Chemical analysis - purity, formulaions, chromatography + required practical chromatograhy Flashcards
what can we to determine if a substance is pure
Melting point and boiling point data can be used to
distinguish pure substances from mixtures.
how to determine if a substance is pure
Pure elements and compounds melt and boil at specific (fixed) temperatures
impure substances melt and boil over a range of temperatures
you can test the purity of a sample by measuring its melting or boiling point and comparing it with the melting or boiling point of the pure substance (which you can find in a data book)
the closer your measured value is to the actual melting or boiling point, the purer your sample is
if we take a substance and heat it, we can measure the melting point and the boiling point, and these will tell us if the substance is pure
what is a pure substance
In chemistry, a pure substance is a single element or compound,
not mixed with any other substance
describe melting and boiling points of pure substances
Pure elements and compounds melt and boil at specific (fixed) temperatures
describe melting and boiling points of impure substances
impure substances melt and boil over a range of temperatures
experiment to test if a substance is pure
results of experiment: http://www.kentchemistry.com/images/links/matter/aim10.5.jpg
results of experiment using pure water
describe the results
We are starting with ice and gradually increasing the temperature
At a certain point, the temperature stops rising. This is the melting point
The melting point in this case is fixed at 0degrees - tells us this is a pure substance
Once the water has melted, the temperature increases again
And at a certain point, the temp. stops rising. this is the boiling point. - the boiling piont in this case is fixed at 100 degrees
BECAUSE BOTH THE MELTING AND THE BOILING POINTS ARE AT SPECIFIC FIXED TEMPERATURES, WE KNOW THAT THIS WATER IS PURE
experiment to test if a substance is IMPURE
results of experiment: https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcTYL2ylbNcZ5h5WmzJEMHXeJmncFwjcL-Djelv96JOt3R9NI_S0
results of experiment using WATER AND AN IMPURITY
describe the results
In this case the water melts and boils over a range of temperatures
This tells us that the water is not pure
An impure sample of the compound would produce a gradual decrease in temperature as it freezes, as shown in the graph below
An impure sample of the compound would produce a gradual increase in temperature as it melts and boils, as shown in the graph below
what is a formulation
A formulation is a complex mixture that has been designed as a useful product
describe the features of a formulation - how is as formulation different to a mixture
In a formulation, the quantity of each component is carefully measured so that the product has the properties we need
In a formulation why is the quantity of each component carefully measured
In a formulation, the quantity of each component is carefully measured so that the product has the properties we need
Examples of formulations
Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and food
How can paper chromatography can be used to identify substances in a mixture
using paper chromatography to identify an unknown substance
benefit of chromatography - can be used to identify an unknown substance.
place a dot of the unknown chemical onto the pencil line and we place the bottom of the paper into a solvent
The solvent now makes its way up the paper and it dissolves the ink (which is in the dot
The ink is now carried up the paper as well
We can see that the chemical is pure as we only have one spot
Note the position the solvent moved to
Measure the distance moved by the unknown chemical. To dot that, measure from the pencil line to the centre of the spot. Record this distance
Measure the distance moved by the solvent. record this distance (measure from pencil line to position reached by the solvent)
Use these numbers to calculate the RF value for the substance
Rf - distance moved by substance / distance moved by solvent
No units for Rf values (f is subscript)
Look this RF value up in a database and that will tell us the identity of the substance
Different compounds have different Rf
values in different solvents,
which can be used to help identify the compounds.
problem with using paper chromatography to identify an unknown substance
Several different substances could have the same Rf value.
We might need to repeat this experiment using a different solvent (to narrow it down further)
If this substance has never been analysed before, then there will not be an Rf value on the database (so further analysis will need to be carried out to identify it)
How does paper chromatography work
Paper chromatography allows us to separate substances based on their different solubilities
suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
e.g. A student has a pack of coloured pens. Wants to know which pens contain only one colour and which pens contain a mixture of colours
Take a piece of special paper called chromatography paper
Draw a pencil line near the bottom
Put a dot of our first colour onto the pencil line
Next to that we put a dot of our second colour
We can do this for several colours, as long as there is enough space on the paper
Place the bottom of the paper into a solvent (a liquid that will dissolve substances)
The solvent now makes its way up the paper and it dissolves the ink (which is in the two coloured dots).
And these are now carried up the paper as well
If a colour forms a single spot, we know that this colour is made from only one colour - it is a single pure colour
If the colour separates into two or more spots, we know that this colour was made from a mixture of two or more different colours