Gases In The Air

Question 1

name the gases in the atmosphere.

Answer 1

Main Gases - Nitrogen (78%), Oxygen (21%), Argon (0.93%), Carbon Dioxide (0.04%)

Question 2

evaluate the different theories about the evolution of the Earth’s early atmosphere.

Answer 2

Volcanic Activity: Early atmosphere likely composed of water vapor, carbon dioxide, and ammonia from volcanic eruptions.

Comet Impact Theory: Some theories suggest that water and other gases were brought by comet impacts.

Biological Evolution: Cyanobacteria started producing oxygen through photosynthesis.

Question 3

explain the increase in atmospheric oxygen since the early Earth.

Answer 3

Photosynthetic organisms, such as cyanobacteria and later plants, produced oxygen.

Question 4

explain the decrease in atmospheric carbon dioxide
since the early Earth.

Answer 4

Photosynthesis - CO2 converted to organic matter and oxygen.

Question 5

give the approximate percentages of the four most
abundant gases in dry air.

Answer 5

Nitrogen: 78%
Oxygen: 21%
Argon: 0.93%
Carbon Dioxide: 0.04%

Question 6

describe an experiment to show the percentage
volume of oxygen in air using reactions of metals (e.g. iron) and non-metals (e.g. phosphorus).

Answer 6

Using Iron - Place iron wool in a measuring cylinder inverted in water. As iron rusts, it reacts with oxygen, and the water level rises, indicating the volume of oxygen consumed.

Using Phosphorus - Burn phosphorus in a closed container. Measure the decrease in air volume as phosphorus reacts with oxygen.

Question 7

determine the approximate percentage by
volume of oxygen in air using a metal or a non-metal.

Answer 7

Measure the initial and final volume of air after the oxidation of iron reaction.

Question 8

describe how carbon dioxide can be made from the
thermal decomposition of metal carbonates.

Answer 8

For example, heating calcium carbonate (CaCO_3) produces calcium oxide (CaO) and carbon dioxide (CO_2).

Equation - CaCO_3 → CaO + CO_2

Question 9

explain observed mass change in terms of the state
symbols in the chemical equation

Answer 9

Mass changes observed when gases are involved are due to the state symbols and conservation of mass.

Question 10

describe the test for carbon dioxide gas.

Answer 10

Limewater Test - Pass CO_2 through limewater (calcium hydroxide solution); it turns milky due to the formation of calcium carbonate.

Question 11

describe the combustion of elements in oxygen including magnesium, hydrogen and sulfur.

Answer 11

Magnesium: Burns with a bright white flame, forming magnesium oxide. Equation: 2Mg + O_2 → 2MgO

Hydrogen: Burns with a pale blue flame, forming water. Equation: 2H_2 + O_2 → 2H_2O

Sulfur -Burns with a blue flame, forming sulfur dioxide. Equation: S + O_2 → SO_2

Question 12

describe the test for oxygen gas.

Answer 12

Relights a glowing splint.

Question 13

describe the test for hydrogen gas.

Answer 13

Pop sound with a lit splint due to formation of water.

Question 14

describe a chemical test for the presence of water using anhydrous copper(Il) sulfate

Answer 14

Chemical Test - Anhydrous copper(II) sulfate turns from white to blue.

Question 15

describe a physical test to show whether a sample of water is pure.

Answer 15

Pure water boils at 100°C and freezes at 0°C.