Chemical Analysis
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In chemistry what is a pure substance
A substance made up of single element or compound, not mixed with any other substances
In everyday language, what is a pure substance
A substance that has had nothing added to it, in its natural state
What are the boiling and melting points like for pure substances
At a fixed (specific) temperature e.g. pure water would boil at 100°C
What are the boiling and melting points like for an impure (mixture) substances
Over a range e.g. salty water would boil from 100 - 105°C
What is a formulation
A mixture that has been designed as a useful product
How are formulations made
By mixing components in carefully measured quantities to get the required properties
Examples of formulations
• Fuels • Cleaning agents • Paints • Medicines • Alloys • Fertilisers • Foods
A method used to separate dyes
Chromatography
What two phases are involved in chromatography
Stationary and mobile phase
What is the stationary phase?
The chromatography paper
What is the mobile phase?
The solvent (usually water but can be any liquid e.g. ethanol)
What must you use when drawing the line on the chromatography paper? Why?
Pencil This is insoluble in the solvent so will not run
Where must the water be in relation to the pencil line? Why?
Water line must be below the pencil line To stop the inks mixing with the solvent
What does the distance the dots move tell you?
How soluble the ink is in the solvent – if it moves to the top it is very soluble
If the dot doesn’t move off the pencil line what does that tell you?
The ink is insoluble in the solvent
Explain how the inks are separated using paper chromatography.
Inks have different solubility’s This means they move up the paper at different speeds/ distances
What is the Rf value
The ratio of the distance moved by a compound to the distance moved by the solvent
How can you express Rf Rf =
distance dot travelled ÷ distance solvent travelled
What value is Rf always below
1
How do we use the Rf values?
They are used to identify substances. Each substance has a specific Rf value so we use these to confirm what is in a mixture.
How many spots would you get for a pure substance
One
How many spots would you get in a mixture
More than one
What is the test for hydrogen
Burning splint Makes squeaky pop noise
What is the test for oxygen
Glowing splint Splint relights