Chemistry (C3) - Identifying and Analysing Substances

This set is for C3 Chemistry, GCSE, AQA "Identifying and Analysing Substances". For this I have used the book by Nigel English as a guide. In this topic there are for sub-topics which are as follows:

Testing for Metal Ions          Testing for Negatively Charged Ions          Titrations          Using Results from Analyses

Testing for Metal Ions

Different metal ions burn different colours when the solution is heated in a Bunsen Burner flame. The results would be:

CRIMSON - Lithium          ORANGE - Sodium          LILAC - Potassium          RED - Calcium          GREEN - Barium

Solutions can be burned by being put on a nichrome wire which must be cleaned between each flame test. It is cleaned by being dipped into hydrochloric acid then held above the flame, this is repeated until the flame's colour is no longer affected by the wire.

Transition metals form coloured compoundsThe colours of hydroxide precipitate when sodium hydroxide is added depend on the metals. 

BLUE - Copper (II)          GREEN - Iron (II)          BROWN - Iron (III)

Aluminium, calcium and magnesium react with sodium hydroxide to produce a white precipitate. However when excess sodium hydroxide is added, the aluminium solution turns colourless.

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Testing for Negatively Charged Ions

All CARBONATE IONS are insoluble in water apart from sodium carbonate which breifly bubblesCarbonate ions react with dilute acids to form carbon dioxide.

This production of carbon dioxide is confirmed by passing the gas through limewater which should produce a white precipitate.

SULPHATE IONS are tested for by adding barium chloride solution to the sulphate solution, this produces a white precipitate of burium sulphate which is insoluble in water.

HALIDE IONS (group 7 ions) are tested for by adding silver nitrate to the halide ion solution which forms a precipitate. Apart from flouride ions, this precipitate is insoluble in water. The colour of the precipitate otherwise depends on the halide ions:

WHITE - Chloride          CREAM - Bromide          YELLOW - Iodide

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Titrations - The Experiment

Titraion  can find out the amount of acid and alkali needed to neutralise a salt and to find out the concerntration of an acid or alkali if the concerntration of the other reactant is known.

The experiment must be accurate so a pipette and burette (with a resolution of 0.1cm³). The acid is usually put in the burette and the acid in a conical flask. 

The standard 25cm³ of acid is measured out more accurately using a 25cm³ pipette. A suitable indicator (usually PHENOLPHTHALEIN) is added to the acid. The indicator is pink when alkaline but colourless when acidic. 

Alkali is added from the burette to the acid until the indicator shows a midpoint between the two. The amount of alkali needed is calculated using thhe difference of start and final amount in the burette. To obtain more precise results, the experiment is repeated.

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Titrations - Calculations

To find the concentration of the known reactant the results can be put into this equation:

Concentration (g/dm³) = mass (g) / volume (dm³)

cm³ can be converted into dm³ by dividing the number in cm³ by 1000.

However, scientists more often use moles/dm³. This can be calculated by the equation:

Conentration (moles/dm³) = Amount (moles) / volume (dm³)

To find the concerntration of the unknown acid or alkali, there are four steps to follow.

1. Write down the balanced equation.

2. Calculate the amount of acid / alkali in moles by dividing concentration by volume.

3. Use steps 1 + 2 to see how many moles of acid or alkali to make the product neutral.

4. Calculate the concentration of the acid or alkali by dividing the answer to 3 by the volume of the acid.

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