Chemical Tests

Question 1

What are cations?

Answer 1

positive ions

Question 2

What tests identify cations (metal ions)?

Answer 2

flame tests

sodium hydroxide (NaOH) solution

Question 3

What colour flame does lithium burn with?

Answer 3

a crimson-red flame

Question 4

What colour flame does sodium burn with?

Answer 4

a yellow-orange flame

Question 5

What colour flame does potassium burn with?

Answer 5

a lilac flame

Question 6

What colour flame does calcium burn with?

Answer 6

a brick-red flame

Question 7

How do you carry out flame tests?

Answer 7

1. clean a platinum wire loop by dipping it in some dilute hydrochloric acid (HCl)
2. hold it up to a flame
3. once you hold the loop in the flame and it burns without any colour, you can dip it into the same you want to test and then put it back in the flame
4. see what colour the flame burns

Question 8

Many metal hydroxides are soluble/insoluble and … out of a solution when formed

Answer 8

Many metal hydroxides are insoluble and precipitate out of a solution when formed

Question 9

How you carry out a test of some metals with sodium hydroxide (NaOH)?

Answer 9

1. add a few drops of sodium hydroxide (NaOH) solution to a solution of your mystery compound in a test tube
2. see what colour the precipitate formed is

Question 10

What colour is a copper (II), Cu²⁺, ion’s precipitate when added to sodium hydroxide? What is the ionic reaction?

Answer 10

a blue precipitate is formed

Cu²⁺ _(aq) + 2OH^- _(aq) → Cu(OH)_{2 (s)}

Question 11

What colour is a iron (II), Fe²⁺, ion’s precipitate when added to sodium hydroxide? What is the ionic reaction?

Answer 11

a sludgy green precipitate is formed

Fe²⁺ _(aq) + 2OH^- _(aq) → Fe(OH)_{2 (s)}

Question 12

What colour is a iron (III), Fe³⁺, ion’s precipitate when added to sodium hydroxide? What is the ionic reaction?

Answer 12

a reddish brown precipitate is formed

Fe³⁺ _(aq) + 3OH^- _(aq) → Fe(OH)_{3 (s)}

Question 13

What does an ammonium compound reacted with sodium hydroxide (NaOH) form?

Answer 13

ammonia

Question 14

Ammonia gas is … (sensory) and can be really harmful to you …

Answer 14

Ammonia gas is smelly and can be really harmful to you eyes

Question 15

How can you test for ammonia gas?

Answer 15

using a damp piece of red litmus paper - the litmus paper must be damp so that the ammonia gas can dissolve and make the colour change

if ammonia is present, the paper will turn blue

Question 16

How can you test whether a substance contains ammonium ions (NH₄^-)?

Answer 16

add some sodium hydroxide to a solution of your mystery substance in a test tube

if there’s ammonia given off this means there are ammonium ions in your mystery substance

Question 17

What are anions?

Answer 17

negative ions

Question 18

How do you test for carbonates (CO₃^2-)? What is the ionic equation for this?

Answer 18

add dilute hydrochloric acid (HCl) to your test sample

if carbonates (CO₃^2-) are present then carbon dioxide will be released

carbonate + acid → carbon dioxide + water

CO₃^2- _(s) + 2H⁺ _(aq) → CO_{2 (g)} + H₂O _(l)

Question 19

How do you test for sulphates (SO₄^2-)? What is the ionic equation for this?

Answer 19

to test for a sulphate ion (SO₄^2-) add dilute hydrochloric acid, HCl, followed by barium chloride solution, BaCl₂

Ba²⁺ _(aq) + SO₄^2- _(aq) → BaSO_{4 (s)}

a white precipitate of barium sulphate means the original compound was sulphate

Question 20

Why is hydrochloric acid (HCl) added to the tests of carbonates and sulphates?

Answer 20

the hydrochloric acid is added to get rid of any traces of carbonate or sulphate ions before you do the test.

both of these would also produce a precipitate, so they would confuse the results

Question 21

How do you test for halides?

Answer 21

to test for chloride (Cl^-), bromide (Br^-​) or iodide ions (I^-​), add dilute nitric acid (HNO₃), followed by silver nitrate solution (AgNO₃)

the nitric acid is added to get rid of the carbonat or sulphate ions before the test

Question 22

What precipitate is formed in the test for chloride ions? What is this ionic equation?

Answer 22

a white precipitate of silver chloride is formed

Ag⁺ _(aq) + Cl^- _(aq) → AgCl _(s)

Question 23

What precipitate is formed in the test for bromide ions? What is this ionic equation?

Answer 23

a cream precipitate of silver bromide is formed

Ag⁺ _(aq) + Br^- _(aq) → AgBr _(s)

Question 24

What precipitate is formed in the test for iodide ions? What is this ionic equation?

Answer 24

a yellow precipitate of silver iodide is formed

Ag⁺ _(aq) + I^- _(aq) → AgI _(s)

Question 25

How do you test for chlorine?

Answer 25

chlorine bleaches damp litmus paper, turning it white

(it may turn red for a moment first though - that’s because a solution of chlorine is acidic)

Question 26

How do you test for oxygen?

Answer 26

oxygen relights a glowing splint

Question 27

How do you test for carbon dioxide?

Answer 27

carbon dioxide turns limewater cloudy - just bubble the gas through a test tube of limewater and watch what happens

Question 28

How do you test for hydrogen?

Answer 28

hydrogen makes a “squeaky pop” with a lighted splint

(the noise comes from the hydrogen burning with the oxygen in the air to form H₂O)

Question 29

How do you test for ammonia?

Answer 29

ammonia turns damp red litmus paper blue

(it also has a very strong smell)

Question 30

How do you test for water?

Answer 30

copper (II) sulphate crystals can be used to test for water

1. when copper (II) sulphate is bound to water (water of crystallisation) it forms blue crystals
2. if you heated the blue hydrated copper (II) sulphate crystals it drives the water off
3. this leaves a white anhydrous copper (II) sulphate powder, which doesn’t have any water bound to it
4. if you then add a couple of drops of water to the white powder you get the blue crystals back again

so to test for water, you add to anhydrous copper (II) sulphate to the solution and see if the white powder turns blue

this test will tell you if water is present in a solution but it won’t tell you if the water is pure

Question 31

How do you test for pure water? Explain this

Answer 31

when a sample is pure it means it’s only made up of one susbstance

this means it has set defined physical properties like boiling point and freezing point

pure water will always boil at 100^oC and freeze at 0^oC - if it doesn’t then the sample isn’t pure water