9a

Question 1

Flame tests

Answer 1

Compounds of some metals burn with a characteristic colour. You can test for various metal ions by carrying out a flame test. This involves putting the substance in a flame and seeing what colour the flame goes. You need to know the colours of the flames produced by the following metal ions:

Question 2

Lithium ions, Li+

Answer 2

Bright red flame.

Question 3

Sodium ions, Na+

Answer 3

Golden yellow / orange flame.

Question 4

Potassium ions K+

Answer 4

lilac flame.

Question 5

Calcium ions, Ca2+

Answer 5

orange-red flame.

Question 6

Copper ions, Cu2+

Answer 6

blue-green flame.

Question 7

Conducting flame tests

Answer 7

To do the test, you first need to clean a nichrome wire loop by dipping it in Tome hydrochloric acid and then rinsing it in distilled water. Then, dip the loop into the sample you want to test and put it into the clear blue part of a Bunsen flame (the hottest bit). Record the colour of the flame.

Question 8

Cations - Precipitation reactions

Answer 8

Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have characteristic colours. This means you can test for some metal ions by adding a few drops of sodium hydroxide solution to a solution of the substance you’re testing. If you get a coloured insoluble hydroxide, you can then tell which metal was in the compound.

Question 9

Results of adding NaOH to solutions of different metal ions

Answer 9

Question 10

Test for ammonium ions

Answer 10

To work out whether a substance contains ammonium ions (NH4+), all you need to do is add some sodium hydroxide solution to a solution of your mystery substance and gently heat it. If ammonia gas is given off, it means that there are ammonium ions in the substance.
You can test for ammonia gas by holding a piece of damp red litmus paper over it. If the gas is ammonia, the litmus paper will turn blue.

Question 11

Testing for carbonate ions - equation

Answer 11

When a carbonate reacts with a dilute acid you get a salt, water and carbon dioxide.

Question 12

Testing for carbonate ions - method

Answer 12

If you add acid to a substance that contains CO,²- ions, the mixture will fizz, 3 giving off a gas that turns limewater milky. The gas is carbon dioxide.

You can test for carbonate ions in a solution by using a dropping pipette to add a couple of drops of dilute acid to a test tube containing your mystery solution and bubbling any gas that’s produced through limewater

Question 13

Testing for halide ions

Answer 13

To test for chloride (Cl-), bromide (Br) or iodide (I-) ions, add dilute nitric acid (HNO3), followed by silver nitrate solution (AgNO3). A chloride gives a white precipitate of silver chloride. A bromide gives a cream precipitate of silver bromide. An iodide gives a yellow precipitate of silver iodide.

Question 14

Testing for halide ions - Reason of adding nitric acid

Answer 14

The nitric acid is added before the silver nitrate solution to get rid of any carbonate ions - they produce a pale precipitate with silver nitrate too, which would confuse the results.

It’s important to use dilute nitric acid for this test rather than hydrochloric acid. Hydrochloric acid would introduce chloride ions to the solution, so a white precipitate would be formed regardless of whether the solution originally contained chloride ions or not.

Question 15

Testing for sulfate ions

Answer 15

To test for sulfate ions in solution, first add some dilute hydrochloric acid to the test sample. Then add some barium chloride solution. If there are sulfate ions in the solution, a white precipitate of barium sulfate will form.

The hydrochloric acid is added to react with and remove any carbonate ions that might be present. If there are carbonate ions present when you add the barium chloride then they’ll also produce a white precipitate and that would confuse your results.

Question 16

Identifying compounds

Answer 16

In the core practical, you’ll be asked to identify the ions in a compound using different tests. lonic compounds are made up of positive and negative ions, so if you’re asked to identify the compound you’ll need to work out what both these ions are. You may need to carry out flame tests, precipitation reactions and tests on gases.

Question 17

Identifying compounds Example

Answer 17

Substance A is a blue-green powder that is soluble in water. When a sample of substance A is held in a Bunsen flame, the flame burns with a blue-green colour. When dilute nitric acid is added to a solution of substance A there is no reaction and no gas is given off, but if silver nitrate solution is then added a white precipitate is formed. Identify substance A.

The blue-green flame suggests that there are copper ions.

There’s no reaction with nitric acid, so there are no carbonate ions.

The white precipitate formed when silver nitrate solution is then added tells us that there are chloride ions present.

So substance A is probably copper chloride.

Question 18

What is flame photometry?

Answer 18

Flame photometry is an instrumental method that allows you to identify ions in a dilute solution. During flame photometry, a sample is placed in a flame. As the ions heat up their electrons become excited. When the electrons drop back to their original energy levels, they transfer energy as light. The light passes through a spectroscope, which can detect different wavelengths of light, to produce a line spectrum.

Since no two ions have the same charge and the same electron arrangement, different ions emit different wavelengths of light. So each ion produces a different pattern of wavelengths, and has a different line spectrum.

Question 19

calibration curve

Answer 19

The emission intensity of the spectrum indicates the concentration of that ion in solution. You can work out concentration from emission intensity using a calibration curve:

e.g.
Flame photometry was carried out on a sample, known to contain calcium ions.

The spectrum known to be emitted by calcium ions had an emission intensity of 4.5. You can use the calibration curve on the right to find the concentration of calcium ions in the sample.

So the concentration of calcium ions in the sample is 0.25 mol dm³

Question 20

flame photometry of mixtures

Answer 20

Flame photometry can also be used to identify different ions in mixtures. This makes it more useful than flame tests, which only work for substances that contain a single metal ion.

Question 21

Flame photometry - numerical form

Answer 21

Flame photometry data can also be given in numerical form.
The mixture contains both the emission line frequencies of a separate solution of ion X and ion Y.

Question 22

Advantages of using Flame photometry

Answer 22

Very sensitive - they can detect even the tiniest amounts of something.
Very fast tests - can be automated which speeds up the process.
Very accurate - unlike manual analysis, they don’t involve human error.

Flame photometry is a good example of an instrumental method that has a clear advantage over chemical tests. Flame tests can’t be used to identify a mixture of ions in solution - only a single metal. However, by using flame photometry, the identities of a mixture of ions in solution can be found.