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Chemical Tests Common Homework Yau-Yan Chai 10P (Flame Tests for Cations,…
Chemical Tests Common Homework Yau-Yan Chai 10P
Gas Tests
oxygen
glowing splint
if it reignites, oxygen is present as oxygen is one of the components required for fire.
hydrogen
a lightened splint
insert the splint into the test tube, if it makes a squeaky pop, ca
ammonium
damp litmus paper
turns from red to blue when ammonia is present
this is because ammonia is alkali
chlorine
damp universal indicator paper
turns red and bleaches
carbon dioxide
lime water
if the water turns from clear to milky white, carbon dioxide is present
Flame Tests for Cations
Li+
produces a red colour
Na+
produces a yellow colour
K+
produces a lilac colour
Ca2+
produces a brick red colour
Cu2+
produces a blue-green colour
How to conduct a flame test:
First, dip a clean wire loop into concentrated hydrochloric acid to remove any excess ions on the wire loop.
Then, dip the wire loop into a solid sample of the compound which you want to test
Next, put the loop into the edge of a blue flame on a bunsen burner instead of a yellow flame so that the colour can be seen easier.
Observe and record the colour produced.
How Can Flame Tests Be Used in Real Life?
They can be used by geologists to identify metal samples and by forensic scientists to make a quick analysis of the elements present at a crime scene.
Cation tests using sodium hydroxide
Fe2+
forms a green precipitate
Fe3+
forms a brown precipitate
Cu2+
forms a blue precipitate
NH4+
produces ammonia gas and water
Can be tested using damp red litmus paper. If present, will turn the red litmus paper blue.
How to perform a cation test using sodium hydroxide:
Prepare a sample of the solution which you want to test for cations in a test tube.
Then, add several drops of dilute sodium hydroxide solution to the sample until a precipitate forms.
If a white precipitate forms, continue to add sodium hydroxide and observe the solution to see if the precipitate dissolves.
Test For Halide Ions
Cl-
Forms a yellow precipitate
Br-
Forms a cream precipitate
I-
Forms a yellow precipitate
How to perform a test for Halide Ions
Prepare of the solution that you want to test in test tube.
add a few drops of dilute nitric acid to the sample
Add a few drops of silver nitrate solution.
Observe and record the colour of any precipitate which forms.
Test for Sulfate and Carbonate Ions
How to test for carbonate ions:
Prepare a sample of the solution you want to test in a test tube.
Add a few drops of any dilute acid. If carbonate ions are present, bubbles should begin to form.
If carbonate ions are present, the gas given off by the bubbles should be carbon dioxide. To test for carbon dioxide use limewater. If carbon dioxide is present, the limewater should turn milky white when the gas is bubbled through it.
How to test for Sulfate ions:
If sulfate ions are present, a white precipitate of barium sulfate should form.
First, prepare a sample of the solution which you want to test in a test tube.
Then, add a few drops hydrochloric acid to remove any carbonate ions as they could also create a white precipitate which would give a false positive result.
Next, add a few drops of barium chloride solution. If sulfate ions are present, a white precipitate will form.