Topic 11
Chemical Tests
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Topic 11
Tests for Gases
Gas
Test
Results
Turns milky
Carbon
Dioxide
Lime water
Sulfur
Dioxide
Acidified Potassium Manganate
(VII)
changes colour from purple to
colourless
Burns with a pop sound
Hydrogen
Lighted splint
Relights
Oxygen
Glowing splint
Bleaches / turns white.
Chlorine
Damp litmus paper
Turns blue.
Ammonia
Damp red litmus paper
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Topic 11
Collection of Gases
Downward Delivery
Upward Delivery
Collection over water
Gas Syringe
For gases denser
than air
For gases less dense
than air
For gas insoluble in water
For any gas
E.g. Chlorine, Sulfur
Dioxide, Carbon
Dioxide
E.g. Ammonia,
Hydrogen
E.g. any gas except
Chlorine & Ammonia
Any gas
Tests for Water
Water can be identified using a chemical test and/or a physical test
Chemical Test for Water:
Anhydrous copper(II) sulfate turns from white to blue on the addition of
water
The equation is:
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Topic 11
Physical Test for Water:
A physical test to see if a sample of water is pure to check its boiling
point
A sample of the liquid is placed in a suitable container such as a boiling
tube and gently heated
Using a thermometer, you can check if the boiling point is exactly 100°C
Any impurities present will usually tend to raise the boiling point and
depress the melting point of pure substance
Flame Tests
The flame test is used to identify the positive metal ion (cations) by the
colour of the flame they produce
Ions from different metals produce different colours
To carry out a flame test:
Dip the loop of an unreactive metal wire such as nichrome or platinum in
concentrated hydrochloric acid then rinse with distilled water to clean it
& remove traces of any ions on it
Hold it in the blue flame of a Bunsen burner until there is no colour
change
Dip the loop into the solid sample / solution and place it in the edge of the
blue Bunsen flame
It is important to place the wire into acid first to prevent contamination
Not doing this might result in two or more ions being present on the wire
meaning the colours will mix
One colour could mask another colour and you will not be able to identify
the ion
The colour of the flame is
observed and used to identify the
metal ion present
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Topic 11
Metal Ion
Flame Colour
Lithium (Li+)
Red
Calcium (Ca²+)
Orange Red
Sodium (Na+)
Yellow
Barium (Ba²+)
Light Green
Copper (Cu²+)
Blue Green
Potassium (K+)
Lilac
Tests for Positive Ions “Cations”
Metal cations in aqueous solution can be identified by the colour of the
precipitate they form on addition of sodium hydroxide
If only a small amount of NaOH is used then normally the metal hydroxide
precipitates
If a precipitate is formed from NaOH then the hydroxide is insoluble in
water
Positive Ion
Effect of aq.
Sodium
Hydroxide
Name of
Precipitate
Ammonium
(NH₄+)
Bubbles of
ammonia
-
Copper (II)
(Cu²+)
blue
precipitate
Copper (II)
Hydroxide
Iron (II)
(Fe²+)
Green
precipitate
Precipitate
Iron (II)
turns brown
Hydroxide
near surface on
standing.
Iron (III)
(Fe³+)
Orange Brown
precipitate
Iron (III)
Hydroxide
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Ionic Equation
Topic 11
Tests for Negative Ions “Anions”
Negatively charged non-metal ions are known as anions
You must be able to describe the tests for the following ions:
Carbonate ions, CO₃²Halide ions, Cl– , Br– , I–
Sulfate ions, SO₄²–
Tests for Carbonate Ions
Carbonate compounds contain the carbonate ion, CO₃²The test for the carbonate ion is:
Add dilute acid
Bubble the gas released through limewater
Limewater turns cloudy if the carbonate ion is present
If a carbonate compound is present then fizzing / effervescence should
be seen as CO₂ gas is produced, which forms a white precipitate of
calcium carbonate when bubbled through limewater:
The white
precipitate turns
limewater cloudy
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Topic 11
Tests for Halide Ions
Halide ions are the negative ions / anions formed by the elements in
Group 7
The test for the halide ions is:
Acidify the sample with nitric acid
Add silver nitrate solution, AgNO₃,
A silver halide precipitate forms if a halide ion is
present
The precipitate is indicated by the state symbol (s)
The colour of the silver halide precipitate depends on the halide ion:
Potassium
Chloride
Silver Nitrate
Potassium
Nitrate
Silver Chloride
The chloride ion forms a white
precipitate of silver chloride
Potassium
Bromide
Silver Nitrate
Potassium
Nitrate
Silver Bromide
The bromide ion forms a cream
precipitate of silver bromide
Potassium
Iodide
Silver Nitrate
Potassium
Nitrate
Silver Iodide
The iodide ions forms a yellow
precipitate of silver iodide
Each silver halide
produces a
precipitate of a
different colour
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Topic 11
Tests for Sulfate Ions
Sulfate compounds contain the sulfate ion, SO42The test for the sulfate ion is:
Acidify the sample with dilute hydrochloric acid
Add a few drops of barium chloride solution
A white precipitate of barium sulfate is formed, if the
sulfate ion is present
The test can also be carried
out with barium nitrate
solution
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