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What percentage of oxygen is in unpolluted dry air?



What percentage of nitrogen is in unpolluted dry air?



What percentage of argon is in unpolluted dry air?



What percentage of carbon dioxide is in unpolluted dry air?



What percentage of other noble gases is in unpolluted dry air?

tiny percentages


Describe an experiment to show the percentage of oxygen in air using copper:

• connect two gas syringes with a silicon tube packed with copper
• one gas syringes should be filled with 100cm3 of air between them. Push this back and forth over the copper as it is heated by a bunsen burner which turns into copper oxide
• ensure that the bunsen is moved along the tube so it heats different copper
• the volume of gas should fall as the oxygen is used up. Wait for it to cool and then work out how much oxygen has been used up.


equation of this experiment?
(an experiment to show the percentage of oxygen in air using copper)

2Cu (s) + O2 (g) ---> 2CuO (s)


What colour is copper oxide?



How can you work out the percentage of oxygen in air from the experiment? (using copper)

When the copper stops turning black and the volume of gas in the syringes stops changing, on cooling there should be around 79cm3 of gas left in the syringes. This means 21% has been used up. Therefore the air contained 21% of oxygen.


Why should you let the apparatus cool before working out the percentage?

When gas is heated it expands and so the readings could show to be higher than usual and therefore be inaccurate. The volume of the gas should be that at room temperature.


When does iron rust?

When in contact with water and oxygen, it will use up the oxygen.


Describe an experiment to show the percentage of oxygen in air using iron:

• place some damp iron wool in the bottom of a test tube.
• the tube is inverted in a beaker of water and the level of water in the tube is marked by a rubber band or permanent marker.
• leave the tube for a week or so for the iron to use up all the oxygen in rusting
• the water level rises in the tube as the oxygen is used up. Mark the new level with another plastic band.
• at the end of the experiment fill the tube with water to the level of each of the plastic bands and pouring it into a small measuring cylinder.
• If the original volume was, say, 15cm3 and the final was 12cm3 then the oxygen used up measured 3cm3.
The percentage of oxygen in the air was 3/15 x 100 = 20%


Describe an experiment to make oxygen in a lab:

• place solid manganese oxide in a side arm boiling tube and place a delivery tube in a beaker of water.
• fill a test tube with water and place it above the delivery tube to collect the oxygen.
• add the hydrogen peroxide solution in a thistle funnel.


What is this method of making oxygen called?

catalytic decomposition of hydrogen peroxide (splitting up hydrogen peroxide using a catalyst)


equation for the catalytic decomposition of hydrogen peroxide:

2H2O2 (aq) ---> 2H2O (l) + O2 (g)


How can you test oxygen?

Oxygen relights a glowing splint


Describe the burning of magnesium in oxygen:

Magnesium burns with a very bright white fame


What does magnesium oxide look like?

A white, powdery ash


Equation for burning magnesium in air:

2Mg (s) + O2 (g) ---> 2MgO (s)


Describe the burning of sulfur in oxygen:

Sulfur burns with a tiny, almost invisible, blue flame
but in OXYGEN it burns much more strongly giving a bright blue flame


Equation for burning sulfur in air:

S (s) + O2 (g) ---> SO2 (g)


What is produced when you burn sulfur in air?

Sulfur dioxide, a colourless and poisonous gas


Describe the burning of carbon in oxygen:

If it is heated very strongly carbon will burn in air or oxygen. It may produce a small yellow-orange flame and perhaps some sparks but this depends on the purity of the carbon.


Equation for burning carbon in air:

C (s) + O2 (g) ---> CO2 (g)


What is produced when you burn carbon in air?

a colourless gas called carbon dioxide


Are metal oxides soluble in water? Do they react with it?

Most metal oxides are not soluble in water and do not react with it.


What will happen if you shake some solid magnesium oxide with water?

It doesn't seem to dissolve but if you test the pH of the mixture you will see that it is slightly alkaline. A very, very small amount does react with the water, forming some magnesium hydroxide. A very small amount of that dissolves in the water making it slightly alkaline.


Equation for reaction of magnesium oxide and water:

MgO (s) + H2O (l) ---> Mg(OH)2 (s and aq)


Do non-metal oxides react with it water? Example?

Many non-metal oxides react with water to give acidic solutions e.g. sulfur doxide reacts to give sulfurous acid

BUT not all do


What non-metal oxides do not react with water to form acidic solutions?

common exceptions are water and carbon monoxide