What are cations?
What tests identify cations (metal ions)?
sodium hydroxide (NaOH) solution
What colour flame does lithium burn with?
a crimson-red flame
What colour flame does sodium burn with?
a yellow-orange flame
What colour flame does potassium burn with?
a lilac flame
What colour flame does calcium burn with?
a brick-red flame
How do you carry out flame tests?
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
Many metal hydroxides are soluble/insoluble and ... out of a solution when formed
Many metal hydroxides are insoluble and precipitate out of a solution when formed
How you carry out a test of some metals with sodium hydroxide (NaOH)?
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
What colour is a copper (II), Cu2+, ion's precipitate when added to sodium hydroxide? What is the ionic reaction?
a blue precipitate is formed
Cu2+ (aq) + 2OH- (aq) → Cu(OH)2 (s)
What colour is a iron (II), Fe2+, ion's precipitate when added to sodium hydroxide? What is the ionic reaction?
a sludgy green precipitate is formed
Fe2+ (aq) + 2OH- (aq) → Fe(OH)2 (s)
What colour is a iron (III), Fe3+, ion's precipitate when added to sodium hydroxide? What is the ionic reaction?
a reddish brown precipitate is formed
Fe3+ (aq) + 3OH- (aq) → Fe(OH)3 (s)
What does an ammonium compound reacted with sodium hydroxide (NaOH) form?
Ammonia gas is ... (sensory) and can be really harmful to you ...
Ammonia gas is smelly and can be really harmful to you eyes
How can you test for ammonia gas?
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
How can you test whether a substance contains ammonium ions (NH4-)?
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
What are anions?
How do you test for carbonates (CO32-)? What is the ionic equation for this?
add dilute hydrochloric acid (HCl) to your test sample
if carbonates (CO32-) are present then carbon dioxide will be released
carbonate + acid → carbon dioxide + water
CO32- (s) + 2H+ (aq) → CO2 (g) + H2O (l)
How do you test for sulphates (SO42-)? What is the ionic equation for this?
to test for a sulphate ion (SO42-) add dilute hydrochloric acid, HCl, followed by barium chloride solution, BaCl2
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)
a white precipitate of barium sulphate means the original compound was sulphate
Why is hydrochloric acid (HCl) added to the tests of carbonates and sulphates?
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
How do you test for halides?
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
What precipitate is formed in the test for chloride ions? What is this ionic equation?
a white precipitate of silver chloride is formed
Ag+ (aq) + Cl- (aq) → AgCl (s)
What precipitate is formed in the test for bromide ions? What is this ionic equation?
a cream precipitate of silver bromide is formed
Ag+ (aq) + Br- (aq) → AgBr (s)
What precipitate is formed in the test for iodide ions? What is this ionic equation?
a yellow precipitate of silver iodide is formed
Ag+ (aq) + I- (aq) → AgI (s)
How do you test for chlorine?
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)
How do you test for oxygen?
oxygen relights a glowing splint
How do you test for carbon dioxide?
carbon dioxide turns limewater cloudy - just bubble the gas through a test tube of limewater and watch what happens
How do you test for hydrogen?
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)
How do you test for ammonia?
ammonia turns damp red litmus paper blue
(it also has a very strong smell)
How do you test for water?
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
How do you test for pure water? Explain this
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 100oC and freeze at 0oC - if it doesn't then the sample isn't pure water