C3.4 Testing for Negative Ions

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  • Created by: Fiona S
  • Created on: 10-04-15 12:36

Testing for Carbonate

1. Carefully add dilute HCl to the substance

2. Carbonates fizz because they give off carbon dioxide

3. Carry out the lime water test

  • Suck up carbon dioxide using a teat pipette
  • Bubble it through a small amount of limewater(IRRITANT) in a small test tube
  • Limewater goes milky/cloudy if carbon dioxide is present

Take Care

  • Don't get any acid in the pipette; it neutralises the lime water
  • Don't get any carbonate in the pipette. If it is white powder it will make the limewater look milky
  • Use as little limewater as you can: it makes the test more sensitive
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Testing for Sulphate

1. Your sulphate needs to be dissolved in distilled water.

2. Acidify the solution with dilute HCl

3. Put about 2cm depth of the acidified sulphate solution in a test tube

4. Add barium chloride solution(TOXIC) drop by drop

5. Look for a dense white precipitate (barium sulphate)

In the sulphate test, you acidify with HCl. You can't use H2SO4 because it contains sulphate ions so will always give a positive result.

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Testing for halide (chloride, bromide, iodide)

1. Your halide needs to be dissolved in distilled water.

2. Put about 2cm depth of halide solution in a test tube.

3. Acidify the solution with dilute HNO3 - take care.

4. Add silver nitrate solution, drop by drop.

5. If a precipitate forms look at its colour to identify your halide.

White Precipitate = Chloride

Cream Precipitate = Bromide

Yellow Precipitate = Iodide

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