Pure Substances and Formulations
A pure substance can be a single element or a single compound. A pure substance is not mixed with
any other substance.
A pure substance melts at a specific fixed temperature. It also has a specific fixed boiling point.
Impure substances melt and boil over a range of temperatures.
Draw graph. Starting with ice and gradually increasing the temperature. At a certain point, the
temperature stops rising. This is the melting point. Once the water has melted, the temperature
increases again. At a certain point, the temperature stops rising. This is the boiling point. Because
both the melting and boiling points are at specific fixed temperatures, we know that this water is
pure.
Draw graph. For a mixture of water and an impurity. The water melts and boils over a range of
temperatures. This tells that the water is not pure.
A formulation is a complex mixture that has been designed as a useful product. In a formulation, the
quantity of each component is carefully measured so that the product has the properties needed.
Formulations include fuels, cleaning products, paints, medicines, alloys, fertilisers and food.
Required Practical: Paper Chromatography
We have a sample of food colouring which is a mixture of chemicals. Going to call this U for
unknown. Also have four known food colourings that it could contain. Going to label these A-D.
Steps:
1. Use a ruler to draw a horizontal pencil line (should be around 2 cm from the bottom) on the
chromatography paper.
2. Mark five pencil spots at equal spaces across the line. Leave at least 1 cm clear at each side.
3. Use a capillary tube (very thin glass tube) to put a small spot of each of the known food
colours and the unknown colour onto the pencil spots. It is important to keep the spots
relatively small this prevents the colours from spreading into each other later.
4. Pour water into a beaker to a depth of 1cm. The water is the solvent.
5. Attach the paper to a glass rod using tape and lower the paper into the beaker. Bottom of
paper should dip into water. The pencil line with the inks must be above the surface of the
water or the water will wash the ink off the line. The sides of the paper must not touch the
side walls of the beaker. If that happens, then it will interfere with the way that the water
moves.
6. Put a lid on the beaker to reduce evaporation of the solvent.
7. The water will move up the paper and the colours will be carried up. During this time, need
to be careful not to move the beaker.
8. Remove the paper when the water has travelled around three-quarters up.
9. Use a pencil to mark the point where the water reached.
10. Hang the paper to dry.
The unknown colour has separated into three spots, telling us that this is a mixture of three colours.
The spots in A, C and D line up with the spots in the unknown colour. This tells us that the unknown
colour is a mixture of colours A, C and D. The unknown colour does not contain colour B.
To identify the chemicals in the colours, calculate the Rf values:
, 1. Measure the distance from the pencil line to the centre of each spot.
2. Measure the distance moved by the water from the pencil line.
3. Put into equation
Rf (no unit) = distance moved by chemical / distance moved by solvent
4. Now this Rf value can be looked up in a database and that will identify the chemical.
Several different chemicals could have the same value. So, we might need to repeat the experiment
using a different solvent to narrow it down further. If the chemical has never been analysed before
then there will not be an Rf value on the database. We would need to carry out further analysis to
identify it.
Testing for Gases
To test for hydrogen, remove the bung and insert a burning splint. Hydrogen gas burns rapidly and
produces a pop sound.
To test for oxygen, use a glowing splint. If oxygen is present, the glowing splint will relight (bursts
into flames).
To test for carbon dioxide, use a limewater. Limewater is an aqueous solution of calcium hydroxide.
Draw some of the gas into a plastic pipette. Bubble the gas through the limewater. If repeated
several times, the limewater may turn cloudy. If the cloudy, then the gas was carbon dioxide.
To test for chlorine, insert damp litmus paper into the mouth of the test tube. Chlorine bleaches the
litmus paper and turns it white.
Flame Tests
One way of identifying a metal ion is to use a flame test.
1. Place a small amount of our chemical onto a wire mounted in a handle.
2. Then place the end of this into a blue Bunsen burner flame. The colour of the flame can be
used to work out the metal ion present.
Lithium produces a crimson flame test.
Sodium produces a yellow flame.
Potassium ion produces a lilac flame.
Calcium produces an orange-red flame.
Copper ion produces a green flame.
The colour of a flame test can be difficult to distinguish, especially if there is only a low
concentration of the metal compound. Sometimes a sample contains a mixture of metal ions which
can mask the colour of the flame. Instead of doing flame tests, scientists often use another
technique called flame emission spectroscopy.
In flame emission spectroscopy, a sample of the metal ion in the solution is placed into a flame. The
light given out is then passed into a machine called a spectroscope. The spectroscope converts the
light into a line spectrum. The positions of the lines in the spectrum are specific for a given metal
ion. This can be used to identify the metal ion in the sample.
, Flame emission spectroscopy can also tell us the concentration of the metal ion. The lines become
more intense at a higher concentration. Flame emission spectroscopy is an example of an
instrumental method (carried out by a machine).
Advantages of instrumental methods:
• Rapid. Flame emission spectroscopy can be used to analyse samples more rapidly than using
flame tests.
