Tests for Ions Flashcards
what are the 4 steps for flame tests?
- dip a nichrome wire loop in concentrated hydrochloric acid
- dip the wire loop in a sample of the compound
- hold the loop in the clear blue part of the Bunsen flame
- observe the colour change in the flame
what colour does the flame turn when lithium is present?
crimson
what colour does the flame turn when potassium is present?
lilac
what colour does the flame turn when sodium is present?
orange-yellow
what colour does the flame turn when calcium is present?
brick red
what colour does the flame turn when barium is present?
green
what colour does the flame turn when copper(II) is present?
blue-green
how do you test for precipitates?
add a few drops of sodium hydroxide solution to a solution of the unknown compound
what colour precipitate will be produced when silver is present, and what is the ionic reaction?
brown (precipitate of silver oxide)
Ag+(aq) + 2OH-(aq) –> Ag2O(s) + H2O(l)
what colour precipitate will be produced when calcium is present, and what is the ionic reaction?
white
Ca 2+(aq) + 2OH-(aq) –> Ca(OH)2 (s)
what colour precipitate will be produced when copper(II) is present, and what is the ionic reaction?
blue
Cu 2+(aq) + 2OH-(aq) –> Cu(OH)2 (s)
what colour precipitate will be produced when lead(II) is present, and what is the ionic reaction?
white
Pb 2+(aq) + 2OH-(aq) –> Pb(OH)2 (s)
what colour precipitate will be produced when iron(II) is present, and what is the ionic reaction?
green
Fe 2+(aq) + 2OH-(aq) –> Fe(OH)2 (s)
what colour precipitate will be produced when iron(III) is present, and what is the ionic reaction?
reddish brown
Fe 3+(aq) + 3OH-(aq) –> Fe(OH)3 (s)
what colour precipitate will be produced when zinc is present, and what is the ionic reaction?
white at first, redissolves in excess NaOH to form a colourless solution
Zn 2+(aq) + 2OH-(aq) –> Zn(OH)2 (s)
then Zn(OH)2(s) + OH-(aq) –> Zn(OH)3 -(aq)
what colour precipitate will be produced when aluminum is present, and what is the ionic reaction?
white at first, redissolves in excess NaOH to form a colourless solution
Al 3+(aq) + 3OH-(aq) –> Zn(OH)3 (s)
then Zn(OH)3(s) + OH-(aq) –> Zn(OH)4 -(aq)
How do you test for carbon dioxide & what does a positive result look like?
bubble gas through a test tube of limewater
limewater turns cloudy
What will happen when hydrochloric acid and carbonates react?
Fizzing as carbon dioxide is given off
How can you test for sulfate ions and what does a positive result look like?
Add dilute HCl followed by barium chloride solution
A white precipitate (of barium sulfate) forms
how can you test for ammonium compounds and what does a positive result look like?
add some sodium hydroxide to the unknown substance in a test tube and gently heat, then insert a damp piece of red litmus paper into the test tube
the paper turns blue
how can you test for hydroxides and what does a positive result look like?
dip a piece of red litmus paper into the solution
paper turns blue
how can you test for chloride ions and what does a positive result look like?
add dilute nitric acid (HNO3) followed by silver nitrate solution (AgNO3)
white precipitate of silver chloride
how can you test for iodide ions and what does a positive result look like?
add dilute nitric acid (HNO3) followed by silver nitrate solution (AgNO3)
yellow precipitate of silver iodide
how can you test for bromide ions and what does a positive result look like?
add dilute nitric acid (HNO3) followed by silver nitrate solution (AgNO3)
cream precipitate of silver bromide
how can you test for nitrate ions and what does a positive result look like?
warm a solution of the unknown substance with sodium hydroxide solution and aluminum foil
(Al reduces the nitrate ions to ammonium ions, NH4 ions then react with OH ions to produce NH3 gas and water)
ammonia is given off
what could produce a false positive when testing for sulfate ions?
if there are any carbonate ions or sulfite ions around first to react with the barium chloride
what could produce a false positive when testing for halide ions?
if any sulfate ions are present
what order should you conduct tests for ions?
- test for carbonates
no CO2 - sulfates
no precipitate - halides
how can you avoid false positives when testing for ions?
add a dilute acid to your solutions which will get rid of any unwanted anions
