8.1 Purity, Formulations, and Chromatography

1. Pure substances

2. Formulations

3. Chromatography

1. Pure Substances

• In chemistry, a pure substance is a single element or compound not mixed with any other substance.

• Pure substances have specific melting and boiling temperatures. These can be used to distinguish pure substances from mixtures.

2. Formulations

• A formulation is a mixture that has been designed as a useful product.

  • Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties. Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilizers, and foods.

3. Chromatography

• Chromatography can be used to separate mixtures.

• Chromatography involves a stationary phase and a mobile phase.

• The ratio of the distance moved by a compound (center of spot from origin) can be expressed as its Rf value:

  • Rf = distance moved by substance/distance moved by solvent

  • When calculating the Rf value remember the solvent will always travel further than the substance so the Rf value can never be greater than 1.

• Different compounds have different Rf values in different solvents, which can be used to help identify the compounds. A pure compound will produce a single spot in all solvents.

8.2 Identification of Common Gases

1. Test for hydrogen

2. Test for oxygen

3. Test for carbon dioxide

4. Test for chlorine

1. Test for Hydrogen

• The test for hydrogen uses a burning splint held at the open end of a test tube of the gas.

• Hydrogen burns rapidly with a pop sound.

2. Test For Oxygen

• The test for oxygen uses a glowing splint inserted into a test tube of the gas.

• The splint relights in oxygen.

3. Test For Carbon Dioxide

• The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water).

• When carbon dioxide is shaken or bubbled through limewater the limewater turns milky (cloudy).

4. Test For Chlorine

• The test for chlorine uses litmus paper.

• When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.

8.3 Identification of Ions by Chemical and Spectroscopic Means (Chemistry only)

1. Flame tests

2. Metal hydroxides

3. Carbonates

4. Halides

5. Sulfates

6. Instrumental methods

7. Flame emission spectroscopy

1. Flame Tests

• Flame tests can be used to identify some metal ions (cations).

  • Lithium compounds result in a crimson flame.

  • Sodium compounds result in a yellow flame.

  • Potassium compounds result in a lilac flame.

  • Calcium compounds result in an orange-red flame.

  • Copper compounds result in a green flame.

• All of these colors are distinctive and can be used to identify these metal ions.

2. Metal Hydroxides

• Sodium hydroxide solution can be used to identify some metal ions (cations).

• Sodium hydroxide is added to a solution of the metal ion to be tested, and a solid precipitate (often coloured) is formed and can be used to identify the metal ion.

• Aluminum, calcium, and magnesium ions form white precipitates when sodium hydroxide solution is added, but only the aluminum hydroxide precipitate dissolves in excess sodium hydroxide solution.

• Colored precipitates:

  • Copper (II) ions form a blue precipitate.

  • Iron (II) ions form a green precipitate.

  • Iron (III) ions form a brown precipitate when sodium hydroxide solution is added.

3. Identifying Carbonates

• Carbonates react with dilute acids to form carbon dioxide gas.

• Carbon dioxide can be identified with limewater.

4. Identifying Halide Ions

• Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid.

  • Silver chloride is white.

  • Silver bromide is cream.

  • Silver iodide is yellow.

5. Identifying Sulfate Ions

• Sulfate ions in solution produce a white precipitate with barium chloride in the presence of dilute hydrochloric acid.

6. Instrumental Methods

• Elements and compounds can be detected and identified using instrumental methods.

• Instrumental methods are:

  • Accurate

  • Sensitive

  • Rapid

7. Flame Emission Spectroscopy

• Flame emission spectroscopy is used to analyze metal ions in solution.

• The sample is put into a flame and the light given out is passed through a spectroscope.

• The output is a line spectrum that can be analyzed to identify the metal ions in the solution and measure their concentrations.