Testing for Unknowns

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22 Terms

1
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Flame Test: Lithium (Li)

Red

2
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Flame Test: Sodium (Na)

Yellow Orange

3
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Flame Test: Potassium (K)

Lilac/Pink

4
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Flame Test: Rubidium (Rb)

Red/Violet

5
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Flame Test: Caesium (Cs)

Blue/Violet

6
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Flame Test: Calcium (Ca)

Orange Red

7
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Flame Test: Strontium (Sr)

Brick Red

8
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Flame Test: Barium (Ba)

Pale Green

9
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Flame Test: Copper (Cu)

Blue/Green

10
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Flame Test: Lead

White/Grey

11
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Test for Halide Ions

Silver Nitrate Test

  • Test done in solution. If starting from solid, sample must first be dissolved in pure water.

  • Add dilute nitric acid to the samples. This is to remove/react away any interfering ions (carbonate or hydroxide ions) that may give a false positive result/precipitation.

  • Add silver nitrate solution to the samples and observe the precipitate.

12
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Silver Nitrate Test: Fluoride Ions

no precipitate

13
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Silver Nitrate Test: Chloride Ions

white precipitate

14
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Silver Nitrate Test: Bromide Ions

cream precipitate

15
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Silver Nitrate Test: Iodide Ions

yellow precipitate

16
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Test for Sulphate Ions

  • Test must be done in solution. If starting from solid, it must first be dissolved in pure water.

  • Acidify the sample with dilute hydrochloric acid.

  • Add barium chloride (BaCl₂).

  • If a sulphate is present, a white precipitate will form (insoluble barium sulphate).

17
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Test for Carbonate Ions

  • Add a dilute acid to the compound (usually dilute hydrochloric acid).

  • If carbonate ions are present, they will react with the acid and produce carbon dioxide, which will be observed as bubbles/effervescence.

  • To test the gas being produced, collect a sample and test with limewater. Carbon dioxide turns limewater cloudy.

18
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Testing Alkenes

  • Shake an alkene with bromine water, or bubble a gaseous alkene through bromine water.

  • Alkenes decolourise bromine water, so the solution will become colourless (from brown/orange).

19
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Testing Aldehydes

To test for aldehydes, you can use Fehling’s solution, Tollens’ Reagent or Benedict’s solution.

Fehling’s Solution

blue solution → brick red precipitate (copper (I) oxide)

Tollens’ Reagent

no visible change → silver mirror (coating of silver on surface of test tube, or black precipitate)

Benedict’s Reagent

blue solution

→ green → yellow (low conc. aldehyde)

→ orange → red ppt. (high conc. aldehyde)

20
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Fehling’s Solution

blue solution → brick red precipitate (copper (I) oxide)

21
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Tollens’ Reagent

no visible change → silver mirror (coating of silver on surface of test tube, or black precipitate)

22
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Benedict’s Solution

blue solution

→ green → yellow (low conc. aldehyde)

→ orange → red ppt. (high conc. aldehyde)