Why must the solvent level start below the baseline?

So the spots dissolve into the solvent as it rises past them, instead of washing straight off the paper into the solvent below.

What is the plate made from in thin layer chromatography?

It is a sheet of glass or plastic coated with a thin, even layer of silica or alumina.

Give an advantage of thin layer chromatography over paper chromatography.

It is faster and gives sharper separation, because the coating is finer and more uniform than paper fibres.

When would a non-aqueous solvent such as ethanol be chosen instead of water?

When the substances being separated do not dissolve in water, so they need an organic solvent to carry them.

OCR GCSE Combined Science Chemistry J250

Chemical Reactions

Topic overview Writing formulae of elements and compounds Balanced equations, and deducing formulae from ions Ionic equations, and state symbols The mole and the Avogadro constant Conservation of mass, and changes of mass in open systems Stoichiometry, and calculating reacting masses Exothermic and endothermic reactions, and reaction profiles Activation energy, and bond energy calculations Oxidation and reduction, by oxygen and by electrons Acids, alkalis and neutralisation The ionic equation for neutralisation, and reactions of acids Dilute and concentrated, weak and strong The pH scale, and hydrogen ion concentration pH as a factor of ten, and measuring pH Products at the electrodes, and molten ionic compounds Electrolysis of aqueous solutions Inert and non-inert electrodes

Elements, Compounds and Mixtures

Topic overview Purity, and melting point data Relative formula mass and empirical formulae Formulations, and separating mixtures Paper and thin layer chromatography Interpreting chromatograms and Rf values Choosing a purification method Metals and non-metals in the Periodic Table Electron arrangement and position in the Periodic Table Ionic, covalent and metallic bonding compared Dot and cross diagrams, and the limits of models Reactions, electron arrangement and Mendeleev's table Carbon's four bonds, and the variety of organic compounds Diamond, graphite, fullerenes and graphene Bond strength, intermolecular forces and predicting state Bulk properties of ionic, molecular and giant structures

Global Challenges

Topic overview Extracting metals by reduction and by electrolysis Biological methods of metal extraction Life cycle assessment Recycling, and the decisions behind it Fractional distillation of crude oil The fractions, and crude oil as a feedstock Dependence on hydrocarbons, and cracking The Earth's early atmosphere, and how oxygen developed The greenhouse effect, and evidence for climate change Effects of carbon dioxide and methane, and air pollutants Producing potable water

Monitoring and Controlling Chemical Reactions

Topic overview Mass of solute, concentration and dilution calculations Measuring the rate of a reaction, and rate graphs Factors affecting rate, and collision theory Surface area and rate Catalysts and how they work Reversible reactions and dynamic equilibrium Changing the conditions of an equilibrium Activation energy, and enzymes as catalysts

Particles

Topic overview The particle model and changes of state Limitations of the particle model How the atomic model changed, and the structure of the atom The size of atoms, and the subatomic particles Calculating protons, neutrons and electrons

Predicting and Identifying Reactions

Topic overview Groups 1, 7 and 0, and why they behave as they do Predicting reactivity from the Periodic Table Reactivity of metals with water and acids Tests for oxygen, hydrogen, carbon dioxide and chlorine

Elements, Compounds and Mixtures

Paper and thin layer chromatography

3 things to know
Paper 9 (Chemistry)

Overview

How a paper chromatogram is set up with a pencil baseline above the solvent, what a thin layer plate is made of and its advantage over paper, choosing a solvent, revealing colourless spots with a locating agent or ultraviolet light, the stationary and mobile phases in paper and thin layer chromatography, and why substances separate.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

When is a non-aqueous solvent chosen, what are the two phases of any chromatography, and what forms each phase in paper chromatography?

Key term

Describe how a paper chromatogram is set up.

Key term

Why is the baseline drawn in pencil rather than in ink?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe how a paper chromatogram is set up.
Model answer
A baseline is drawn near the bottom of the paper, spots of the mixture are placed on it, and the paper is stood in solvent so that the solvent level is below the baseline and rises through the paper.
2
Why is the baseline drawn in pencil rather than in ink?
Model answer
Because pencil is insoluble in the solvent, so it will not travel up the paper or add spots of its own.
3
Why must the solvent level start below the baseline?
Model answer
So the spots dissolve into the solvent as it rises past them, instead of washing straight off the paper into the solvent below.
4
What is the plate made from in thin layer chromatography?
Model answer
It is a sheet of glass or plastic coated with a thin, even layer of silica or alumina.
5
Give an advantage of thin layer chromatography over paper chromatography.
Model answer
It is faster and gives sharper separation, because the coating is finer and more uniform than paper fibres.
6
When would a non-aqueous solvent such as ethanol be chosen instead of water?
Model answer
When the substances being separated do not dissolve in water, so they need an organic solvent to carry them.

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Chemical Reactions

Elements, Compounds and Mixtures

Global Challenges

Monitoring and Controlling Chemical Reactions

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