25 - Qualitative Analysis: Tests for Ions

Question 1

Why is it important that the test for any ion is unique?

Answer 1

The results of a test must let you determine which ion is present, rather than being uncertain about which ion it is.

Question 2

How are flame tests carried out?

Answer 2

1) Light a Bunsen burner (to blue flame)

2) Pick up small sample of test using a wire loop

3)Hold sample above Bunsen burner and observe flame colour

Question 3

What substance is the wire loop cleaned in before testing each sample?

Answer 3

Hydrochloric acid

Question 4

What colour does Lithium make?

Answer 4

Red

Question 5

What colour does Sodium make?

Answer 5

Yellow

Question 6

What colour does Potassium make?

Answer 6

Lilac

Question 7

What colour does Calcium make?

Answer 7

Orange-red

Question 8

What colour does Copper make?

Answer 8

Blue-green

Question 9

What does the flame photometer measure?

Answer 9

The light intensity of the flame colours produces by metal ions

Question 10

What can data from a flame photometer be used for?

Answer 10

• To identify metal ions in a sample
• Determine the concentration of metal ions in dilute solutions

Question 11

How do we identify which metal ion is present in an emission spectrum?

Answer 11

Each metal ion produces a unique emission spectrum

Question 12

What is an emission spectrum?

Answer 12

Light given off by a substance, split into its component colours or wavelengths

The metal present in a sample is identified by comparing its
spectrum with reference spectra

Question 13

What is a calibration curve?

Answer 13

Graph with the readings from a machine plotted against known amounts of a substance

Question 14

How do we determine concentration when using a flame photometer to test for an metal ion?

Answer 14

A reading is taken from the flame photometer for different concentrations of a metal ion in solution.

Readings are used to plot a calibration curve

Question 15

Why might we use instrumental methods of analysis instead of simple laboratory tests?

Answer 15

They are faster, more accurate and more sensitive than simple chemical tests

Question 16

How do you test for metals?

Answer 16

Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble (so they appear as
precipitates)

Question 17

What colour does the metal ion Iron (II) make?

Answer 17

Green

Question 18

What colour does the metal ion Iron (III) make?

Answer 18

Brown

Question 19

What colour does the metal ion Copper make?

Answer 19

Blue

Question 20

What colour does the metal ion Calcium make?

Answer 20

White

Question 21

What colour does the metal ion Aluminium make?

Answer 21

White

Question 22

What is the chemical test for Ammonia?

Answer 22

• Damp red litmus paper –> blue
• Hydrogen chloride gas (from hydrochloric acid) held near it –> forms white smoke of ammonium chloride

Question 23

How do you distinguish between aluminium ions and calcium ions?

Answer 23

If you add excess sodium hydroxide solution:

• The aluminium hydroxide precipitate reacts to form a colourless solution
• The calcium hydroxide precipitate is unchanged

Question 24

How can we identify negatively charged ions in a solution?

Answer 24

The formation of different precipitates allows us to identify different negatively charged ions in solution

Question 25

How do we test for **Carbonate Ions**?

Answer 25

We add dilute acid (usually hydrochloric acid) to the carbonate ions. Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound. Bubbling is usually Co2

Question 26

How do we identify the gas produced whilst testing for **Carbonate Ions**?

Answer 26

Limewater is used to confirm that the gas is carbon dioxide

Question 27

What *acid* do we use to test for **Carbonate Ions**?

Answer 27

The test for carbonate ions works whatever acid is added

Question 28

How do we test for **Sulfate Ions** in a solution?

Answer 28

Barium ions react with sulphate ions to form insoluble white barium sulphate 1) Add a few drops of dilute hydrochloric to the sample 2) Add a few drops of dilute barium chloride solution A white precipitate forms if sulphate ions are present

Question 29

Why do you add **dilute hydrochloric acid** to a solution when testing for **sulphate ions**?

Answer 29

This acidifies the solution and removes carbonate ions that might also give a precipitate in the test

Question 30

How do we test for **Halide ions**?

Answer 30

Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates 1) Add a few drops of dilute nitric acid to the sample 2) Add a few drops of dilute silver nitrate solution Observe and record the colour of any precipitate that forms.

Question 31

What **precipitate colour** does the halide ion **Chloride** make?

Answer 31

White

Question 32

What **precipitate colour** does the halide ion **Bromide** make?

Answer 32

Cream

Question 33

What **precipitate colour** does the halide ion **Iodide** make?

Answer 33

Yellow

Question 34

# Core practical How can you *identify the ions* in an **unknown salt**?

Answer 34

You need to carry out: * Flame tests * Hydroxide precipitate tests * Test for ammonium ions * Test for carbonate ions * Test for Sulfate ions * Test for Halide ions

Question 35

# Core practical Describe the **flame tests** for **metal cations**?

Answer 35

1) Light a Bunsen burner and open the air hole to give a hot blue flame 2) Pick up a small sample of a solid salt (using a clean wire loop or damp wooden splint) 3) Hold the sample in the edge of the flame 4) Observe and record the flame colour

Question 36

# Core practical Describe the **Hydroxide precipitate test** for **metal cations**?

Answer 36

1) Dissolve a small amount of salt in a test tube using distilled water 2) Add a few drops of dilute sodium hydroxide solution (one drop at a time) 3) Record the colour of any precipitate formed 4) If a white precipitate forms, add excess dilute sodium hydroxide solution (to distinguish between aluminium and calcium ions)

Question 37

# Core practical Describe how to test for **Ammonium ions**?

Answer 37

1) Dissolve a little solid salt in a test tube using distilled water 2) Add dilute sodium hydroxide solution & warm gently 3) Remove from the flame. Hold a piece of damp red litmus paper near the mouth of the test tube. Record what happens to its colour

Question 38

# Core practical Describe how to test for **Carbonate ions**?

Answer 38

1) Put a little solid salt in a test tube + add a few drops of dilute acid 2) Record whether any effervescence occurs 3) Use limewater to check that any bubbles contain Co2

Question 39

# Core practical Describe how to test for **Sulfate ions**?

Answer 39

1) Dissolve a little solid salt in a test tube using distilled water 2) Add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution. Record whether a white precipitate forms

Question 40

# Core practical Describe how to test for **Halide ions**?

Answer 40

1) Dissolve a little solid salt in a test tube using distilled water 2) Add a few drops of dilute nitric acid, then a few drops of silver nitrate solution. Record the colour of any precipitate formed