8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 1/12

Testing for negative ionsAims: To use qualitative chemical tests to identify

some of the ions present in a solution

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 2/12

Qualitative chemical tests

• These tests are simple test tube reactions thatcan help us to identify unknown substances

• They tell us nothing about the quantities of

substances – just YES or NO!• Examples you have seen before are:

Indicators for acids/ bases

Bromine water decolourised by alkenesLime water for carbon dioxide

Squeaky pop for hydrogen

Glowing splint relights for oxygen

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 3/12

You are going to learn how to testfor negative ions ions

These are:

Carbonate ions, CO32- 

Sulfate ions, SO42- 

Nitrate ions, NO3- 

Halide ions, Cl-, Br-, I- (these are ions of G7 elements)

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 4/12

Solid carbonate compounds• Metal carbonate compounds break down on

heating (thermal decomposition)• This produces the metal oxide and carbon

dioxide gas• The gas can be tested with limewater to confirm

it’s identity as carbon dioxide – the lime waterwill turn cloudy.

• Two of these also have characteristic colourchanges!!

• Copper carbonate changes fro

m green to black• Zinc carbonate changes from white to yellow 

(but changes back when it cools!)

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 5/12

• Carbonate compounds react with acid to form asalt, carbon dioxide and water.

• CaCO3 + 2HCl ↓ CaCl2 + CO2 + H2O

• So if we add acid to a solution containing

carbonate ions it will fizz.• The gas can be tested with limewater to confirm

it’s identity as carbon dioxide – the lime waterwill turn cloudy.

And in solutions

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 6/12

Testing for Sulfate ions

• First we add some hydrochloric acid toreact with and remove any carbonate ions.

• Then add a few drops of Barium chloridesolution.

• A white precipitate indicates that sulfateions are present.

• The precipitate is insoluble Barium sulfate

Ba2+

(aq) + SO42-

(aq)  ↓ BaSO4(s)

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 7/12

Testing for Halide ions

• First add some Nitric acid solution to reactwith and remove any carbonate ions

• Then add a few drops of Silver Nitratesolution

• A precipitate indicates the presence ofhalide ions

• The colour of the precipitate formed showswhich halide ion was present

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 8/12

Testing for Halide ions• A White precipitate ↓ Chloride ions

•  A Cream precipitate ↓ Bromide ions

•  A Yellow precipitate ↓ Iodide ions

• The precipitates are the silver halides:

 AgCl AgBr AgI

• In practice the last 2 can be very hard to tell apart!

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 9/12

Testing for Nitrate ions

• First add a small amount of aluminiumpowder.

• Then add a few drops of sodium hydroxidesolution and warm GENTLY.

• If nitrate ions are present they will be

reduced to ammonia.• Test for this using damp red litmus paper – it will turn blue.

• It also smells, A LOT!

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 10/12

Try it yourselves!

• You are provided with a mystery solutionlabelled, X.

• Your assignment is to identify the ions in solution

X using a range of chemical tests.• You should record the tests that you carry out

and what the results are.

• Use your observations to identify the ions in X• This is chemical detective work or

FORENSIC CHEMISTRY

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 11/12

The identity of X

• You should now be able to identify the ionsin X

• Be prepared to explain how you reachedyour decision

• Would your evidence stand up in court?

8/3/2019 Testing for Negative Ions

http://slidepdf.com/reader/full/testing-for-negative-ions 12/12

The identity of X

• You should now be able to identify the ionsin X

• Be prepared to explain how you reachedyour decision

• Would your evidence stand up in court?