Chemical analysis

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

Analysing pure substances and mixtures 

• The melting point and boiling point of a substance can be used to determine if it is pure. Pure substances have a spacific fixed point at which they melt or boil.
• These fixed points are recorded in data books or data bases.
• The melting point of a substance can be found using simple apparatus:

The melting and boiling points of mixtures will vary depending on the composition of the mixture 

• A mixture does not have a sharp melting or boiling point, instead it changes over a range of temperatures.
• The difference in melting and boiling points can be used to distinguish between a pure substance and a mixture.
• Impurities tend to lower the melting point of a substance and raise its boiling point.
• The size of the difference compared to the pure substance depends on the amount of impurities mixed with it.

A formulation is a mixture that has been designed to produce a useful product. 

Some examples of this include:

• Medicines 
• Paints
• Washing-up liquids 
• Cosmetics
• Feuls 
• Alloys 

The composition of the formulation will be designed to give the product the right properties for its purpose

Chrmatography is a technique used to separate and identify a mixture of substances, such as food, dyes and inks.

The technique involves a mobile phase and stationary phase.

The mobile phase moves through the stationary phase carrying the componants in the mixture with it.

Each componant in the mixture has a different attraction for the mobile phase and the stationary phase.

A strong attraction between the componant and the mobile phase compared to the stationary phase will allow it to travel further in a given time.

A strong attraction for the stationary phase will mean that it does not travel as far in the same time. 

                                             Rf = Distance moved by substance                                                                                                  Distance moved by solvent 

Hydrogen

A lighted wooden splint makes a popping sound in a test tube of hydrogen.

Oxygen

A glowing wooden splint relights in a test tube of oxygen.

Carbon dioxide

A lighted wooden splint goes out in a test tube of carbon dioxide but this happens with other gases, too. It is better to bubble the test gas through limewater - calcium hydroxide solution. Carbon dioxide turns limewater cloudy white.

Chlorine

Chlorine has a characteristic sharp, choking smell. It also makes damp blue litmus paper turn red, and then bleaches it white. Chlorine makes damp starch-iodide paper turn blue-black.

Sodium Hydroxide solutions can be used to identify some metal ions (cations)

Solutions of aluminium, calcium and magnesium ions form white precipitates when sodium hydroxide solution is added but only the aluminium hydroxide percipitate dissolves in excess sodium hydroxide solution. 

Solutions of Copper (II), iron (II) and iron (III) form coloured preciptates when sodium hydroxide solution is added 

Copper (II) forms a blue precipitate, Iron (II) a green precipitate and Iron (III) a brown precipitate.