36
37
Chapter 5 - Acids and alkalis 5a - Making acid
and alkali solutions
Experiment 6.1
The reaction of magnesium with an acid
• magnesium strip
• vinegar
• test tube
• matches
In Experiment 5.11 you found that vinegar is acidic. It contains an acid
called ethanoic acid.
Add a 2cm piece of magnesium strip to 2cm of vinegar in a test tube.
Tiny bubbles of hydrogen gas can be seen coming from the surface of the
magnesium. This is hydrogen gas. Warm the test tube so that there is a
vigorous reaction.
Loosely
put one of the red caps (not a cork) into the
test tube and stand the test tube in the test tube rack. Wait for 30 seconds
(count to 30). Light a match, remove the cap and immediately hold the
match flame in the top of the test tube. There is a very tiny explosion as
the hydrogen gas quickly burns with a POP. This is the test for hydrogen
gas. If this doesn’t happen repeat the experiment.
NOTE: It is perfectly safe to burn a test tube full of hydrogen gas, but
you must NEVER try this experiment in a bigger container. You could
break the container and do damage.
The word equation for the reaction that has taken place is: magnesium
+ ethanoic acid hy magnesium ethanoate.
Experiment 6.2
The reaction of zinc with an acid
• granulated zinc
• sodium hydrogen
sulphate solution
• 2 test tubes
• funnel
• filter paper
• crystallising dish
Add 2 pieces of granulated zinc to 2cm of your sodium hydrogen
sulphate solution (made in Experiment 5.1) in a test tube. Warm the test
tube. Bubbles of hydrogen gas will be seen coming from the surface of
the zinc. Warm the solution to increase the rate of the reaction. You can
try capping the tube and collecting and burning the hydrogen as in
Experiment 6.1, but the reaction here is not so vigorous and you will
probably be unsuccessful.
Keep periodically warming the tube and keep it in the test tube rack for
half an hour or so when no more hydrogen should be being formed.
This means that all the acid has been used up.
The word equation for the reaction that has taken place is: zinc +
sodium hydrogen sulphate hy sodium su
zinc sulphate.
Filter off the remaining zinc, wash it and return it to its container. The
sodium sulphate and zinc sulphate are in the filtrate. Pour this into a
crystallising dish and let the water evaporate off in a warm place leaving
a mixture of sodium sulphate and zinc sulphate crystals.
Experiment 5.12
Neutralisation of an acid with an alkali using
universal indicator
• universal indicator paper
• citric acid
• sodium hydrogen
carbonate (sodium
bicarbonate)
• sodium hydrogen
sulphate solution
• sodium carbonate
solution
• 2 test tubes
• conical flask
• 2 crystallising dishes
• dropping pipette
The indicator chemicals in a piece of indicator paper can be dissolved in
water and used in solution. Tear a piece of universal indicator paper into
several pieces and put them into a clean conical flask. Add 2cm of water
from a test tube. Gently shake to dissolve the indicator from the paper, a
green solution will be formed. Stand the conical flask on a piece of white
paper so the colour of the indicator shows clearly.
Dissolve ½ measure of citric acid in 5cm of water in a test tube and
pour the solution into a crystallising dish. Dissolve ½ measure of
sodium hydrogen carbonate (sodium bicarbonate) in 5cm of water in a
second test tube and pour this solution into a second crystallising dish.
Using the dropping pipette add 10 drops of the sodium hydrogen
carbonate to the indicator. What colour is the indicator?
Wash the dropping pipette and use it to add the citric acid solution DROP
BY DROP into the conical flask. Gently shake the flask after each drop
and note the colour of the indicator. Suddenly the colour will change over
one or two drops and remain the same as more citric acid is added.
What colour is the solution now?
Repeat the experiment using sodium hydrogen sulphate solution instead
of citric acid and sodium carbonate solution instead of sodium hydrogen
carbonate. What are the colours of the indicator in sodium carbonate
solution and in sodium hydrogen sulphate solution?
Try to explain what has happened as the indicator has changed colour
from its original colour, to that with sodium hydrogen carbonate, that
with citric acid, that with sodium carbonate and that with sodium
hydrogen sulphate.
Chapter 5 - Acids and alkalis
5b - Testing for acids and alkalis
Chapter 6 - Chemical reactions of acids and alkalis
6a - Reactions of acids with metals
Dropping
pipette
1 drop
of liquid
Indicator
Acids contain hydrogen. When metals react with acids the hydrogen is released as hydrogen gas.
The word equation for the reaction that takes place is: Acid + metal hy salt.
