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What is the chemical meaning of a ‘pure’ substance?



What is the every day meaning of a ‘pure’ substance?

In Chemistry: A single element or compound that is not mixed with any other substance.


In everyday language: A substance that hasn’t had anything added to it, e.g. pure milk.


What data can be used to prove a substance is ‘pure’ and not a mixture?

Melting and Boiling points (because pure substance melt and boil at specific temperatures)


What is a formulation?

A mixture that has been designed as a useful product.


Give three examples of formulations

Any three from:


Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.


Name the two phases in chromatography.

• Stationary Phase

• Mobile Phase


In Chromatography, how do you calculate the Rf value for a substance?

R= distance moved by substance / distance moved by solvent


What will the chromatogram of a pure substance look like?

A pure substance will only show a single spot on the chromatogram.


Explain how paper chromatography can be used to separate a mixture.

• A substance is added to the chromatography paper (stationary phase).

• Molecules within a substance dissolve in the solvent and move up the paper (mobile phase).

• The molecules within the substance separate because different chemicals spend different amounts of time in the mobile and stationary phases and therefore move different distances up the paper.


Describe how you would test for Hydrogen.

• Collect gas in a test tube

• Hold a burning splint at the open end of a test tube.

• You will hear a ‘pop’ sound if Hydrogen is present


Describe how you would test for Oxygen.

• Collect gas in a test tube

• Insert a glowing splint into the test tube

• The splint will relight if oxygen is present


Describe the test for Carbon dioxide.

• Bubble the gas through limewater (calcium hydroxide solution)

• If the limewater turns milky (cloudy) then carbon dioxide is present


Describe the test for Chlorine.

• Collect gas in a test tube

• Place damp litmus paper in the test tube

• If chlorine is present, the litmus paper is bleached and turns white


Separate Q. What type of ions are metal ions?



Separate Q. What would the flame test results be for the following metal compounds?

a) Lithium

b) Sodium

c) Potassium

d) Calcium

e) Copper

a) Crimson flame

b) Yellow flame

c) Lilac flame

d) Orange-red flame

e) Green flame


Separate Q. If a sample contains a mixture of metals, why might it be difficult to identify the metals using a flame test?

Some flame colours can be masked by others


Separate Q. Describe the results you would see when adding sodium hydroxide to compounds containing the following metals:

a) Aluminium

b) Calcium

c) Magnesium

d) Copper (II)

e) Iron (II)

f) Iron (III)

a) Aluminium: White precipitate

b) Calcium: White precipitate

c) Magnesium: White precipitate

d) Copper (II): Blue Precipitate

e) Iron (II): Green Precipitate

f) Iron (III): Brown Precipitate


Separate Q. Name the solution added to metal compounds to test for metal ions.

Sodium hydroxide solution


Separate Q. Aluminium, calcium and magnesium all form white precipitates when sodium hydroxide is added. If sodium hydroxide is added in excess, which precipitate will dissolve?

Aluminium hydroxide


Separate Q. Complete the word equations for the reaction of sodium hydroxide with:

a) Calcium nitrate

b) Copper sulfate

a) calcium nitrate + sodium hydroxide → calcium hydroxide + sodium nitrate


b) copper sulfate + sodium hydroxide →  copper hydroxide + sodium sulfate


Separate Q. Describe the test for a carbonate.

• Add dilute acid to the unknown compound.

• Bubble gas produced through limewater.

• If limewater turns milky (cloudy) then the compound is a carbonate.


Separate Q. Describe the test for Halide ions.

• Add silver nitrate and dilute nitric acid to your unknown solution.

• If a precipitate is produced, the unknown solution contains Halide ions.

• Silver chloride is white, silver bromide is cream, silver iodide is yellow.


Separate Q. Complete the word equations for the following reactions:

Silver nitrate + Barium chloride →

Silver nitrate + Barium bromide →

Silver nitrate + Barium iodide →

Silver nitrate + Barium chloride → Silver chloride + Barium nitrate

Silver nitrate + Barium bromide → Silver bromide + Barium nitrate

Silver nitrate + Barium iodide → Silver iodide + Barium nitrate


Separate Q. Describe the test for sulfate ions.

• Add barium chloride and dilute hydrochloric acid to your unknown solution.

• A white precipitate will form if sulfate ions are present.


Separate Q. Give three advantages of using instrumental methods to identify substances.

• Fast

• Accurate

• Sensitive


Separate Q. Name the instrumental method that can be used to identify metal ions in solutions.

Flame emission spectroscopy


Separate Q. Describe how flame emission spectroscopy works.

• Sample is put in a flame

• Light emitted by flame is passed through a spectroscope

• Line spectrum output is produced by the machine

• This line spectrum is used to identify the metal ions and measure their concentration in the solution.


Separate Q. Identify the metal ion present in the solution using the flame emission spectrometry data below.

The unknown metal ion is Ion Y because the line spectrum for the unknown metal ion matches the reference spectrum for Ion Y.