Gases in the Atmosphere

Composition of the Earth's Atmosphere

• The Earth's atmosphere consists mainly of:
  • Nitrogen (N₂): 78%
  • Oxygen (O₂): 21%
  • Other gases: 1% (mainly Argon (Ar), traces of other noble gases, and Carbon Dioxide (CO₂) at 400 ppm or 0.04%)

Discovery of Oxygen

• Space Shuttle Oxygen Supply:
  • Question about how space shuttles carry their oxygen supply.
• Liquid Oxygen Transport:
  • Question about why oxygen is transported as a liquid.
• Mercury Heated in Air:
  • Question about what happens to mercury when heated in air.
• Priestley's Observation:
  • Question about what Priestley thought was given off when substances burned in oxygen.
• Lavoisier's Experiment:
  • Question about which non-metallic element Lavoisier first burned in air to test his oxygen theory.
• Magnesium Heating:
  • Question about what happens to the mass of a piece of magnesium after strong heating in air.
• Scheele's Method:
  • Question about what type of compound Scheele heated to collect oxygen gas.
• Modern Uses of Oxygen:
  • Question about two modern uses for oxygen.
• Key Scientists:
  • Question about naming the three key scientists and their nationalities in the discovery of oxygen.

Experiment OXID/1: Determining the Percentage of Oxygen in Air

• Apparatus:
  • Two gas syringes connected to a glass tube containing excess copper granules.
  • Total initial volume of air in both syringes: $100 cm^3$.
• Procedure:
  • Air is passed over heated copper in a Bunsen flame.
  • The volume of gas is measured after no further change is observed and the apparatus has cooled.
• Measurements:
  • Initial volume of air: $100 cm^3$
  • Final volume of air: to be recorded
  • Volume decrease: to be calculated
• Calculations:
  • Percentage of oxygen in air: Calculated using the volume decrease.
• Questions:
  • Ensuring unreacted copper: Why it's important to have some unreacted copper left over at the end of the reaction (Correct option: C - So that we know that all the Oxygen has been used up).
  • Cooling the apparatus: Why it's important to let the apparatus cool before measuring the final volume (Correct option: C - Because the hot gas expands and will make the readings on the syringe appear larger than they should).
  • Replacing copper with magnesium: Why this experiment wouldn't work if copper were replaced with magnesium (Correct option: B - Magnesium will also react with nitrogen and not just oxygen).

Experiment OXID/2: Percentage of Air Used Up When Iron Rusts

• Process:
  • Iron rusts, removing oxygen from the air.
  • Steel wool is placed in a measuring cylinder, soaked in water, and inverted over a beaker of water.
  • The setup is left for a week to allow the iron to rust.
• Measurements:
  • Initial volume of air: to be recorded in $cm^3$
  • Final volume of air: to be recorded in $cm^3$
  • Volume of air used up: to be calculated in $cm^3$
  • Percentage of oxygen in air: to be calculated
• Phosphorus Experiment:
  • Phosphorus reacts with oxygen in the air, forming solid phosphorus oxide ($P<em>4O</em>{10}$).
  • This reduces the pressure, causing the water level in a bell jar to rise by about 20%.
  • Reaction: phosphorus + oxygen → phosphorus oxide

Experiment OXID/4: Reactions of Elements with Oxygen

• Test for oxygen gas: Described.
• Process:
  • Elements react with oxygen to form oxides.
  • A sample of the element is heated in a strong flame and transferred to a gas jar containing oxygen.
  • Distilled water is added to the gas jar to dissolve any oxide produced.
  • Universal indicator is added to the solution to determine the pH.
• Results Table:
  • Elements tested: Carbon, Sodium, Sulfur, Magnesium.
  • Observations: To be recorded for each element.
  • Color and pH of solution: To be recorded for each element.
• Observations:
  • Note differences between oxides formed by metals and non-metals.
• Balanced Equations:
  • Write balanced equations for the reactions between sodium, sulphur, magnesium, and carbon, including state symbols.

Experiment HYD/2: Combustion of Hydrogen

• Apparatus:
  • Draw a neat labeled diagram of the apparatus used in this experiment.
• Observations:
  • Describe the appearance of the liquid that collects in the test-tube.
  • Describe how the liquid is tested and the result of the test.
  • Explain whether this test proves that the liquid is pure.
  • How to use a physical test to show that the liquid is pure water.

Greenhouse Gases

• Greenhouse Gases:
  • Name at least two gases that can be described as greenhouse gases.
  • Explain how these gases are produced.
• Effect on Climate:
  • What effect does the build-up of greenhouse gases have on the world's climate?
• Polar Ice Caps:
  • Given that polar ice caps contain a significant amount of dissolved carbon dioxide and methane, what are the potential problems of polar ice caps melting?

Experiment OXID/6: Thermal Decomposition of Copper(II) Carbonate

• Procedure:
  • A test-tube with approximately 0.4g of copper(II) carbonate is pre-weighed.
  • The tube is connected to a gas syringe.
  • The test-tube is heated until the copper carbonate has completely decomposed.
  • The apparatus is allowed to cool, and the volume of gas given off is measured.
  • The mass of the test-tube + solid after heating is reweighed.
• Measurements and Calculations:
  • Mass of test-tube + copper carbonate before heating: to be recorded in grams (g).
  • Volume of gas given off: to be recorded in cubic centimeters ($cm^3$).
  • Mass of test-tube + solid after heating: to be recorded in grams (g).
  • Mass loss in the reaction: to be calculated in grams (g).
  • Mass of the gas given off in the reaction: to be calculated in grams (g).
  • Density of the gas (mass/volume): to be calculated in g/$cm^3$.
• Gas Identification:
  • Density of gases provided: Oxygen (0.0013 g/$cm^3$), Carbon dioxide (0.0018 g/$cm^3$), Carbon monoxide (0.0012 g/$cm^3$).
  • Identify the gas from the densities.
• Solid Residue:
  • What is the solid remaining in the test-tube?
• Equations:
  • Write a word equation for the reaction that has occurred.
  • Write a balanced symbol equation for the reaction that has occurred.
• Confirmation:
  • Heat a fresh sample of copper(II) carbonate and test the gas produced with limewater.
  • Observe if the limewater turns cloudy.
• Observations Table:
  • Substance: Copper(II) carbonate, Lime water.
  • Observations: Before reaction, After reaction.
• Questions:
  • Evidence of a chemical reaction: Give two pieces of evidence.
  • Gas produced: Which gas is produced in the reaction?
  • Test for the gas: What is the test for this gas?