Specification Point 1.11:
Understand how a chromatogram provides information about the composition of a mixture
Composition of a Mixture
The composition of an ink using paper chromatography
Composition of a mixture:
Chromatography paper is taken. A sample (unknown) is placed along the pencil line. Other known elements are placed beside the sample along the same line.
- Chromatograms will show the composition of a mixture as the different coloured substances (components) will spread apart as they will have different solubilities so will travel at different rates
- A pure substance will only produce one spot on the chromatogram during paper chromatography
- In the diagram shown, red, blue and yellow are three pure substances, whilst the sample on the left is a mixture of all three
Specification Point 1.12:
Understand how to use the calculation of Rf values to identify the components of a mixture
Rf Values to Identify Mixture Components
Using Rf values to identify components of a mixture
Retardation factor (Rf) values:
Used to identify the components of mixtures
- The Rf value of a particular compound is always the same
- Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
Rf value = distance moved by compound ÷ distance moved by solvent
Specification Point 1.13:
Practical: investigate paper chromatography using inks/food colourings
The paper chromatography of ink and plant dye
To separate substances that have different solubilities in a given solvent (e.g different coloured inks that have been mixed to make black ink).
- Pencil line is drawn on chromatography paper and spots of ink / dye is placed on it.
- Paper is lowered into a bucket of solvent, allowing the solvent to travel up the paper, taking some of the coloured substances with it.
- Allow chromatography to occur until the solvent reaches the top of the paper.
- As the solvent travels up the paper, different substances will have different solubilities so will travel at different rates, causing the substances to spread apart.
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Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.
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