Gases in the Atmosphere

This lesson covers:

  1. The composition of gases in the atmosphere 
  2. How magnesium, hydrogen, and sulfur burn in oxygen 
  3. How thermal decomposition of metal carbonates produces carbon dioxide

Composition of the atmosphere


Diagram showing the composition of gases in the atmosphere with nitrogen at 78 percent, oxygen at 21 percent, argon at 1 percent, and carbon dioxide at 0.04 percent.

The atmosphere is the layer of gases that surrounds the earth. The abundance (amount) of each gas in the atmosphere has changed over time, but at the moment it is almost all nitrogen and oxygen:


78% nitrogen (N2)

21% oxygen (O2)

Almost 1% Argon (Ar)

0.04% carbon dioxide (CO2)


Burning magnesium, hydrogen and sulfur


When you burn (combust) something in air, it reacts with the oxygen in the atmosphere to form an oxide. These oxides can be acidic, neutral, or alkaline.

Magnesium

Illustration showing burning magnesium, resulting oxide powder, and a beaker of blue solution.
  • Burns with a bright white flame.
  • Forms a white powder of magnesium oxide.
  • Magnesium oxide is alkaline when it's dissolved in water.
  • 2Mg(s) + O2(g) ➔ 2MgO(s)

Hydrogen

Illustration showing burning hydrogen forming an oxide.
  • Burns easily, produces a pale blue flame
  • Burning hydrogen forms water vapour (which has a neutral pH)
  • 2H2(g) + O2(g) ➔ 2H2O(g)

Sulfur

Diagram showing the burning of sulfur forming a gas and then a red liquid.
  • Burns with a pale blue flame.
  • Produces sulfur dioxide.
  • Sulfur dioxide is acidic when dissolved in water.
  • S(s) + O2(g) ➔ SO2(g)

Thermal decomposition of metal carbonates produces CO2

'Thermal decomposition' means to break something down using heat. This makes sense because thermal refers to heat and decomposition means to decompose or break down.

When you heat a metal carbonate it will break apart into a metal oxide and carbon dioxide:


Metal carbonate  ➔  metal oxide + carbon dioxide

Diagram showing the thermal decomposition of metal carbonates into metal oxides and carbon dioxide.

As an example, if you heat copper(II) carbonate (a green powder), it will decompose into copper(II) oxide, which is black, and carbon dioxide, which is a colourless gas:


CuCO3(s) ➔ CuO(s) + CO2(g)


Illustration showing the thermal decomposition of copper(II) carbonate into copper(II) oxide and carbon dioxide.

You can see this experimentally by heating copper(II) carbonate and collecting the carbon dioxide that is given off in a test tube. You'll slowly see the powder turn from green to black.

What percentage of the atmosphere is oxygen?

1 %

78 %

21 %

0.04 %

0

/

1

What is the name of the most abundant gas in the atmosphere?

Nitrogen

0

/

1

What is the percentage abundance of CO2 in the atmosphere?

0.04

%

%

0

/

1

What colour flame does magnesium give when burned in oxygen?

Orange/yellow

Pale blue

Purple

White

0

/

1

Illustration of a pale blue flame against a black background.

Which substance(s) burn with a pale blue flame?


(Select all that apply)

Hydrogen

Sulfur

Magnesium

0

/

2

What does magnesium form when burned in oxygen? 

(Give the name in words, not symbols)

Magnesium oxide

0

/

1

What does sulfur form when burned in oxygen? 

(Give the name in words, not symbols)

Sulfur dioxide

0

/

1

Two flasks showing magnesium oxide solution with pH indicators, one turning purple indicating alkaline and one turning red indicating acidic.

Is magnesium oxide acidic or alkaline when dissolved in water?

Acidic

Alkaline

0

/

1

Is sulfur dioxide acidic or alkaline when dissolved in water?

Acidic

Alkaline

0

/

1

If you heat a metal carbonate, it will produce a metal and dioxide.

oxide
carbon

0

/

2

What is the colour change when copper(II) carbonate decomposes to copper(II) oxide?

Brown ➔ green

Black ➔ green

Green ➔ black

Green ➔ brown

0

/

1