• Sensitive. Flame emission spectroscopy will work even on a tiny sample of metal compound.
• Accurate. Flame emission spectroscopy is more likely to identify a metal ion correctly than
using a flame test.
Metal Hydroxide Precipitates
We can test for certain metal ion using their reaction with sodium hydroxide solution.
Solutions of calcium ions, magnesium ions and aluminium ions. If sodium hydroxide solution is
added to these ions then they all produce a white precipitate. That is a problem as we cannot
distinguish between these three tests. If excess sodium hydroxide solution is added, then the
aluminium precipitate redissolves. We would need to do flame tests to work out which one is
calcium.
Calcium nitrate + sodium hydroxide → sodium nitrate + calcium hydroxide
Ca (NO3)2 (aq) + 2NaOH (aq) → 2NaNO3 (aq) + Ca(OH)2 (s)
We can tell that calcium hydroxide is a precipitate as it has the state symbol for a solid.
Magnesium nitrate + sodium hydroxide → sodium nitrate + magnesium hydroxide
Mg(NO3)2 (aq) + 2NaOH (aq) → 3NaNO3 (aq) + Mg(OH)2 (s)
Aluminium nitrate + sodium hydroxide → sodium nitrate + aluminium hydroxide
Al(NO3)2 (aq) + 2NaOH (aq) → 3NaNO3 (aq) + Al(OH)3 (s)
Copper II ions react with sodium hydroxide to form a blue precipitate of copper (II) hydroxide.
Copper (II) nitrate + sodium hydroxide → sodium nitrate + copper hydroxide
Cu(NO3)2 + 2NaOH (aq) → 2NaNO3 (aq) + Cu(OH)2 (s)
Iron (II) ions react with sodium hydroxide to form a green precipitate of iron (II) hydroxide.
Iron (II) nitrate + sodium hydroxide → sodium nitrate + iron (II) hydroxide
Fe(NO3)2 (aq) + 2NaOH (aq) → 2NaNO3 (aq) + Fe(OH)2 (s)
Iron (III) ions react with sodium hydroxide to form a brown precipitate of iron (III) hydroxide.
Iron (III) nitrate + sodium hydroxide → sodium nitrate + iron (III) hydroxide
Fe(NO3)3 (aq) + 3NaOH → 3NaNO3 (aq) + Fe(OH)3 (s)
Identifying Non-metal Ions
Testing for the carbonate ion:
1. Add dilute acid to sample.
A pure substance can be a single element or a single compound. A pure substance is not mixed with
any other substance.
A pure substance melts at a specific fixed temperature. It also has a specific fixed boiling point.
Impure substances melt and boil over a range of temperatures.
Draw graph. Starting with ice and gradually increasing the temperature. At a certain point, the
temperature stops rising. This is the melting point. Once the water has melted, the temperature
increases again. At a certain point, the temperature stops rising. This is the boiling point. Because
both the melting and boiling points are at specific fixed temperatures, we know that this water is
pure.
Draw graph. For a mixture of water and an impurity. The water melts and boils over a range of
temperatures. This tells that the water is not pure.
A formulation is a complex mixture that has been designed as a useful product. In a formulation, the
quantity of each component is carefully measured so that the product has the properties needed.
Formulations include fuels, cleaning products, paints, medicines, alloys, fertilisers and food.
Required Practical: Paper Chromatography
We have a sample of food colouring which is a mixture of chemicals. Going to call this U for
unknown. Also have four known food colourings that it could contain. Going to label these A-D.
Steps:
1. Use a ruler to draw a horizontal pencil line (should be around 2 cm from the bottom) on the
chromatography paper.
2. Mark five pencil spots at equal spaces across the line. Leave at least 1 cm clear at each side.
3. Use a capillary tube (very thin glass tube) to put a small spot of each of the known food
colours and the unknown colour onto the pencil spots. It is important to keep the spots
relatively small this prevents the colours from spreading into each other later.
4. Pour water into a beaker to a depth of 1cm. The water is the solvent.
5. Attach the paper to a glass rod using tape and lower the paper into the beaker. Bottom of
paper should dip into water. The pencil line with the inks must be above the surface of the
water or the water will wash the ink off the line. The sides of the paper must not touch the
side walls of the beaker. If that happens, then it will interfere with the way that the water
moves.
6. Put a lid on the beaker to reduce evaporation of the solvent.
7. The water will move up the paper and the colours will be carried up. During this time, need
to be careful not to move the beaker.
8. Remove the paper when the water has travelled around three-quarters up.
9. Use a pencil to mark the point where the water reached.
10. Hang the paper to dry.
The unknown colour has separated into three spots, telling us that this is a mixture of three colours.
The spots in A, C and D line up with the spots in the unknown colour. This tells us that the unknown
colour is a mixture of colours A, C and D. The unknown colour does not contain colour B.
