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Oxygen is a very important gas. About one fifth of the Earth’s atmosphere is oxygen. All animals
need it to live, it is used by our bodies to “burn” food in our cells. In Chapter 8 metals heated in
air reacted with oxygen to form metal
oxides
.
Experiment 9.14
The proportion of oxygen in air
• iron filings
• test tube
• small glass
Wet the inside of a test tube and then sprinkle in some iron filings
so they stick to the glass. Turn the tube upside down in 1cm of
water in a small glass. Leave it for 2 or 3 days. When the level
of water inside the test tube no longer changes estimate the
proportion of the test tube containing water. What has happened to
the oxygen in the air inside the test tube? What proportion of air is
oxygen? What is the main gas left in the test tube? What has
happened to the iron filings?
Experiment 9.15
The rusting of iron
• 4 nails
• sodium chloride
• 2 test tubes
• 2 crystallising dishes
Get 4 bright shiny nails. Put one nail in an absolutely dry test tube
and put the cap on the test tube. Put the second nail in a test tube
with some water that has been boiled. Put the third nail in a
crystallising dish with some water. Put the fourth nail in another
crystallising dish with some water in which has been dissolved
1 measure of sodium chloride.
Leave them all for a few days. Describe what has happened to each
of the nails. From the results of this experiment what do you think is
necessary for iron to rust?
Experiment 9.16
Making a glowing splint
• lolly stick
Oxygen gas is needed for a substance to burn. In pure oxygen most
things burn very vigorously in oxygen. This is the basis of the
common test for oxygen gas; it relights a glowing splint.
Get a wooden splint ready for this test by lighting the end of a lolly
stick in the burner flame and then blowing it out. The wood will still
glow red. If you leave it in air the red glow soon goes out. If put into
oxygen, however, the red glow increases and the wood relights.
You will use this glowing splint in the next experiment.
Experiment 9.11
The reaction of copper sulphate with ammonia
• copper sulphate
• ammonia solution
• test tube
• dropping pipette
Put ¼ measure of copper sulphate into a clean dry test tube and
add a very small amount of water (about ½ cm). Warm the tube to
dissolve the copper sulphate and let it cool. Add ammonia solution
drop by drop using the dropping pipette. A blue-white precipitate of
copper hydroxide is formed. Continue adding ammonia with the
dropping pipette. The copper hydroxide precipitate dissolves to give
a deep blue solution.
Experiment 9.12
The reaction of iron sulphate with ammonia
• iron sulphate
• ammonia solution
• test tube
• dropping pipette
Dissolve ¼ measure of iron sulphate in 2cm of water in a test tube.
Cool the solution under a running tap if you have heated it to
dissolve the iron sulphate. Add ammonia solution with the dropping
pipette. A dark green precipitate of iron(II) hydroxide is formed.
Filter off the precipitate and leave it for an hour or so. What happens
to it?
Experiment 9.13
The reaction of aluminium potassium sulphate with ammonia
• aluminium potassium
sulphate
• ammonia solution
• test tube
• dropping pipette
Dissolve ½ measure of aluminium potassium sulphate in 1cm of
water in a test tube. Cool the solution if you have heated it. Add
ammonia solution with the dropping pipette. A barely visible
precipitate of aluminium hydroxide is formed. What colour is it?
Experiment 9.10
Tests on ammonia solution
• ammonia solution
• universal indicator paper
Put a drop of the ammonia solution onto a piece of universal
indicator paper. What colour is the paper? Does this show that
ammonia is an acid or an alkali?
Smell the ammonia cautiously. DO NOT TAKE A DEEP BREATH -
it has a distinctive strong smell.
Ammonia is used in bottles of “smelling salts” which some people
use when they feel faint or giddy. Do you think that smelling
ammonia would make you feel more awake?
Chapter 9 - The chemistry of some gases 9c - Oxygen
Chapter 9 - The Chemistry of some gases
9b - Ammonia
Test tube
Iron filings
Small
glass
Water
