.
  • Suspension: A heterogeneous mixture containing insoluble particles floating in a liquid (e.g., muddy water, chalk powder in water).

Types of Solutions

Type of Solution Description
Dilute Solution Contains a small amount of dissolved solute in a large volume of solvent. Can still dissolve more solute.
Concentrated Solution Contains a large amount of dissolved solute in a given volume of solvent. Can still dissolve a small amount of solute.
Saturated Solution Contains the maximum amount of dissolved solute at a given temperature. Cannot dissolve any more solute; excess solute settles at the bottom.

Factors Affecting Rate of Solubility

  • Temperature of Solvent: Higher temperatures increase particle kinetic energy, raising solubility rate.
  • Rate of Stirring: Stirring spreads solute particles faster throughout the solvent.
  • Size of Solute Particles: Smaller particles have a larger total surface area exposed to the solvent, dissolving faster.

Organic Solvents

Water is a universal solvent, but non-polar substances require organic (non-aqueous) solvents:

  • Alcohol: Used to dissolve iodine, perfume, and varnish.
  • Acetone: Used to dissolve nail polish and cosmetics.
  • Turpentine: Used to dissolve paint and wax.
  • Kerosene: Used to dissolve grease and oil stains.
  • Ether: Used to extract oils and fats.

5.3 Water Purification and Water Supply

Water Purification Methods

  1. Boiling: Kills microorganisms but does not remove suspended solids or dissolved minerals.
  2. Chlorination: Kills harmful bacteria/microorganisms by adding chlorine, but does not remove impurities or dissolved salts.
  3. Filtration: Removes insoluble suspended particles, but does not remove microorganisms or dissolved substances.
  4. Distillation: Produces pure water (distilled water) free from microorganisms, suspended solids, and dissolved minerals.

Water Supply System Treatment Steps

  1. Filtration Screening: Removes large debris like twigs and leaves.
  2. Coagulation: Alum is added to clump suspended particles together; Slaked lime is added to reduce acidity.
  3. Flocculation: Flocs form as clumps grow larger.
  4. Sedimentation: Suspended clumps settle to the bottom of the tank.
  5. Sand Filtration: Removes remaining fine suspended particles.
  6. Chlorination & Fluoridation: Chlorine is added to kill bacteria; Fluoride is added to prevent dental caries.
  7. Storage & Distribution: Clean water is pumped to elevated storage reservoirs for public distribution.
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