Lesson Notes
Physical Sciences Grade 10
Chemical Solutions
LESSON
7
Teacher Guide
Colourful Cations
This lesson shows how cations can be identified. The lesson shows clear footage of flame tests and shows how to test
for ammonium ions too.
Lesson Outcomes
By the end of this lesson, you should be able to:
• Identify an unknown cation by using analytical
methods
Lesson Notes
In this lesson we will investigate how to detect the
presence of different cations in a solution. We are
going to use a technique called the flame test to identify
different cations.
First, we use three samples of ionic salts: sodium
chloride, sodium nitrate and sodium sulphate. Notice that
the anions differ but the cation is sodium in each case.
We dip a wire into acid to clean it and then place the
clean wire into the blue flame of the bunsen burner.
We now use the wire to place a few crystals of sodium
chloride into the flame. We do this by dipping the wire
into the salt so that some crystals stick to it before placing
them into the flame. We repeat this procedure with all
three the salts.
We find that the different anions in the salts do not
influence the flame colour and we can conclude that all
sodium salts burn with a yellow flame.
Next we test potassium nitrate, lithium carbonate and
copper chloride. We use the flame test method again and
we find that the potassium salt burned with a lilac flame,
the lithium salt burned with a red flame and the copper
salt burned with a blue-green flame.
We can summarise these findings as follows:
• Sodium salts burn with a yellow flame
• Potassium salts burn with a lilac flame
• Lithium salts burn with a red flame
• Copper salts burn with a blue-green flame
Some cations are more complex and cannot be identified
by using a flame test – the ammonium cation is an
example of such a complex cation. Two simple chemical
tests can be used to identify an ammonium cation in an
ionic salt.
For our experiment we use ammonium sulphate. We
place some calcium hydroxide and some ammonium
sulphate in a tube and shake the tube to mix the
powders. We then heat the powders gently and hold a
piece of damp litmus paper at the mouth of the test tube.
Curriculum Links
LO 1: Scientific Inquiry and Problem-solving Skills
AS 3: Solving problems
LO 2: C
onstructing and Applying Scientific
Knowledge
AS 1: Recall prescribed concepts
AS 3: Apply scientific knowledge
The litmus paper turns blue.
This is a good indication that a gas is being given of and
that this gas is soluble in water. Ammonia gas is highly
soluble in water and will turn litmus paper blue. We can
conclude that, if ammonia gas is given off when a salt is
heated, it must be produced by an ammonium ion in the
salt sample.
The chemical equation for the thermal decomposition is:
(NH4)2SO4 + Ca(OH)2 CaSO4 + 2NH3 + 2H2O
We can repeat the initial steps of our previous experiment
and while heating the test tube we can bring the stopper
of a bottle of concentrated hydrochloric acid (instead of
the litmus paper) close to the mouth of the test tube. You
will see thick white fumes forming.
This is a second positive test for the presence of
ammonia. The chemical equation for this reaction is:
NH3 (g) + HCℓ (g) NH4Cℓ (s)
You can also identify ammonia gas by simply smelling it!
This is of course not a scientifically sound test, but it is
important to know that ammonia gas has a very sharp
and distinctive smell that catches your nose and makes
your eyes water.
Ammonia-based products can be found in most
households. It is commonly found in cleaning detergents
and smelling salts.
?
TASK
Describe what tests you would conduct to confirm
the presence of copper sulphate ions in a solution.
133