criteria of purity a) describe paper chromatography and interpret chromatograms b) describe how...

Criteria of Purity a) Describe paper chromatography and interpret

chromatograms b) Describe how chromatography techniques can be

applied to colourless substances.c) Identify substances and test their purity by melting

point and boiling point determination and by paper chromatography.

d)Explain the importance of purity in substances in everyday life. Eg. in foodstuffs and drugs.

Chromatography

method of separating and identifying mixtures;

used to separate the solutes in a mixture

Eg. to identify artificial dyes in food

can be used to identify very tiny amounts of substances

Chromatography

Chromatography

Chromatography

principle involved depends upon the different solubilities of the substances in the mixture in the solvent used – the most soluble solute will be the most soluble solute will be carried quickly up the paper carried quickly up the paper with the solvent while the least with the solvent while the least soluble solute will be left soluble solute will be left behindbehind..

Steps involved

Chromatography

a dye strongly adsorbed onto paper, and not very soluble in solvent will be left behind

a dye weakly adsorbed onto paper, and very soluble in solvent will be carried furthest

simulation

Chromatography

ensure that the spots are above the spots are above the solventsolvent; and the solvent just touches the tip of the paper

solvent front

Start line drawn with

pencil

Chromatography - uses

To separate (a) pigments from plants; (b) dyes from ink; (c) amino acids from proteins for analysis.

To identify poisons and drugs, or to detect traces of unlawful dyes or other additives in foodstuffs.

To determine the purity of a given substance.

Criteria of Purity

Pure substance- substance with no other substances mixed

with them- has fixed (sharp) melting point and boiling

point

- Mixture

• A mixture does not have definite properties.

• It melts or boils over a range of temperatures

Checking the Purity

Checking its melting point Checking its boiling point

No two substances have the same pair of melting and boiling pts. Hence melting and boiling pts. can be used to identify a substance.

Using chromatography

Determination of boiling point

Checking purity - with mpt. & bpt

Impurites will lower the melting point and raise the boiling point

For an impure substance, the temperature does not stay constant during boiling/melting.

Checking purity - using chromatography

A pure substance will give only one spot in the chromatogram.

Pure substance

Mixture

Mixtures

Pure substances

Melting/Boiling pt.

Chromatography

Can be differentiate

d by

Has sharp melting/boiling point

Melts/boils over a

range of temperatur

es

Produces only one spot

Produces more than

one spot