Purity, Formulations & Chromatography (AQA GCSE Chemistry)
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What is a pure substance?
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What is a pure substance?
A pure substance is a single element or compound that contains no other substances.
What is the difference between a pure substance and a mixture?
A pure substance consists of only substance (this can be an element or a compound), while a mixture contains two or more different substances that are not chemically combined.
True or False?
Salt dissolved in water is a pure substance.
False.
A solution of salt dissolved in water is a mixture.
State what is meant by the term melting point.
The melting point is the temperature at which a solid turns into a liquid.
What is a boiling point?
The boiling point is the temperature at which a liquid turns into a gas.
What property can be used to distinguish a pure substance from a mixture?
Melting point and boiling point can be used to distinguish a pure substance from a mixture.
True or False?
Pure substances have a range of melting and boiling points.
False.
Pure substances have sharp, specific melting and boiling points, while mixtures have a range of melting and boiling points.
How can melting and boiling points distinguish between pure substances and mixtures?
Pure substances have specific and sharp melting and boiling points, while mixtures have a range of melting and boiling points.
Define a formulation.
A formulation is a mixture designed as a useful product.
Give an example of a formulation.
Examples of formulations include:
Fuels
Cleaning agents
Paints
Medicines
Alloys
Fertilisers
Foods
True or False?
Many products are complex mixtures in which each chemical has a particular purpose.
True.
Many products are complex mixtures in which each chemical has a particular purpose.
How is a formulation made?
A formulation is made by mixing the components in carefully measured quantities to ensure that the product has the required properties.
True or False?
Formulations are compounds.
False.
Formulations are mixtures.
What is paper chromatography used for?
Paper chromatography is used to separate substances that have different solubilities in a given solvent, such as identifying the components of a mixture.
What does a single spot on a chromatogram indicate?
A single spot on a chromatogram indicates that the substance is pure.
What should the base line on a chromatogram be drawn with?
The baseline on a chromatogram should be drawn with a pencil.
True or False?
The solvent level should start above the pencil line on a chromatogram.
False.
The solvent level should not start above the pencil line, as this would ruin the chromatogram.
How many substances are in sample D?
There are 3 substances in D.
True or False?
Multiple spots on a chromatogram indicate a pure substance.
False.
Multiple spots on a chromatogram indicate that the substance is a mixture.
True or False?
If two substances are the same, they will produce identical chromatograms.
True.
If two substances are the same, they will produce identical chromatograms.
Which colours from A-E does black food colouring contain?
The black food colouring contains A, E and an unknown.
Why should the baseline (starting line) in chromatography be drawn in pencil?
The baseline (starting line) should be drawn in pencil because it will not contaminate the results / will not run.
State the equation to calculate the retention factor (Rf value).
The equation for the retention factor (Rf value) is:
What does an Rf value closer to 1 indicate?
An Rf value close to 1 indicates that the component is very / highly soluble in the solvent used.
True or False?
The Rf value of a compound is always the same, regardless of the solvent used.
False.
The Rf value of a compound depends on the solvent used.
If the solvent is changed, the Rf value will change.
What is the purpose of calculating Rf values in paper chromatography?
The purpose of calculating Rf values is to allow chemists to identify unknown substances.
Why does an Rf value have no units?
An Rf value does not have units because it is a ratio.
What does the Rf value range between?
The Rf values ranges between 0 and 1.
Identify the mobile and stationary phase in the chromatography experiment below.
The mobile phase is the water, and the stationary phase is the chromatography paper.
True or False?
If two substances are the same, they will produce identical chromatograms.
True.
If two substances are the same, they will produce identical chromatograms.