Particles and purificartion

Question 1

Describe dependence of rate of diffusion on molecular mass (treated qualitatively)

Answer 1

The rate of diffusion depends on the relative molecular mass. Substances with high Mr diffuse more slowly

Question 2

Describe paper chromatography

Answer 2

Chromatography is a method used to separate pigments using filter pape. The colours separate if the pigments have different solubilities in solvent or pigments have different degrees of attraction for the filter paper

Question 3

Interpret simple chromatograms

Answer 3

• Draw a pencil base line
• Place a spot of of the concentrated dye mixture on the base line using pipette
• The filter paper is put in a jar with the solvent
• Make sure the solvent level is below the level of spots (otherwise the dye will wash off)
• As the solvent moves up the paper, dyes in mixture will be separated

Question 4

Interpret simple chromatograms, including

the use of Rf values

Answer 4

• Chrom. can be used to indentify colourless compounds
• Carry out the same way but this time mark a line near the top to show where the solvent has reached
• The paper will the be dried and sprayed with locating agent - to make colourless compounds visible
• The compounds on a chromatogram can be used can be identified using Rf value
• Rf value = distance from base line to the centre of spot/ distance of solvent front from base line

Question 5

Identify substances and assess their purity

Answer 5

melting and boiling

Question 6

Methods of purification

Answer 6

• Filtration
• Decanting and centrifugation
• Crystallisation
• Solvent extraction
• Simple distillation
• Fractional distillation
• Evaporation
  (pg 12 to 15)

Question 7

Why use pencil in paper chromatography?

Answer 7

Ink is a mixture of dyes => It will be separated by the chromatography solvent