1.2 Elements, Compounds and Mixtures

REVISION NOTES

IGCSE Edexcel Chemistry

1.2.1 Understand how to classify a substance as an element, compound or mixture

1.2.2 Understand that a pure substance has a fixed melting and boiling point, but that a mixture may melt or boil over a range of temperatures

1.2.3 describe these experimental techniques for the separation of mixtures:

Summary of Separation Technique

Filtration

Crystallisation

Simple Distillation

Fractional Distillation

PAPER CHROMATOGRAPHY

This technique is used to separate substances that have different solubilities in a given solvent
It can be used to identify the impurity of a sample
Stationary phase: chromatography paper
Mobile phase: solvent

Step 1: A pencil line is drawn on chromatography paper

Step 2: Sample is spotted on the pencil line

Step 3: The paper is then lowered into the solvent container (make sure that the pencil line sits above the level of the solvent)

Step 4: The solvent travels up the paper, taking some of the coloured substances with it

Step 5: Different substances have different solubilities so will travel at different rates, causing the substances to spread apart

KEY POINTS

Pencil is used as ink would run into the chromatogram along with the samples
Make sure that the pencil line sits above the level of the solvent, so the samples don’t wash into the solvent container
Substances with higher solubility will travel further than the others

1.2.4 Understand how a chromatogram provides information about the composition of a mixture

If two or more substances are the same, they will produce identical chromatograms
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots

The sample on the left would be a mixture as it is separated into three spots (three components)
The samples on the right would be pure substance as they have only one spot (one component)

1.2.5 understand how to use the calculation of R_f values to identify the components of a mixture

RETENTION FACTOR (Rf) VALUE

Used to identify the components of mixtures
- The R_f value of a particular compound is always the same
- But it depends on the solvent used
Calculating the Rf value allows chemists to identify unknown substances
- By comparing the R_f values with the reference values of known substances under the same conditions

Calculation

1.2.6 Practical: investigate paper chromatography using inks/food colourings

AIM

Investigate how paper chromatography can be used to separate and identify a mixture of food colourings

EQUIPMENT LIST

METHOD

Result Table

RESULTS

As solvent travels up the paper, different substances have different solubilities so will travel at different rates, causing substances to spread apart
This will show the different components of the ink/colouring