Module 3 Section 1- Test for Ions Flashcards
How to test for carbonates
1) Add a dilute acid eg dilute hydrochloric acid.
2) if carbonates are present, carbon dioxide will be released (effervescence + bubbling)
3) you can test for carbon dioxide using limewater. Bubble the gas through a test tube and see if the limewater turns cloudy.
Ionic equation of reaction between carbonate and acid
CO3 2- (s) + 2H+ (aq) -> CO2 (g) + H2O (l)
Carbonate + acid -> carbon dioxide + water
How to test for Sulfates ?
1) add dilute HCl
2) then add barium chloride solution, BaCl2
3) if you get a white precipitate, it’s barium Sulfate
Ionic equation for reaction between barium chloride and Sulfate solution
Ba2+ (aq) + SO4 2- (aq) -> BaSO4 (aq)
What is the test for halides
1) add nitric acid
2) then add silver nitrate solution
3) if chlorine, bromine or iodide are present a precipitate will form.
Colour of precipitates for halogen test
Silver chloride - white
Silver bromide - cream
Silver iodide - yellow
How can you tell the precipitates are different from each other?
You can test their solubility in ammonia
Which halogen precipitates are soluble in ammonia
Silver chloride - dissolves in both dilute and concentrated ammonia
Silver bromide - only dissolves in concentrated ammonia
Silver iodide- does not dissolve in ammonia
How can you test for ammonium compounds?
1) Ammonia gas is alkaline so you can use damp red litmus paper. If anmonia is present, the paper will turn blue.
2) when testing if a substance contains ammonium ions (NH4+), add sodium hydroxide solution and warm the mixture.
3) if ammonia is given off (use damp red litmus paper) there means there is ammonium ions in the solution.
Ionic equation for reaction of ammonium ions and sodium hydroxide?
NH4+ (aq) + OH- (aq) -> NH3(g) + H2O (l)
Example reaction of ammonium chloride and sodium hydroxide
NH4Cl (aq) + NaOH (aq) -> NH3 (g) + H2O(l) + NaCl (aq)
What order should you carry out the tests
Test for carbonates -> test for Sulfates -> test for halides
Why do we carry out the tests in a specific order
1) barium carbonate is also insoluble. If you’re testing for Sulfate ions, you need to make sure no carbonate ions around first.
2) Sulfate ions also produce a precipitate with silver nitrate so you have to make sure there are not Sulfate ions before carrying out the halide test.
How to avoid false positive tests?
Add dilute acid to the test solutions. This gets rid of any lurking anions that we do not want.
