Chemical Analysis

Question 1

In chemistry, what is a pure substance?

Answer 1

It is a single element/single compound that contains no other substances. Or, a pure substance that is not mixed with any other substance.

Question 2

In everyday language, what is a pure substance?

Answer 2

Something that is clean or natural and which nothing else has been added to it.

Question 3

How can pure substances be distinguished from mixtures?

Answer 3

Pure substances melts at a fixed temperature and at a specific boiling point.

Impure substances melt over a range of temperatures.

Question 4

What is a formulation?

Answer 4

A complex mixture that has been designed as a useful product. The quantity is carefully measured to get desired properties.

Question 5

What are some examples of formulations?

Answer 5

Examples include, fuels, cleaning products, paints, medicine, alloys, fertilisers and foods. Very important in the pharmaceutical industry.

Question 6

‘Describe how chromatography can be used to separate a mixture of food colourings’ (6)

Answer 6

1. Use a ruler to draw a horizontal line 2cm from the bottom.
2. Mark 5 pencil dots on the line equal distances away from each other, at least 1cm away.
3. Use a capillary tube to put a small spot of the known food colours and the unknown colours onto the pencil spots.
4. Pour water in a beaker to depth of 1cm, water is the solvent.
5. Attach the paper to a glass rod using tape and lower the paper into the beaker.
6. Water will will move up the paper, with the ink - do not touch.
7. Remove paper when the water is 3/4 way up and draw a line where the water ended.

Question 7

How do we measure an Rf value? And how do we find the chemical using the Rf Value?

Answer 7

Rf value = distance moved by chemical/distance moved by solvent.

We can look up the rf value in the database to find identity.

Question 8

‘Describe how paper chromatography is used to separate mixtures’ (5)

Answer 8

1. Take a piece of chromatography paper, draw a pencil line near the bottom.
2. Place a dot on the first colour, place dot on the second colour.
3. Place bottom of paper in the solvent, the solvent will make it ways up the paper, dissolving the ink in the dots.
4. The ink is carried up the paper.
5. If we see one more dot on the line, it must be a mixture.

Question 9

What are the two phases in chromatography? Why are they like this?

Answer 9

1. Paper is the stationary phase because it does not move.
2. Solvent is the mobile phase because the solvent moves.

Question 10

Why does paper chromatography work? And which attracted chemicals will move less up the paper?

Answer 10

It works because each chemical in the mixture will be attracted to the stationary phase to a different extent, the stronger the attraction, the less it will move up the paper.

Question 11

Why do we use a pencil in chromatography to draw the dots/line and not ink? (1)

Answer 11

We use pencil as pen ink would move up the paper.

Question 12

‘Describe the test for Hydrogen, what is the result?’ (2)

Answer 12

Remove bung and insert a burning splint, if hydrogen is present it will produce a pop sound.

Question 13

‘Describe the test for Oxygen, what is the result?’ (2)

Answer 13

Glowing splint and place into the tube, if oxygen is present the splint will relight.

Question 14

‘Describe the test for carbon dioxide, what is the result?’ (2)

Answer 14

Take our unknown gas (we assume hydrogen) and place/bubble gas through limewater, if present it will turn cloudy.

Question 15

What is limewater?

Answer 15

It is an aqueous solution of calcium hydroxide.

Question 16

‘Describe the test for chlorine. What is the result?’ (2)

Answer 16

Damp litmus paper into test tube - chlorine bleaches the litmus paper and it will turn white.

Question 17

What colour flame is a lithium ion?

Answer 17

Crimson

Question 18

What colour flame is a sodium ion?

Answer 18

Yellow

Question 19

What colour flame is a pottasium ion?

Answer 19

Lilac

Question 20

What colour flame is a calcium ion?

Answer 20

Orange-red flame

Question 21

What colour flame is a copper ion?

Answer 21

Green flame

Question 22

What are the issues with the flame tests?

Answer 22

Colour can be difficult to distinguish if their is a low concentration of the metal compound.
If there is a mixture of metal ions it can mask the flame.

Question 23

‘Describe how flame emission spectroscopy can be used to analyse metal ions in a solution’ (4)

Answer 23

1. Sample metal ion in solution is placed in flame.
2. Light given out is passed onto a spectroscope
3. Convers light into the line spectrum.
4. Position in the line spectrum are specific for a given metal ion we can use this to identify it and tell us the concentration of the ion.

Question 24

What is flame an emission spectroscopy an example of? What are the advantages of these methods?

Answer 24

It is an example of an instrumental method.
They are rapid, quicker than flame tests.
Sensitive, so will work on a small sample on metal counds.
Accurate, more likely to identify a metal ion correctly then using a flame test.

Question 25

How can we test for magnesium, calcium and aluminium ions? What colour does it form?

Answer 25

Add them to sodium hydroxide solution, and it forms a white precipitate.

Question 26

How can aluminium hydroxide be distinguished from calcium hydroxide and magnesium oxide? (How can the calcium ion be distinguished?)

Answer 26

Add excess sodium hydroxide solution and the precipitate will redissolve. We will need to do a flame test for the calcium.

Question 27

What is the written equation for the metal hydroxide reactions? (Provide state symbols)

Answer 27

Metal Nitrate (aq) + Sodium Hydroxide (aq) to form Sodium Nitrate (aq) + Metal Hydroxide (s)

Question 28

What are the colours of the copper(II) ions, Iron (II) ions and iron (III) ions precipitate?

Answer 28

Copper (II) - Blue Iron (II) - Green Iron (III) - Brown

Question 29

'Describe the test for carbonate ions'(3) (Hint; CO2)

Answer 29

1. Add dilute acid to our sample. 2. If more carbonate present, acid will react to make carbon dioxide. We will see fizzing but this does not prove it. 3. Bubble the gas through limewater, this proves we have carbon dioxide, meaning we started with a carbon ion.

Question 30

'Describe the test of the halide ions' (3)

Answer 30

1) Add dilute nitric acid to sample. 2) Add dilute silver nitrate solution. 3) Halide ions produce a precipitate. Each halide makes a different coloured precipitate.

Question 31

What colours do the chloride,bromide,iodide ions form?

Answer 31

1. Chloride - White precipitate of silver chloride. 2. Bromide - Cream precipitate of silver bromide. 3. Iodide Ions - yellow precipitate of silver iodide.

Question 32

'Describe the test for the Sulfate Ions' (3)

Answer 32

1. Add dilute hydrochloric acid to our sample. 2. Add barium chloride solution. 3. If present we will see a white precipitate.