C8 Chemical Analysis

In Chemistry, a pure substance can be ?

A single element

A single commpound

A pure substance is not mixed with any other substance

Impure substances

Melt and boil over a range of temperatures

What is a Formulation ?

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 products has the properties we need

Examples of Formulations

Fuels

cleaning products

medicines

alloys

fertilisers

Food

What does Paper Chromatography allow us to do ?

Allows us to seperate substances based on their solubilities

How paper chromatography works

1. We take a piece of chromatography paper and we draw a pencil line near the bottom

2. We then put a dot of our first colour onto the pencil line and next to that we put our second colour (we can do this for several colours if there is enough space on the paper)

3. We then place the bottom of the paper into the solvent (a solvent is a liquid that will dissolve substances)

4. The solvent moves up the paper and it dissolves the ink in the two coloured dots and these are carried at the paper aswell

What is the stationary phase in paper chromatography

The paper as it does not move

What is the mobile phase in paper chromatography

The solvent as it can move

A pure compound will ?

Produce a single spot in all solvents

The compounds in a mixture may ?

Separate into different spots depending on the solvent

Why do we draw our starting line in pencil

If we drew the line in pen, the pen ink would move up the paper, with the solvent

Chromotography RF value Calculation

distance moved by substance/distance moved by solvent

(RF values do NOT have units)

Using paper chromatography to work out colours in food colouring

1. Use a ruler to draw a horizontal pencil line on the chromatography paper. The line should be 2cm from the bottom of the paper

2. We then mark five pencil spots at equal spaces across the line and leave at least 1cm clear at each side

3. We then use a capilary tube to put a small spot of each of the known food colours and the unknown colour onto the pencil spots (its important that the spots are relatively small as it prevents the colours from spreading into each otheer

4. We then pour water into a beaker to a depth of 1cm, in this case water is the solvent.

5. We then attach the paper to a glass rod using tape and lower the paper into the beaker. The bottom of the paper should dip into water

6. At this stage, the water will move up the paper and the colours will be carried up. During this time we must be careful to not to move the beaker

7. Remove the paper when the water has travelled around three quarters up. At this point we use a pencil to mark where the water reached and then we hang the paper to dry

3 key points for chromatography

1. The pencil line with the spots of ink must be above the surface area of the water, otherwise the water will wash the ink off the line

2. The sides of the paper must not touch the side walls of the beaker, if it happens then it will interfere with the way the water moves

3. We usually put a lid on the beaker to reduce evaporation of the solvent

Hydrogen Gas Test

Use a burning splint held at the open end of a test tube of the gas which creates a squeaky pop sound, this is because hydrogen burns rapidly

Oxygen Gas Test

Uses a glowing splint inserted into a test tube of the gas to which the splint relights (bursts into flames)

Carbon Dioxide Gas Test

Bubble the gas through the lime water (an aqueous solution of calcium hydroxide) to which it will turn cloudy

Chlorine Gas Test

Use a damp litmus paper. Chlorine bleaches the litmus paper and turns it white