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Chapter 5 - Acids and alkalis
5b - Testing for acids and alkalis
Chapter 5 - Acids and alkalis
5b - Testing for acids and alkalis
Experiment 5.5
To show that acids have a sour taste and are
neutralised by alkalis
• a lemon
• citric acid
• sodium hydrogen
carbonate (sodium
bicarbonate)
• a plate
• an egg cup
Normally you should NEVER TASTE a chemical. However some
chemicals that are in our food are obviously safe to taste. One such
chemical is citric acid. This is the acid in most citrus fruits such as
oranges and lemons. It is also put into many sour tasting sweets and
fizzy drinks.
An alkali which we can eat is sodium hydrogen carbonate (sodium
bicarbonate). This is used in cooking to make cakes rise and in
stomach powders that are taken if you have indigestion.
Squeeze a lemon and taste the juice. Does it taste sharp and sour?
This is due to the citric acid in the lemon.
Put a little citric acid and a little sodium hydrogen carbonate (sodium
bicarbonate) onto a plate. Wet a clean finger, dip it into the citric acid
and taste it. Does it have the same sharp taste as the lemon juice?
Now do it again and immediately after putting the citric acid on your
tongue dip your finger in the sodium hydrogen carbonate and taste it.
Has the sharp citric acid taste disappeared?
You can do a similar experiment by putting a little lemon juice into an
egg cup. Taste the lemon juice and then add a little sodium hydrogen
carbonate (sodium bicarbonate) and taste again. Keep doing this
until the sharp taste of the lemon juice has disappeared.
Why do you think citric acid has this name?
Experiment 5.7
Home made indicators - red cabbage & turmeric
• citric acid
• sodium hydroxide solution
• red cabbage
• turmeric
• saucepan
• beaker
• conical flask
Many common vegetables and flowers contain acid-alkali indicators.
The substance that gives red cabbage its colour is an indicator.
Chop up a little red cabbage and gently boil it in water for 10 minutes
or so. Let the deep purple water cool and pour some
into your beaker.
Dissolve ½ measure of citric acid in 2cm of water in a test tube and
pour it into the conical flask. Add 1cm of red cabbage water. What
colour is the solution? Add 2cm of your sodium hydroxide solution.
What colour is the indicator now? As in the previous experiment you
can continually change the colour backwards
and forwards as the solution changes from acidic to alkaline.
Repeat the experiment using a solution of the spice turmeric as the
indicator. What are the colours of turmeric in acid and alkalis?
Experiment 5.8
Other home made indicators
• citric acid
• sodium hydroxide
solution
• beetroot
• blackberry juice
• red rose or red carnation
• saucepan
• beaker
• conical flask
Other coloured vegetables and flowers can be used as indicators.
Beetroot indicator is made in an identical way to that used for red
cabbage in Experiment 5.7, and can be tested as in Experiment 5.7.
Blackberry juice can also be used as an indicator. A red rose or a
red carnation can be boiled in a little water in a saucepan. Let the
water cool and use it as an indicator.
For most of these vegetable and flower indicators the colour in acid
is usually red. In alkali the colour may be yellow, blue, green or
purple.
Experiment 5.6
Using litmus to test for acids and alkalis
• litmus blue
• citric acid
• sodium hydrogen
carbonate (sodium
bicarbonate)
• test tube
Dissolve a “pinch” (less than ¼ measure) of litmus blue in 2cm
of water in a test tube. Warm the tube to dissolve the litmus blue.
Add ¼ measure of citric acid. The blue colour changes to red.
Now add ½ measure of sodium hydrogen carbonate (sodium
bicarbonate) and shake the tube. Does the colour go back to blue?
If not add more sodium hydrogen carbonate until it does.
The litmus is red in acidic solution and blue in alkaline solution.
It is acting as an
acid-alkali indicator
.
