Testing for Positive Ions 1
The flame test; some metal ions produce colours when put into a flame:
lithium will burn a bright red flame
sodium will burn a golden yellow flame
potassium will burn a lilac flame
calcium will burn a brick red flame
barium will burn a green flame
Most hydroxides of metals that have ions with +2 or +3 charges are insoluble in water. When sodium hydroxide is added to solutions of these ions a precipitate forms. We can show these reactions in an ionic equation:
AL3+(aq) + 3OH-(aq) --> AL(OH)3(S)
When sodium hydroxide is added to a substance with: Aluminium, calcium and magnesium ions form white precipitates. Aluminium hydroxide dissolves when excess sodium hydroxide is added
Testing for Positive Ions 2
Copper(ii) hydroxide. A blue precipitate is formed
Iron(ii) hydroxide. A dirty green precipitate is formed
Iron(iii) hydroxide. A reddish brown precipitate is formed
We can test for ammonium ions by adding sodium hydroxide to a solution to form ammonia. If the solution is warmed, ammonia gas is given off and turns damp litmus paper blue
test for Negative Ions
There are 4 tests for negative ions:
Carbonate ions: Add dilute hydrochloric acid to the substance to see if it fizzes, if this gas turns lime water milky, then the solution contains carbonate ions.
Halide Ions: Add dilute nitric acid and then silver nitrate solution. Chloride ions give a white precipitate, bromide ions a cream percipient and iodide ions a yellow precipitate.
Sulfate Ions: Add dilute hydrochloric acid and then barium chloride solution. If a white precipitate forms, sulfate ions are present.
Nitrate Ions: Add a little aluminum powder and then add sodium hydroxide solution. Gently warm and test the gas given off with damp red litmus paper. If it turns blue, ammonia was produced and nitrate ions a present.
Carbonates also produce CO2 When they decompose on heating but this can be more difficult to detect. Copper carbonate is green and when heated it decoposes to black copper oxide.Zinc carbonate is white and decomposes to form zinc oxide, which is yellow when hot but turns white when cool.
Testing for Organic Substances
All organic substances burn or charr when heated, we can tell if an element or compound is organic if it has carbon in it. We can find the empirical formula of an organic compounds by measuring the amount to water and carbon dioxide given off when we burn them.
Organic compounds that contain a carbon-carbon double bond are called 'unsaturated' we can find if they are by reacting them with bromide and iodine. Bromine water becomes colourless in these reactions to show if the compound is unsaturated or saturated. Iodine solution is used in titration's to find the number of double bonds in organic molecules such as fats and oils.
We can work out the empirical formula of an organic compound by burning it and measuring the amounts of the formed products. For example, an organic substance Z contains hydrogen and carbon. A sample of Z is burnt in an excess of oxygen, producing 1.80g of water and 3.52g of carbon dioxide. We can work out the empirical formula for Z.
(Go to next slide for the solution)
Combustion Analysis Solution
- Firstly, calculate the moles of carbon dioxide:
The relative atomic mass of carbon dioxide is 12 + (2 x 16) = 44g
Amount of carbon dioxide = 3.52 ÷ 44 = 0.08 moles
- Then calculate the moles of water:
The relative atmoic mass of water is (2 x 1) + 16 = 18g
Amount of water = 1.80 ÷ 18 = 0.1 moles
- Each molecule of carbon dioxide formed requires one carbon atom from a molecule of Z. So for every mole of carbon dioxide formed, Z must contain one mole of carbon atoms
Amount of carbon atoms in sample of Z = 0.08 moles
- Similarly, every molecule of water formed requires two hydrogen atoms from Z. So for every mole of water formed, Z must contain two moles of hydrogen atoms
Amount of hydrogen atoms in sample of Z = 0.1 x 2 = 0.2 moles
- So Z contains carbon atoms and hydrogen atoms in the ratio:
0.08 : 0.20 = 2:5
- The empirical formula of Z is then C2H5
Instrumental methods are used to quickly and accurately detect and identify elements and compounds. They can be detected by mass absorption spectroscopy and atomic absorption spectroscopy.
Mass absorption spectroscopy: This can detect very small amounts of about 40 elements, mainly metals in samples smaller than 0.03cm3
Atomic absorption spectroscopy: These can be used to identify and measure the elements in a substance and also find the ratios of isotopes of an element that can be used to calculate relative atomic masses.