AQA Triple Chemistry | Analysis and Synthesis

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  • Created by: Sam Nunn
  • Created on: 12-03-13 11:55

Tests For Positive Ions

-Flame tests involve placing a chemical in a bunsen burner flame, as some have characteristic colours.

-Flame Colours

Lithium   Crimson

Sodium   Yellow

Potassium   Lilac

Barium   Green

Calcium    Red

Magnesium  Colourless

-You can use sodium hydroxide solution to identify positive ions. Aluminium, calcium and magnesium all form a white precipitate. If add more solution the aluminium precipitate will dissolve buy calcium and magnesium will not.

-Sodium Hydroxide Precipitate Colours

Copper (II)  Blue

Iron (II)       Green

Iron (III)      Red/Brown

Tests For Negative Ions

-If we add a dilute acid to a carbonate it fizzes and produces carbon dioxide gas, allowing us to identify it as a carbonate. The gas will turn limewater cloudy.

-To see whether chloride, bromide or iodide ions are present in a solution you need to add dilute nitric acid and then silver nitratre solution. The nitric acid will remove any carbonate ions which would interfere with the test.

-Sliver Nitrate Precipitate Colours

Chloride  White

Bromide  Cream

Iodide      Yellow

-We can test for sulfates by adding dilute hydrochloric acid followed by barium chloride solution. The hydrochloric acid will remove any carbonate ions which would interfere with test results. A white precipitate tells us that sulfate ions are present (barium sulfate).

Titrations

-An acid and an alkali will also react together and neutralise each other. They form salt and water in the process.

-If you start off with more acid than alkali, the solution will not be fully neutralised, as there is not enough alkali in the solution. Titrations allow us to find out exactly how much of an acid or alkali is needed to completely neutralise a solution. 

-The point at which the two chemicals have completely neutralised one another is called the end point. We show this by using a chemical indicator.

-Titration Method:

.Measure a known volume of alkali into a conical flask using a pipette, washed with distilled water

.Add your indicator to the solution in the flask

.Pour the acid into the burette and place the flask on a white tile beneath teh burette

.Open the tap to allow a slow stream of acid to enter the flask, swirling the flask constantly until the two solutions have mixed.

.Record the reading of acid on the burette and calculate the volume of acid used

.Repeat at least three times for accurate and reliable results, calculating an average

Titration Calculations

-The concentration of a solute in a solution is the number…

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