Identification of species (chemistry only)

PAG 5

Core Practicals
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Apparatus and techniques that the practical must use or cover

Use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products including gas tests, flame tests, precipitation reactions, and the determination of concentrations of strong acids and strong alkalis.

Safe use of appropriate heating devices and techniques including use of a Bunsen burner and a water bath or electric heater.

Risk Asessment

As a general rule, eye protection (goggles) must be worn for all practicals.

hazard possible harm precaution
barium chloride solid
 harmful if inhaled, toxic if swallowed
 use dilute solutions, get your teacher to use it
silver nitrate solution
 serious eye irritation, skin irritation
 avoid skin contact by using dropper bottles/wearing gloves

This risk assessment is provided as an example only, and you must perform your own risk assessment before doing this experiment.

Apparatus

Each group will need:

Bunsen burner
heat‐resistant mat
test tubes
test tube rack
test tube holder
flame test loops
spatula
dropping pipettes

barium chloride solution
hydrochloric acid
silver nitrate solution
nitric acid
sodium hydroxide solution
distilled water
samples of salts to test (labelled A-D)

Method

Flame tests for metal cations

  1. light a bunsen burner and open the air hole to give a hot blue flame
  2. pick up a small sample of solid salt using a clean wire loop
  3. hold the sample in the edge of the flame
  4. observe and record the flame colour
  5. wash the wire loop with dilute hydrochloric acid before continuing any other flame tests

Hydroxide precipitate tests for metal cations

  1. dissolve a little solid salt in a test tube using distilled water
  2. add a few drops of dilute NaOH solution dropwise
  3. record the colour of any precipitate formed
  4. if a white precipitate forms add excess dilute NaOH and see it disappears to leave a clear solution

Testing for ammonium ions

  1. dissolve a little solid salt in a test tube using distilled water
  2. add dilute NaOH solution and heat gently
  3. remove the flame, and then hold a piece of damp red litmus paper near the mouth of the test tube
  4. record any change in colour

Testing for carbonate ions

  1. put little solid salt in a test tube and add a few drops of dilute acid
  2. record whether any bubbling (effervescence) occurs
  3. use limewater to check that any bubbles contain carbon dioxide

Testing for sulfate ions

  1. dissolve a little solid salt in a test tube using distilled water
  2. add a few drops of dilute hydrochloric acid and then a few drops of barium chloride solution
  3. record if a white precipitate forms

Testing for halide ions

  1. dissolve a little solid salt in a test tube using distilled water
  2. add a few drops of dilute nitric acid and then a few drops of silver nitrate  solution
  3. record the colour of any precipitate formed

click here if you need to check back on your tests for salts

Results and Analysis

salt solution test observation ion present
A flame test yellow flame
A hydrochloric acid and barium chloride solution added
 white precipitate
B sodium hydroxide added
 blue precipitate
B  nitric acid and silver nitrate solution added
 white precipitate

Use the results to identify the ions present in each salt, and then to name the salts.

A is sodium sulfate.
B is copper(II) chloride.

Exam Question and Model Answer

Describe a series of experiments to distinguish between the following compounds: sodium chloride, sodium carbonate, potassium chloride, potassium iodide, potassium sulfate.

[6 marks]

Level 1 (1-2 marks)

First, do a flame test on all the chemicals. Depending on which elements are present will mean they turn different colours.
Then test them by adding dilute nitric acid (then silver nitrate).
Then test by adding barium chloride - will make a precipitate if sulfate is present.

Level 2 (3-4 marks)

Flame Tests
sodium compounds will give a yellow colour
potassium compounds give a lilac colour

Dilute nitric acid, then silver nitrate
carbonates will fizz (the gas produced turns limewater cloudy)
chlorides will produce a white precipitate
iodides will produce a yellow precipitate
sulfates does not produce a precipitate (adding barium chloride produces a white precipitate)

Level 3 (5-6 marks)

First, do a flame test on all the chemicals. Depending on which elements are present will mean they turn different colours:
sodium chloride, and sodium carbonate will give a yellow colour
potassium chloride, potassium iodide, potassium sulfate will give a lilac colour

Then test them by adding dilute nitric acid (then silver nitrate). Carbonates will fizz, chlorides turn white, iodides turn yellow.
sodium carbonate will fizz (the gas produced turns limewater cloudy)
sodium chloride, potassium chloride will produce a white precipitate
potassium iodide will produce a yellow precipitate
potassium sulfate does not produce a precipitate (adding barium chloride produces a white precipitate)