The Discovery of Oxygen & Air Composition

• Oxygen occurs naturally in the air, a mixture of gases.

Composition of Dry Air

• Nitrogen: 78%, the most abundant and mainly inert gas.
• Oxygen: 21%, essential for respiration and combustion.
• Argon: 1%, is an inert noble gas.
• Carbon Dioxide: 0.04%, important for photosynthesis and acts as a greenhouse gas.
• Other gases and varying amounts of water vapor are also present.
• Oxygen is crucial for life because it supports breathing in animals and combustion.

Phlogiston Theory

• Before oxygen was discovered, scientists believed in the Phlogiston Theory.
• Phlogiston was thought to be a substance in all materials that could burn.
• Burning was believed to release phlogiston.
  • A burning candle releases phlogiston.
  • A log turns to ash as phlogiston escapes.
• The theory was wrong because metals gain mass when burned (e.g., magnesium forming magnesium oxide).
• If phlogiston was escaping, the metal should get lighter, but it got heavier.

Discovery of Oxygen

• Lavoisier disproved the Phlogiston Theory in the 1770s.
• Combustion and rusting need a specific gas from the air.
• This gas increases in mass, proving something was added, not lost like phlogiston.
• Lavoisier named this gas "oxygen," meaning "acid producer."
• Water is made of oxygen and hydrogen.
• Modern definition of elements were developed.
• Law of Conservation of Mass was established, stating matter cannot be created or destroyed.

Uses of Gases

• Oxygen is used in medical breathing support, welding, and rocket fuel.
• Nitrogen is used to make fertilizers, in food packaging, and as a coolant (liquid nitrogen).
• Argon is used in light bulbs and welding.
• Carbon dioxide is used in fizzy drinks, fire extinguishers, and photosynthesis.

Oxides

• Acidic oxides are formed by non-metals and dissolve in water to form acids (e.g., CO2 forms carbonic acid, SO2 forms sulfuric acid).
• Basic oxides are formed by metals and dissolve in water to form bases (e.g., calcium oxide forms limewater).
• Neutral oxides are neither acidic nor basic, like carbon monoxide (CO) and water (H2O).

Fire Triangle

• Fire Triangle: consists of oxygen, heat, and fuel.
  • Oxygen supports combustion.
  • Heat provides the energy to start a fire.
  • Fuel is the substance that burns.
• Removing any one of these elements will stop a fire.
• Fuel: Any material that can be burned to release energy, e.g., wood, petrol, and gas.

Combustion

• Complete combustion occurs when there is plenty of oxygen, forming carbon dioxide and water.
• Incomplete combustion occurs when there is not enough oxygen, producing carbon monoxide (a toxic gas) and soot (carbon particles), and releases less energy.

Air Pollution from Burning Fossil Fuels

• Fossil fuels (coal, oil, gas) contain carbon, hydrogen, and sulfur.
• When burned, they produce:
  • Carbon dioxide contributes to global warming.
  • Carbon monoxide is a poisonous gas that reduces oxygen in blood.
  • Sulfur dioxide forms acid rain, damaging forests and buildings.
  • Nitrogen oxides contribute to smog and acid rain.

Alternative Energy

• Renewable energy sources do not run out and are environmentally friendly.
  • Solar energy converts sunlight into electricity.
  • Wind energy uses wind turbines to generate electricity.
  • Hydroelectric power uses flowing water to generate power.
  • Geothermal energy uses heat from the Earth's core.
• These sources reduce reliance on fossil fuels and decrease pollution.

Rusting

• Rusting occurs when iron reacts with oxygen and water to form iron oxide (rust).
• Prevention methods include painting/coating, galvanizing (coating with zinc), and using oil or grease to prevent air and water contact.

Experimental Variables

• Independent variable: The factor you change in an experiment.
• Dependent variable: The factor you measure in response.

Sample Size

• A larger sample size reduces errors and improves accuracy.
• Repeating experiments and averaging results ensures reliability.