Gases in Atmosphere
Specification Reference
Gases in the Atmosphere
1. Approximate Percentages of Gases in Dry Air
4 Most Abundant Gases By Volume:
Nitrogen (N₂): Approximately 78%
Oxygen (O₂): Approximately 21%
Argon (Ar): Approximately 0.93%
Carbon Dioxide (CO₂): Approximately 0.04%
Variable gases: Water vapor because it depends on humidity
2. Determining Percentage of Oxygen in Air
Calculation of Percentage Oxygen = %Oxygen = (Volume decrease/Initial volume) x 100
Methods to Determine Percentage by Volume of Oxygen:
Using Metals (e.g., Iron):
React iron with oxygen in the air to form iron oxide.
Measure the decrease in volume of air due to oxygen consumption.
The water level in the burette will rise because the iron filings react with the oxygen in the
burette. The water rises to replace the oxygen that has reacted.
The burette water level should be read from at eye level using the bottom of the meniscus (curve of liquid)
Damp iron wool is placed inside a measuring cylinder.
The cylinder is inverted over water in a trough.
The initial volume of trapped air is recorded.
The iron reacts slowly with oxygen to form iron oxide (rust).
Water rises inside the cylinder.
The final volume of gas is recorded after several days.
% O2 in air = (change in height of water in the burette/initial volume of air in the burette) x 100
Using Non-Metals (e.g., Phosphorus):
Burn phosphorus in a closed container of air.
Measure the decrease in volume of air and relate it to the volume of oxygen consumed.
A small piece of phosphorus is placed on a heatproof dish.
It is ignited inside a sealed container over water.
Phosphorus reacts rapidly with oxygen.
White smoke of phosphorus(V) oxide forms.
Water rises as oxygen is removed.
Initial and final volumes are measured
The water level in the bell jar rises because the combustion of phosphorus uses up oxygen.
The water rises to replace the volume of oxygen used up.
Using Copper (Gas Syringe Method)
Two gas syringes connected by a glass tube containing copper turnings or powder, heated by a Bunsen burner.
Fill one syringe with a known volume of air (e.g., 100cm³) while the other is empty.
Heat the copper strongly while passing the air across it repeatedly using the plungers.
The oxygen reacts with the copper to form black copper(II) oxide (CuO)
3.Continue until the volume of gas stops decreasing.
4.Allow the apparatus to cool to room temperature before taking the final volume reading
3. Combustion of Elements in Oxygen
Types of Elements Discussed:
Magnesium:
Burns brightly with a white flame to form magnesium oxide to form a white powder (magnesium oxide) in an exothermic oxidation reaction
Magnesium + Oxygen → Magnesium Oxide
2Mg(s) + O2 (g) → 2MgO (s)
Hydrogen:
Burns with a pale/light blue flame that produces water
Hydrogen + Oxygen → Water
2H2(g) + O2 (g) → 2MgO (s)
Sulfur:
It burns with a blue flame to produce a colourless, poisonous gas (sulphur dioxide)
Sulphur + Oxygen → Sulphur dioxide
S (s) + O2 (g) → SO2 (g)
5. Carbon Dioxide as a Greenhouse Gas
Characteristics of Carbon Dioxide:
Known as a greenhouse gas.
Increasing levels of CO₂ in the atmosphere are linked to climate change, necessitating the understanding of its role in global warming.
6. Practical Experiment on Oxygen Percentage
Determine Approximate Percentage of Oxygen in Air:
Utilize either a metal or non-metal that reacts with oxygen (similar to above methods) to conduct the experiment.
Tests for Gases
1. Tests for Specific Gases
Hydrogen:
Test: Pop Test - A burning splint makes a characteristic 'pop' sound in the presence of hydrogen gas.
Oxygen:
Test: Glowing Splint Test - A glowing splint reignites in a sample of oxygen.
Carbon Dioxide:
Test: Limewater Test - Carbon dioxide turns limewater milky due to the formation of calcium carbonate.
Ammonia:
Test: Smell Test - Ammonia has a pungent smell; it turns moist red litmus paper blue.
Chlorine:
Test: Bleaching Test - Chlorine gas bleaches litmus paper and has a distinct sharp smell.