Analysis Flashcards
Test for hydrogen
Lighted spill is held to the mouth of the tube. The hydrogen explodes with a squeaky pop. The hydrogen combines explosively with oxygen in the air to make water.
For example, in the test tube is acid and a piece of magnesium. And then you do the above experiment to see if hydrogen is present.
Test for oxygen
A glowing splint is put into the tube containing the gas. Oxygen relights a glowing splint
Test for carbon dioxide
The carbon dioxide is bubbled through limewater. The carbon dioxide turns the limewater milky/chalky/cloudy. Limewater is calcium hydroxide solution. The carbon dioxide reacts with it to form a white precipitate of calcium carbonate.
Ca(OH)2 + CO2 —> CaCO3 + H2O
Calcium hydroxide + carbon dioxide —> calcium carbonate + water
Test for Ammonia
Ammonia gas’ colour
Smell
Test
Ammonia gas has no colour - it is colourless
Ammonia gas has a characteristic pungent smell
Ammonia gas will turn moist litmus paper from red to blue. It is alkaline in water and it’s pH is 11.5
Ammonia will form a white smoke in the presence of hydrogen chloride gas. The white smoke is ammonium chloride.
Hydrogen chloride is in the test tube (along with the gas), a Bunsen burner is heating it up. Once the litmus paper is over the test tube, it will turn blue and ammonium chloride is formed
Test for chlorine
Colour
Smell
Test
Chlorine gas is green/yellow in colour
Chlorine gas has a pungent choking smell
Chlorine gas turns moist litmus paper from blue to red, it is acidic
After turning red, the paper is then bleached white. Chlorine is the only gas that has a bleaching affect.
Test for positive ions
Flame tests can be used to identify metal ions. Lithium, sodium, potassium and calcium produce distinctive colours in flame tests.
To carry out a flame test:
- Use a clean wire. First the wire loop should be dipped in concentrated hydrochloric acid and heated to clean it. Then it should be dipped in the acid again before dipping in the metal compound.
- Hold the loop in the roaring blue flame of a Bunsen Burner.
- Use the colour of the Bunsen flame to identify the metal ion in the compound.
The metal ions and their flame colour
Lithium Sodium Potassium Calcium Copper
Lithium - red Sodium - yellow Potassium - lilac Calcium - orange-red Copper (I)- blue-green -
Test for ammonium
To test for ammonium, add aqueous sodium hydroxide to the sample and gently warm the mixture.
In the test tube is ammonium chloride and sodium hydroxide.
A pungent smelling gas is produced if ammonium ions are present. The gas produced turns damp red litmus paper blue. This gas is ammonia.
The litmus paper turns blue if ammonium is present
Ionic equation:
NH4^+ + OH^- -> NH3 + H2O
Reactions with sodium hydroxide
Most metal hydroxides are insoluble. Therefore adding an aqueous solution of sodium hydroxide drop wise to aqueous solutions of metal salts will result in a precipitate being formed. Some metal ions form a coloured precipitate with sodium hydroxide
The colour of precipitate for the metal ions when mixed with sodium hydroxide
Copper (II)- Blue
Iron (II) - Green
Iron (III) - Brown
Test for water
To show that a liquid is pure water, we can measure the freezing point (exactly 0 degrees Celsius) and it’s boiling point (exactly 100 degrees Celsius) under normal atmospheric pressure.
To show that a liquid contains water, we can add the liquid to anhydrous copper sulphate which turns from white to blue.
Equation: CuSO4 + 5H2O —> CuSO4.5H2O
Test for negative ions - Halides (chloride, bromide and iodide)
Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid. First we add dilute nitric acid to remove any carbonate ions which would form a precipitate with the silver ions and interfere with the test. Then silver nitrate solution is added. If a precipitate forms then there are halides present.
First we add the dilute nitric acid to the halide (e.g. sodium chloride) to remove any carbonate ions (which would form a precipitate with the silver ions and interfere with the test). Then silver nitrate is added. If a precipitate forms then there are halides present
Word equation: sodium chloride + silver nitrate —> sodium nitrate + silver chloride
Colour of precipitates for the halide ions
Chloride - white
Bromide - cream
Iodide - yellow
Ionic equations
An ionic equation is a chemical equation in which the species are written out as their ions. For example, instead of NaCl, it is written as Na^+ and Cl^-
Ionic equations are used for displacement reactions that occur in aqueous solution.
The ionic equation for the reaction of the halide ions with silver nitrate is:
Ag^+ + X^- —> AgX
Sulfates
We can test for sulphate ions by adding dilute hydrochloric acid followed by barium chloride solution. The dilute hydrochloric acid is added first to remove carbonate ions that would form a precipitate with the barium ions. Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
Word equation:
Barium chloride + sodium sulphate —> barium sulphate + sodium chloride
Ionic equation:
Ba^2+ + SO4^2- —> BaSO4
