4.8.1 - PURITY, FORMULATIONS AND CHROMATOGRAPHY

what is a pure substance (in chemistry)

a single element or compound, not mixed with any other substance

what is a pure substance in everyday language

a substance that has had nothing added to it, so it is unadulterated and in its natural state

what are characteristics of pure elements and compounds

• melt and boil at specific temperatures

how could you distinguish between pure and impure substances

• pure substances melt and boil at specific and sharp temperatures

• boiling points

• melting points

• GENERALLY, impure substances have lower melting points and higher boiling points than the pure substance

why do impure substances have a range of boiling and melting points

• they consist of different substances 

how could you carry out melting point analysis

• use melting point apparatus which allows you to slowly heat a small amount of the sample, making it easier to observe the exact melting point

• compared to data tables

• the closer the measured value is to the actual melting or boiling point then the purer the sample is

what are formulations

a mixture that has been designed as a useful product

what are characteristics of a formulation

• a mixture that has been designed as a useful product

• complex mixtures in which each chemical has a particular purpose.

• made by mixing the components in carefully measured quantities to ensure that the product has the required properties.

how are formulations made

• mixing the components in carefully measured quantities to ensure that the product has the required properties.

why are formulations important

• very important in the pharmaceutical industry where the exact ratios of each component must be precise

• changing the formulation of a particular medicine chemists can make sure it delivers the drug to the correct part of the body at the right concentration, that it’s safe to consume and has an adequately long shelf life

what are examples of formulationsFF

FFF CAMP

• fuels, fertilisers and food

• cleaning agents, alloys, medicines and paints

what is chromatography used for

• to separate mixtures

• can give information to help identify substances

what are the two phases of chromatography

mobile - solvent

stationary phase - the paper

what is the mobile phase

the solvent in which the sample molecules can move, which in paper chromatography is liquid e.g. water or ethano

what is the stationary phase

the chromatography paper itself

what does separation in chromatography depend on

• the distribution of substances between the phases

what is the Rf value (not the formula but actual definition)

the ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent

Rf value formula

distance moved by substance / distance moved by solvent

what happens with different compounds in different solvents

they have different Rf values (which can help identify the compounds)

what will happen to compounds within a mixture during chromatography

• the compounds in a mixture may separate into different spots depending on the solvent

what will happen to a pure substance during paper chromatography

• a pure compound will produce a single spot in all solvents

how does paper chromatography separate mixtures

• the components have different solubilities and different adhesion

• the solvent travels up the paper by capillary action, taking some of the coloured substances with it

• different substances have different solubilities so will travel at different rates, causing the substances to spread apart

• those substances with higher solubility will travel further than the others

• this separates different pigments in a coloured substance

describe what happens during chromatography in terms of the phases

• the substances constantly move between the mobile and stationary phases

• the mobile phase moves through the stationary phase and anyhing dissolved in the mobile phase moves with it

• chemicals that spend more time in the mobile phase than the stationary phase will move further through the stationary phase

• components in a mixture will normally separate through the stationary phase

what does the amount of time molecules spend in each chromatography phase depend on

• how soluble they are in the solvent

• how attracted they are to the paper

what is the objective of the chromatography practical

investigate how paper chromatography can be used to separate and tell the difference between coloured substances

what equipment is needed for the chomratography practical

• A 250 cm³ beaker

• A wooden spill

• A rectangle of chromatography paper

• Four known food colourings labelled A–D

• An unknown mixture of food colourings labelled U

• Five glass capillary tubes

• Paper clip

• Ruler & pencil

what is the method for the chromatography practical

1. Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper

2. Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line

3. Use the fifth tube to put a small spot of the unknown mixture U on the line

4. Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil

5. Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots

6. Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper should dip into the solvent

7. Allow the solvent to travel undisturbed at least three quarters of the way up the paper

8. Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line

9. Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent

10. For each food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot

11. Record your results in a table and repeat the experiment 3 times

what are hazards, risks and precautions

hazard - harmful solvent

risks - skin irritation and breathing difficulties

precaution - avoid skin contact (gloves) and use a fume cupboard

what is a chromatogram

the results of separating mixtures by chromatography.