Identification of ions by chemical and spectroscopic means (8.3)

Question 1

What are two ways that can be used to test for metal ions (cations)?

Answer 1

• flame test
• adding sodium hydroxide solution to metal ion

Question 2

In a flame test, what do lithium compounds result in?

Answer 2

a crimson flame

Question 3

In a flame test, what do sodium compounds result in?

Answer 3

a yellow flame

Question 4

In a flame test, what do potassium compounds result in?

Answer 4

a lilac flame

Question 5

In a flame test, what do calcium compounds result in?

Answer 5

an orange-red flame

Question 6

In a flame test, what do copper compounds result in?

Answer 6

a green flame

Question 7

What happens of a sample containing a mixture of ions is used in a flame test?

Answer 7

some flame colours can be masked so all metal ions cannot be identified (distinguished)

Question 8

3 needed

When sodium hydroxide solution is added, which solutions form white precipitates?

Answer 8

Solutions of:

• aluminium ions
• calcium ions
• magnesium ions

Question 9

Out of aluminium, calcium and magnesium hydroxide precipitates, which is the only solution that will redissolve in excess sodium hydroxide solution?

Answer 9

only the aluminium hydroxide precipitate dissolves

Question 10

When NaOH solution is added, what do compounds contain copper(II) ions form?

Answer 10

a blue precipitate

Question 11

When NaOH solution is added, what do compounds contain iron(II) ions form?

Answer 11

a green precipitate

Question 12

When NaOH solution is added, what do compounds contain iron(III) ions form?

Answer 12

a brown precipitate

Question 13

When NaOH is added to compounds containing certain metal ions, certain precipitates will be formed. What are these precipitates? What state will they be at room temperature (after reaction?

Answer 13

insoluble hydroxides - so will be solid at room temperature

Question 14

What is the word, symbol and ionic equation for when sodium hydroxide solution is added to copper sulphate?

Answer 14

copper sulfate + sodium hydroxide → copper hydroxide + sodium sulfate

CuSO_4(aq) + 2NaOH_(aq) → Cu(OH)_2(s) + Na₂SO_4(aq)

Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s)

Other reactions are similar to this, just swap out copper for other metal ion (forms hydroxide) and sulfate for other negative ion (forms sodium salt)

Question 15

Carbonates react with dilute acids to produce what?

Answer 15

carbon dioxide gas - it is seen when it fizzes

Question 16

Halide ions in solution produce what with silver nitrate solution in the presence of dilute nitric acid?

Answer 16

precipitates

Question 17

When do halide ions produce precipitates?

Answer 17

When in solution and silver nitrate solution is added in the presence of dilute nitric acid

Question 18

What colour is silver chloride precipitate?

Answer 18

White

Question 19

What colour is silver bromide precipitate?

Answer 19

cream

Question 20

What colour is silver iodide precipitate?

Answer 20

yellow

Question 21

What do sulfate ions in solution produce when barium chloride solution is added in the presence of dilute hydrochloric acid

Answer 21

a white precipitate

Question 22

What is needed for sulfate ions to produce a white precipitate?

Answer 22

Add barium chloride solution in the presence of dilute hydrochloric acid

Question 23

What are instrumental methods used to do?

Answer 23

Detect and identify elements and compounds

Question 24

3 needed

What are the advantages of instrumental methods compared with chemical tests?

Answer 24

Instrumental methods are more accurate, sensitive and rapid

• sensitive as can detect even tiny amounts of a substance*
• rapid as they are fully automated*

Question 25

What is an example of an instrumental methods?

Answer 25

Flame emission spectroscopy

Question 26

What is flame emission spectroscopy used to do?

Answer 26

to analyse metal ions in solutions to identify them and to measure their concentrations

Question 27

Describe how a flame emission spectroscopy works?

Answer 27

The sample is put into a flame and the light given out is passed through a spectroscope.

The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.

Question 28

How would you identify the metal ions in a substance using the results of a spectroscope given the results of normal metal ions?

Answer 28

by comparing its spectrum with reference spectra (all metal ions have unique line spectrum)

If they match, it must contain that metal ion

Note: it could contain multiple metal ions, in which all lines on the spectra will appear on the spectrum of the spectroscope

Question 29

What is the main advantage of flame emission spectroscopy over flame tests?

Answer 29

If a sample contains multiple different metal ions, the spectrum will show lines for all of them

Question 30

How would you determine the concentration of metal ions from the results on a spectroscope?

Answer 30

Look at lines on spectrum

The intensity of the lines represents the concentration of that specific metal ion (i.e. the higher the intensity, the higher the concentration)

Question 31

What does sensitive mean?

Answer 31

The ability to detect a much smaller amount of the substance

Question 32

Solutions will only effervesce/fizz when dilute acid is added to (…) solutions

Answer 32

carbonate