Identifying Inorganic Ions: RP11, Period 3 Oxides & Metal-Aqua Ions (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about Required Practical 11, testing Period 3 oxides, identifying ions and the reactions of metal-aqua ions.

All four are inorganic test-and-observe practicals. A reagent goes into the sample, the observation is recorded exactly, and that observation is what tells you what you have.

Required Practical 11 uses simple test-tube reactions to identify transition metal ions in aqueous solution. Testing Period 3 oxides reacts the elements with oxygen and then tests the pH of the oxides that form. Identifying ions covers the test-tube reactions for the positive and negative ions in the specification. The reactions of metal-aqua ions put copper, iron and aluminium ions against three different bases. What joins them is that in each one the reagent goes in, the observation is recorded exactly, and that observation is the result: a precipitate and its colour, a gas, or a pH.

We'll start with Required Practical 11 and the three tests it runs. Then the Period 3 oxides. After that the ion tests, and last the metal-aqua ions.

Required Practical 11 is carrying out simple test-tube reactions to identify transition metal ions in aqueous solution. Three unknown solutions each go through three tests. First, sodium hydroxide is added dropwise until it is in excess, and those tubes are then stood in a beaker of hot water for about ten minutes. Second, sodium carbonate solution is added to a fresh sample of each. Third, silver nitrate solution is added and the tubes are left to stand for about ten minutes.

The results are the observations from those three tests: the initial colour of each solution, the colour of each precipitate, and whether anything changes on standing. Taken together they identify the ions present.

Period 3 elements from sodium to sulfur react with oxygen. Sodium, magnesium and aluminium are all highly reactive and glow brightly when heated in oxygen. Silicon, phosphorus and sulfur form molecular compounds with covalent bonding, and white phosphorus catches fire spontaneously in air. In the lab, sodium, magnesium and sulfur are lowered into a combustion jar of oxygen on a deflagrating spoon, and aluminium is oxidised with tongs over a Bunsen burner.

The oxides that form are then added to water and the pH is tested with a pH probe, pH paper or universal indicator. Sodium and magnesium oxides are basic, sulfur oxides are acidic and aluminium oxide is amphoteric, meaning it can act as both an acid and a base. Here the observation is the pH reading, and it is what tells you the character of the oxide.

The ion tests apply the same idea across the specification. For positive ions there are Group 2 ions and the ammonium ion. Group 2 ions are tested by adding sodium hydroxide dropwise until it is in excess. Ammonium ions are tested by warming the solution with sodium hydroxide in a water bath and holding damp red litmus paper at the mouth of the tube, where it turns blue in the presence of ammonia gas.

For negative ions there are halides, hydroxide, carbonate and sulfate. Nitric acid followed by silver nitrate gives a white precipitate for chloride, cream for bromide and pale yellow for iodide. Hydroxide turns red litmus blue. Carbonate reacts with acid to give carbon dioxide, which turns limewater milky. Sulfate, after acidifying with hydrochloric acid, gives a white precipitate of barium sulfate with barium chloride. In every one of these, the colour or the gas is the identification.

This practical takes copper(II), iron(II), iron(III) and aluminium ions and puts each against three bases: sodium hydroxide, ammonia solution and sodium carbonate solution. Ten drops of the metal-aqua ion go into a clean dry test tube, ten drops of the base are added and the tube is shaken, and the observations are noted. A further ten drops are then added so that the base is in excess, and any additional change is recorded.

The notes are explicit that correct vocabulary matters here: you need to distinguish solutions from precipitates in what you write, and to say what changes again on adding the excess. Some changes take a few minutes, so it is worth going back to the tubes rather than emptying them straight away. As in the other three practicals, the written observation is the result.

In the sulfate test, hydrochloric acid goes in before the barium chloride. That is to remove any carbonate which may be present, because a carbonate would also produce a precipitate and interfere with the results.

Required Practical 11 identifies transition metal ions from three test-tube reactions: sodium hydroxide with a water bath, then sodium carbonate, then silver nitrate.

Testing Period 3 oxides reacts the elements with oxygen and tests the pH of the oxides, with sodium and magnesium oxides basic, sulfur oxides acidic and aluminium oxide amphoteric.

The ion tests identify Group 2 and ammonium ions, and halide, hydroxide, carbonate and sulfate ions, by a precipitate colour, a gas or an indicator.

The metal-aqua ion reactions put copper, iron and aluminium ions against three bases, before and after excess.

In every one of them, what you see is the result.