Hi, heres all the information you need to know about all the tests in additional science to pass your exam.
Firstly we have to know the difference between Qualitative and Quantitative to grab those easy marks.
Qualitative tests show you what the substance is, a non numerical result.
Quantitative tests show you how much of the substance there is, a numerical result.
Tests For Positive Ions
Flame Tests:
Compounds of metals give certain colours when heated, here are these colours:
Sodium, Na+ gives an orange/yellow colour
Potassium, K+ gives a LILAC flame, the word LILAC is very important as you will not get the mark for anything else
Calcium, Ca2+ gives a brick-red flame
Copper, Cu2+ gives a blue-green flame
Now we move on top the precipitate tests,
Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have a characteristic colour.
In this test you add a few drops of sodium hydroxide solution to a solution of your mystery compound- all in the hope of forming an insoluble salt.
The colour of the salt gives away what metal is in the compound.
Calcium, Ca2+ = White precipitate
Copper (II), Cu2+ = Blue
Iron (II), Fe2+ = Sludgy Green
Iron (III), Fe3+ = Reddish brown
Aluminium, Al3+= White a first but then dissolves in excess NaOH to form a colourless solution.
Ammonium, NH4= No precipitate but when heated gives off ammonia which has a distinctive smell like hair dye.
Ionic Equations show just the useful bits of the reactions
Example: Ca2+ (aq) + 2OH- (aq) --> Ca(OH)2(s)
Ionic equations are half of full equations and only show the useful bits of the equation.
Now we move on to the tests for negative ions
Negative Ions
Hydrochloric acid helps to detect sulphites and carbonates
Carbonates give of CO2 when reacted with dilute HCl
You can then test for CO2 using limewater.
Sulphites give of SO2 when reacted with dilute HCl
You then test for SO2 by using potassium dichromate paper in which the SO2 turns it from orange to green.
Test for Sulphates whit HCl and Barium Chloride
Sulphate ions produce a white precipitate
To test for a sulphate add dilute HCl, followed by barium chloride
A white precipitate of barium sulphate means the original compound was a sulphate
(The hydrochloric acid is added to get rid of any traces of sulphite or carbonate ions both would produce a similar precipitate so it would confuse the results.
Test for Halides (Cl-, Br-, I-) with Nitric Acid and Silver Nitrate
To test for chloride, bromide or iodide ions, add dilute nitric acid ( HNO3), followed by silver nitrate (AgNO3)
A chloride gives a white precipitate of silver chloride.
A bromide gives a cream precipitate of silver bromide.
An Iodide gives a yellow precipitate of silver iodide.
The acid is only added to get rid of any carbonate or sulphite ions that could affect the results.
Tests for Acids and Alkalis
Indicators
Litmus: Blue litmus turns red if the solution is acidic
Red litmus paper turns blue if the solution is alkali
Phenolphthalein : Pink in alkalis but colourless in acids
Methyl orange: yellow in alkalis, red in acids
Tests
Squeaky pop test- test for Acid
When an acid reacts with a metal hydrogen gas is given off. Only metals more reactive than hydrogen will give of the gas.
Test for Alkali
Heat the substance with Ammonium salt to test for Alkalis. Hydroxides give off Ammonia gas when heated gently with an ammonium compound.
Firstly we have to know the difference between Qualitative and Quantitative to grab those easy marks.
Qualitative tests show you what the substance is, a non numerical result.
Quantitative tests show you how much of the substance there is, a numerical result.
Tests For Positive Ions
Flame Tests:
Compounds of metals give certain colours when heated, here are these colours:
Sodium, Na+ gives an orange/yellow colour
Potassium, K+ gives a LILAC flame, the word LILAC is very important as you will not get the mark for anything else
Calcium, Ca2+ gives a brick-red flame
Copper, Cu2+ gives a blue-green flame
Now we move on top the precipitate tests,
Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have a characteristic colour.
In this test you add a few drops of sodium hydroxide solution to a solution of your mystery compound- all in the hope of forming an insoluble salt.
The colour of the salt gives away what metal is in the compound.
Calcium, Ca2+ = White precipitate
Copper (II), Cu2+ = Blue
Iron (II), Fe2+ = Sludgy Green
Iron (III), Fe3+ = Reddish brown
Aluminium, Al3+= White a first but then dissolves in excess NaOH to form a colourless solution.
Ammonium, NH4= No precipitate but when heated gives off ammonia which has a distinctive smell like hair dye.
Ionic Equations show just the useful bits of the reactions
Example: Ca2+ (aq) + 2OH- (aq) --> Ca(OH)2(s)
Ionic equations are half of full equations and only show the useful bits of the equation.
Now we move on to the tests for negative ions
Negative Ions
Hydrochloric acid helps to detect sulphites and carbonates
Carbonates give of CO2 when reacted with dilute HCl
You can then test for CO2 using limewater.
Sulphites give of SO2 when reacted with dilute HCl
You then test for SO2 by using potassium dichromate paper in which the SO2 turns it from orange to green.
Test for Sulphates whit HCl and Barium Chloride
Sulphate ions produce a white precipitate
To test for a sulphate add dilute HCl, followed by barium chloride
A white precipitate of barium sulphate means the original compound was a sulphate
(The hydrochloric acid is added to get rid of any traces of sulphite or carbonate ions both would produce a similar precipitate so it would confuse the results.
Test for Halides (Cl-, Br-, I-) with Nitric Acid and Silver Nitrate
To test for chloride, bromide or iodide ions, add dilute nitric acid ( HNO3), followed by silver nitrate (AgNO3)
A chloride gives a white precipitate of silver chloride.
A bromide gives a cream precipitate of silver bromide.
An Iodide gives a yellow precipitate of silver iodide.
The acid is only added to get rid of any carbonate or sulphite ions that could affect the results.
Tests for Acids and Alkalis
Indicators
Litmus: Blue litmus turns red if the solution is acidic
Red litmus paper turns blue if the solution is alkali
Phenolphthalein : Pink in alkalis but colourless in acids
Methyl orange: yellow in alkalis, red in acids
Tests
Squeaky pop test- test for Acid
When an acid reacts with a metal hydrogen gas is given off. Only metals more reactive than hydrogen will give of the gas.
Test for Alkali
Heat the substance with Ammonium salt to test for Alkalis. Hydroxides give off Ammonia gas when heated gently with an ammonium compound.
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