Tests For Cations & Anions Flashcards
What do flame tests identify?
(Positive) metal ions
Flame test: what colour is lithium?
Red
Flame test: what colour is sodium?
Yellow
Flame test: what colour is potassium?
Lilac
Flame test: what colour is calcium?
Orange-red
Flame test: what colour is copper?
Blue-green
How to carry out a flame test (3)
Clean a nichrome wire loop by dipping in hydrochloric acid and rinsing in distilled water
Dip wire loop into sample of metal compound and put loop in blue part of Bunsen flame
Record flame colour
What methods are there to test for cations?
Flame test
Coloured precipitate with NaOH
NaOH to ammonium ions
How to carry out coloured precipitate cation test (2)
Add a few drops of sodium hydroxide solution to a solution of mystery compound
If a hydroxide precipitate forms, use its colour to tell which metal was in the compound
Coloured precipitate test: what colour is aluminium?
White at first but then redissolves in excess NaOH to form a colourless solution
Coloured precipitate test: what colour is calcium?
White
Coloured precipitate test: what colour is copper?
Blue
Coloured precipitate test: what colour is iron II?
Green
Coloured precipitate test: what colour is iron III?
Brown
What test for cations do you do after the flame test and the coloured precipitate test?
Add NaOH to ammonium ions to test for ammonia
How do you test for ammonia ions? (3)
Add NaOH to substance and gently heat it
If ammonia gas is given off there are ammonium ions in the substance
Test for ammonia gas - turns damp red litmus paper blue
How to test for halide ions (method + outcomes)
Add dilute nitric acid followed by a few drops of silver nitrate solutution
A chloride gives a white precipitate (silver chloride)
A bromide gives a cream precipitate (silver bromide) An iodide gives a yellow precipitate (silver iodide)
Silver nitrate solution, halide test: what colour precipitate is chloride?
White
Silver nitrate solution, halide test: what colour precipitate is bromide?
Cream
Silver nitrate solution, halide test: what colour precipitate is iodide?
Yellow
How to test for carbonates (3)
Add dilute acid
If carbonate ions are present the mixture will fizz because carbon dioxide is produced
Test for carbon dioxide by bubbling it through limewater which turns milky
How to test for sulphate ions (3)
Add dilute hydrochloric acid
Then add barium chloride solution
If sulphate ions are present in the solution, a white precipitate (barium sulphate) forms
Sulphate ions, barium chloride test: why do you add hydrochloric acid?
To stop any precipitation reactions not involving sulphate ions from taking place
