P2C25 - Qualitative + Test for ions

Question 1

What flame does copper burn?

Answer 1

Blue green

Question 2

What flame does potassium burn?

Answer 2

Lilac

Question 3

What flame does lithium burn?

Answer 3

Red

Question 4

What flame does sodium burn?

Answer 4

Yellow

Question 5

What flame does calcium burn?

Answer 5

Orange red

Question 6

Why is it problematic for interpreting the results of a flame test if a sample is made up of multiple ions?

Answer 6

If the sample consists of a mixture of metal ions, the flame colours of some may be masked, which can lead to incorrect interpretations.

Question 7

What are the steps for the flame test practical?

Answer 7

1.Submerge a nichrome wire loop in dilute hydrochloric acid to make sure that it is clean.
2. Place the nichrome wire loop into the sample to be tested.
3. Hold the nichrome wire loop in a Bunsen burner’s blue flame.
4. Observe any change in colour of the Bunsen flame.

Question 8

What can be found in aqueous solutions of metal compounds?

Answer 8

Metal ions

Question 9

What happens when metal ions react with sodium hydroxide solution? (NaOH)

Answer 9

They can precipitate out of it

Question 10

Which colour of precipitate is formed by aluminium, calcium and magnesium?

Answer 10

White

Question 11

Whats the test for Ammonia?

Answer 11

Ammonia gas can be identified by placing damp red litmus paper at the mouth of the test tube.
The litmus will turn blue if ammonia gas is present.

Question 12

Whats the test for carbonates?

Answer 12

A reaction between a carbonate and a dilute acid gives a salt, carbon dioxide and water.

E.g. Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water

As carbon dioxide is produced, we can use the standard carbon dioxide test to work out if an unknown substance is a carbonate.

To recap, this test involves bubbling the gas through limewater. If the solution turns cloudy, the gas is carbon dioxide.

Question 13

Whats the test for sulphates?

Answer 13

1. Add dilute hydrochloric acid to an unknown solution. - This removes carbonate ions.
   (These could disrupt test results by forming a precipitate with the barium ions added in step 2.)
2. Add barium chloride to the same solution.
   If sulfates are present, the white precipitate barium sulfate will form:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

Question 14

How do we test for halide ions?

Answer 14

1. Add dilute nitric acid to an unknown solution. - This removes carbonate ions.
   (These could disrupt test results by forming a precipitate with the silver ions added in step 2.)
2. Add silver nitrate to the same solution.
   If halide ions are present, they will form a precipitate with the silver ions:

Ag+(aq) + Cl-(aq) → AgCl(s) (silver chloride is white)
Ag+(aq) + Br-(aq) → AgBr(s) (silver bromide is cream)
Ag+(aq) + I-(aq) → AgI(s) (silver iodide is pale yellow)

Question 15

What’s the certain order of tests to find cations?

Answer 15

Flame test.
Coloured precipitates with sodium hydroxide.
Testing the gas evolved for ammonium.

Question 16

What’s the certain order of tests to find anions?

Answer 16

Test for carbonate - test for carbon dioxide gas to confirm a positive result.
Test for sulfates - the presence of a white barium sulfate precipitate confirms a positive result.
Test for halide ions - the colour of silver halide corresponds to halide anion in a positive result.

Question 17

Why do the tests for anions have to be carried out in a logical order?

Answer 17

If any HCl or BaCl2 is added to test for carbonate or sulfate, there will be chloride ions in the solution.
These will be tested and there will be a false positive result for chloride ions.

Question 18

What are the advantages of instrumental methods?

Answer 18

Greater sensitivity and accuracy.
Quicker results
Able to analyse tiny samples.

Question 19

What are the disadvantages of instrumental methods?

Answer 19

The instruments are expensive.
Special training to operate the instruments is needed.
The results are only useful when compared to data from known substances.

Question 20

What is flame emission spectroscopy?

Answer 20

An instrumental method used to work out the identity and concentration of the metal ions present in a solution.

Question 21

What are the steps to flame emission spectroscopy?

Answer 21

1. Place sample of metal solution being tested into a flame. Light will be given off.
2. The light that is given off is captured by a spectroscope.
   This instrument generates a line spectrum by distinguishing between light with different wavelengths.