Chemical analysis

What is a pure substance?

A single element or a single compound

Not mixed with any other substance

Key feature of pure substances

• Melts at a specific fixed boiling point

• Specific fixed boiling point

Key feature of impure substances

Melt and boil over a range of temperatures

How do we use the melting and boiling point on a graph to find if a substance is pure?

Goes up in fixed amounts

What does a impure substances graph look like?

What is a formulation?

A complex mixture that has been designed as a useful product

• Each component is carefully measured to be exactly what’s needed

What does paper chromatography allow us to do?

Separate substances based on their different solubilities

What are the stages to chromatography?

• Take a piece of chromatography paper and drew a pencil line near the bottom

• Put a dot of the first solution onto the line and a dot of the second solution

• Place the paper in a solvent (dissolves substances)

• The solvent makes its way up the paper and carries the dots up the paper

What is the stationary phase?

The stationary phase because it does not move

What is the mobile phase?

The solvent because it moves

What do the results show?

If the dot of solution remains one dot it is a pure substance

If the dot of solution becomes more then one dot it is a mixture

Why does paper chromatography work?

Different substance have different solubilities

Why is the starting line drawn in pencil?

The pen ink would move up the paper

How can paper chromatography be used to identify a unknow substance?

• Follow the usual steps

• Find where the solvent moved to and where the dot moved

• Measure the distance moved by the unknown chemical (pencil line to dot)

• Measure the distance moved by the solvent (pencil line to solvent dot)

• Calculate the Rf

• Compare this to the database

What is the equation for the Rf?

Rf = distance moved by substance / distance moved by solvent

What are the problems with finding unknow solvents?

• Several different substances could have the same Rf value

• If its never been analysed before, then there will not be an Rf value

What is the test for hydrogen gas?

• Insert burning split into gas

• Hydrogen burns rapidly and produces a pop sound

What is the test for oxygen gas?

• Insert a glowing splint into the gas

• It is oxygen if it relights

What is the test for carbon dioxide gas?

• Draw some of the gas into a plastic pipette

• Bubble the gas through limewater

• Repeat several times

• If its carbon dioxide it turns cloudy

What is limewater?

An aqueous solution of calcium hydroxide

What is the test for chlorine gas?

• Insert damp litmus paper into the gas

• Chlorine bleaches the litmus paper and turns it white