Carry out chemical tests according to procedure
Prerequisite Knowledge
Notes
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Scan through the entire practical paper.
For each part, write down: (1) what can the test be used to identify? (2) what are the observations that you should look out for?
e.g. Add sodium hydroxide solution, dropwise with shaking, until you observe no further change.
(1) Test for cations. (2) Look out for (I) colour of ppt and (II) whether ppt dissolves in excess sodium hydroxide. -
If a gas is expected to be formed, try to anticipate prepare the test for gas in advanced.
e.g. To some solid S in a test tube, add dilute hydrochloric acid to a depth of 2 cm. Test for any gas evolved.
Since an acid was added, the gas produced must be either hydrogen or carbon dioxide (recall the reactions of acids).
If the solid is grey/ silvery, it is likely to be a metal → get ready lighted wooden splint to test for H2 upon addition of acid.
If the solid is white or other colours, it is likely to be a carbonate → get ready limewater-delivery tube set-up to test for CO2 upon addition of acid. -
When adding aq. NaOH or aq. NH3 for the testing of cations, it must be done dropwise and look out for any ppt formed.
You may miss any ppt formed if aq. NaOH or aq. NH3 is added too fast. -
What is the difference between gentle and strong heating?
Gentle heating is carried out with the bunsen burner airhole half-open (luminous flame).
It is usually done on solutions and you need to hover the test tube over the flame to avoid the solution boiling over. Once you feel vibration in the test tube, briefly move it away from the flame otherwise the solution will spurt out.
Strong heating is carried out with the bunsen burner airhole fully-open (non-luminous flame).
It is usually done on solids and you need to place the test tube over the flame for an extended period (no need to hover). If you don't heat the solid strong enough, you may miss out on some expected observations.
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