2.2 Methods of Separating & Purifying Substances (Edexcel GCSE Chemistry)
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What is the difference between a pure substance and a mixture?
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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.
What is a cooling curve?
A cooling curve is a graph that shows how a substance changes from a gas to a liquid to a solid as temperature decreases.
What does a horizontal line on a cooling curve indicate?
A horizontal line on a cooling curve indicates where a substance is changing from a gas to a liquid or from a liquid to a solid.
What is a pure substance?
A pure substance is a single element or compound that contains no other substances.
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.
State the use of melting point analysis.
Melting point analysis is used to assess the purity of a substance.
X represents the melting point of substances A and B.
Is substance A pure or impure?
Substance A is impure because it melts over a range of temperatures.
What does simple distillation separate?
Simple distillation is used to separate:
a liquid and soluble solid from a solution (e.g., water from a solution of salt water)
a pure liquid from a mixture of liquids
State what is meant by the term simple distillation.
Simple distillation is a technique used to separate a liquid from a solution (e.g., water from a salt solution) by boiling the solution and condensing the vapour.
What is fractional distillation?
Fractional distillation is a technique used to separate two or more liquids that have different boiling points from a mixture.
What does filtration separate?
Filtration separates an undissolved solid from a mixture of solid and liquid.
What is crystallisation used for?
Crystallisation is used to separate a dissolved solid from a solution.
In terms of the solid (solute), what is important about the solvent?
It is important that the solid (solute) is much more soluble in hot solvent than in cold.
Which property does paper chromatography depend upon?
Paper chromatography depends upon the solubility of different substances.
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 is the mobile phase in paper chromatography?
The mobile phase in chromatography is the solvent / liquid that travels up the paper.
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.
True or False?
In paper chromatography, a pure substance produces more than one spot.
False.
In paper chromatography, a pure substance only produces one spot.
Which technique separates solutions with more than one type of dissolved solid (solute)?
The technique that separates solutions with more than one type of dissolved solid is chromatography.
What does a single spot on a chromatogram indicate?
A single spot on a chromatogram indicates that the substance is pure.
State the equation to calculate the retention factor (Rf value).
The equation for the retention factor (Rf value) is:
Rf =
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?
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?
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.
What does the Rf value range between?
The Rf values ranges between 0 and 1.
What should the initial line on a chromatogram be drawn with?
The initial line 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?
If two substances are the same, they will produce identical chromatograms.
True.
If two substances are the same, they will produce identical chromatograms.
Why does an Rf value have no units?
An Rf value does not have units because it is a ratio.
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.
Which colours from A-E does black food colouring contain?
The black food colouring contains A, E and an unknown.
What is the first step in the purification water?
The first step in the purification of water is filtration to remove large particles.
What is desalination?
Desalination is the process of removing salt from seawater to make it suitable for drinking and irrigation.
True or False?
Distillation is used in water purification.
False.
Distillation is not used in water purification.
What happens during sedimentation in water treatment?
Sedimentation allows heavy particles to settle at the bottom of the water treatment tanks.
What is the purpose of chlorination?
Chlorination is the process of adding chlorine to water to disinfect it by killing bacteria and other pathogens.
True or False?
Filtration removes dissolved impurities from water.
False.
Filtration removes large particles but not dissolved impurities.
What is reverse osmosis?
Reverse osmosis is a water purification process that uses a semi-permeable membrane to remove ions, molecules, and larger particles.
Why is deionised water used in chemical analysis?
Deionised water is used as the ions could react with the substances under analysis and would give the experiment a false result.
Why is making sea water potable expensive?
The process is expensive as a lot of energy is required to heat the large volumes of water to 100 ºC.
Define the term potable water.
Potable water is water that is clean enough for human consumption.