To identify the chemicals in the colours, calculate the Rf values:
, 1. Measure the distance from the pencil line to the centre of each spot.
2. Measure the distance moved by the water from the pencil line.
3. Put into equation
Rf (no unit) = distance moved by chemical / distance moved by solvent
4. Now this Rf value can be looked up in a database and that will identify the chemical.
Several different chemicals could have the same value. So, we might need to repeat the experiment
using a different solvent to narrow it down further. If the chemical has never been analysed before
then there will not be an Rf value on the database. We would need to carry out further analysis to
identify it.
Testing for Gases
To test for hydrogen, remove the bung and insert a burning splint. Hydrogen gas burns rapidly and
produces a pop sound.
To test for oxygen, use a glowing splint. If oxygen is present, the glowing splint will relight (bursts
into flames).
To test for carbon dioxide, use a limewater. Limewater is an aqueous solution of calcium hydroxide.
Draw some of the gas into a plastic pipette. Bubble the gas through the limewater. If repeated
several times, the limewater may turn cloudy. If the cloudy, then the gas was carbon dioxide.
To test for chlorine, insert damp litmus paper into the mouth of the test tube. Chlorine bleaches the
litmus paper and turns it white.
Flame Tests
One way of identifying a metal ion is to use a flame test.
1. Place a small amount of our chemical onto a wire mounted in a handle.
2. Then place the end of this into a blue Bunsen burner flame. The colour of the flame can be
used to work out the metal ion present.
Lithium produces a crimson flame test.
Sodium produces a yellow flame.
Potassium ion produces a lilac flame.
Calcium produces an orange-red flame.
Copper ion produces a green flame.
The colour of a flame test can be difficult to distinguish, especially if there is only a low
concentration of the metal compound. Sometimes a sample contains a mixture of metal ions which
can mask the colour of the flame. Instead of doing flame tests, scientists often use another
technique called flame emission spectroscopy.
In flame emission spectroscopy, a sample of the metal ion in the solution is placed into a flame. The
light given out is then passed into a machine called a spectroscope. The spectroscope converts the
light into a line spectrum. The positions of the lines in the spectrum are specific for a given metal
ion. This can be used to identify the metal ion in the sample.
, Flame emission spectroscopy can also tell us the concentration of the metal ion. The lines become
more intense at a higher concentration. Flame emission spectroscopy is an example of an
instrumental method (carried out by a machine).
Advantages of instrumental methods:
• Rapid. Flame emission spectroscopy can be used to analyse samples more rapidly than using
flame tests.
• Sensitive. Flame emission spectroscopy will work even on a tiny sample of metal compound.
• Accurate. Flame emission spectroscopy is more likely to identify a metal ion correctly than
using a flame test.
Metal Hydroxide Precipitates
We can test for certain metal ion using their reaction with sodium hydroxide solution.
Solutions of calcium ions, magnesium ions and aluminium ions. If sodium hydroxide solution is
added to these ions then they all produce a white precipitate. That is a problem as we cannot
distinguish between these three tests. If excess sodium hydroxide solution is added, then the
aluminium precipitate redissolves. We would need to do flame tests to work out which one is
calcium.
Calcium nitrate + sodium hydroxide → sodium nitrate + calcium hydroxide
Ca (NO3)2 (aq) + 2NaOH (aq) → 2NaNO3 (aq) + Ca(OH)2 (s)
We can tell that calcium hydroxide is a precipitate as it has the state symbol for a solid.
Magnesium nitrate + sodium hydroxide → sodium nitrate + magnesium hydroxide
Mg(NO3)2 (aq) + 2NaOH (aq) → 3NaNO3 (aq) + Mg(OH)2 (s)
Aluminium nitrate + sodium hydroxide → sodium nitrate + aluminium hydroxide
Al(NO3)2 (aq) + 2NaOH (aq) → 3NaNO3 (aq) + Al(OH)3 (s)
Copper II ions react with sodium hydroxide to form a blue precipitate of copper (II) hydroxide.
Copper (II) nitrate + sodium hydroxide → sodium nitrate + copper hydroxide
Cu(NO3)2 + 2NaOH (aq) → 2NaNO3 (aq) + Cu(OH)2 (s)
Iron (II) ions react with sodium hydroxide to form a green precipitate of iron (II) hydroxide.
Iron (II) nitrate + sodium hydroxide → sodium nitrate + iron (II) hydroxide
Fe(NO3)2 (aq) + 2NaOH (aq) → 2NaNO3 (aq) + Fe(OH)2 (s)
Iron (III) ions react with sodium hydroxide to form a brown precipitate of iron (III) hydroxide.
Iron (III) nitrate + sodium hydroxide → sodium nitrate + iron (III) hydroxide
Fe(NO3)3 (aq) + 3NaOH → 3NaNO3 (aq) + Fe(OH)3 (s)
Identifying Non-metal Ions
Testing for the carbonate ion:
1. Add dilute acid to sample.
