Chemistry C3.4

C3 4.1

A few positive ions can be identified by burning them or by using sodium hydroxide solution. Some metal ions change colour when put into a flame. 

Lithium turns crimson red. Sodium turns yellow. Potassium turns lilac. Calcium turns red. Barium turns green.

However, hydroxides of most metals that have positive charges of 2 and 3 are insoluble in water and therefore form a precipitate when added to sodium hydroxide. When it does, the colour can tell us what metal ions are present.

Aluminium, calcium and magnesium ions make a white precipitate, however if enough is added aluminium hydroxide dissolves. Copper (II) hydroxide is blue. Iron (II) hydroxide is green. Iron (III) hydroxide is brown. 

C3 4.2

To test for carbonate ions you could add dilute hydrochloric acid to a substance to see if it fizzes. If it does, then check to see if the gas makes limewater milky. If it does that too, it contains carbonate ions.

To test for halide ions you add dilute nitric acid and silver nitrate solution. Chloride ions give a white precipitate. Bromide ions give a cream precipitate. Iodide ions give a yellow precipitate.

To test for sulfate ions you add dilute hydrochloric acid and then barium chloride solution. If a white precipitate is made, sulfate ions are present.

These processes are so specific because some of the different solutions and acids might react to form a precipate without the metal ions reacting. For example, we use dilute nitric acid with silver nitrate solution instead of hydrochloric acid, because it would form a precipitate.

C3 4.3

When a acid and an alkali mix together, a neutralisation reaction occurs. The amount of solutions needed can be found by doing a titration. An alkali and a indicator are placed in a pipette and measured accurately before putting it into a conical flask. A burette containing a acid is added at small amounts each time. When we reach the end colour, we say that the end point has been reached. The volume of acid used is found from the initial and final burrette readings.

C3 4.4

Titration calculations are always confusing, therefore I shall reveal 3 equations to help you through this process:

• Volume in dm cubed (cm cubed/1000) times concentration in mol/dm cubed = amount of moles
• Volume in dm cubed times concentration in g/dm cubed = mass, in grams
• Mass, in grams, divided by the molecular mass of a substance = the amount of moles.

These will definitely help you. For example, if a question reads: "50cm cubed of solution was made using 5.6g of potassium hydroxide, KOH. What is its concentration in g/dm3 and mol/dm3?" you know that you must use the second equation because it gave us the mass in grams and not…