Chemical Analysis

What is a pure substance?

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

What is an element?

A substance containing only one type of atom.

What is a compound?

A substance containing two or more different elements chemically bonded together.

How can melting point data be used to identify a pure substance?

A pure substance melts at a specific, fixed temperature, whereas an impure substance usually melts over a range of temperatures.

How can boiling point data be used to identify a pure substance?

A pure substance boils at a specific, fixed temperature, whereas an impure substance usually boils over a range of temperatures.

What does it suggest if a substance melts over a range of temperatures?

The substance is likely to be impure.

What does it suggest if a substance boils over a range of temperatures?

The substance is likely to be impure.

Why can melting point and boiling point data distinguish pure substances from mixtures?

Pure substances have specific melting and boiling points, whereas mixtures contain different substances and therefore usually melt or boil over a range of temperatures.

What is a formulation?

A mixture that has been designed as a useful product.

How are formulations made?

Components are mixed in carefully measured quantities so that the product has the required properties.

Why are the components of a formulation carefully measured?

To ensure the final product has the required properties.

Give examples of formulations.

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

Why is an alloy a formulation?

It is a mixture of metals designed to have particular properties.

Why can an alloy be more useful than a pure metal?

Alloys can have improved properties, such as being harder than the pure metals.

What is chromatography used for?

To separate mixtures and provide information that can help identify substances.

What are the two phases in chromatography?

The stationary phase and the mobile phase.

What is the stationary phase?

The phase that does not move during chromatography.

What is the mobile phase?

The phase that moves through or across the stationary phase.

In paper chromatography, what is the stationary phase?

The chromatography paper.

In paper chromatography, what is the mobile phase?

The solvent.

What causes substances to separate during chromatography?

Different substances have different affinities for the stationary and mobile phases.

What happens to a substance with a stronger attraction to the stationary phase?

It moves more slowly with the mobile phase.

What happens to a substance with a stronger attraction to the mobile phase?

It travels further with the mobile phase.

How is a paper chromatography experiment set up?

A start line is drawn near the bottom of the paper and the mixture is placed on the line.

Why should the start line be drawn near the bottom of the chromatography paper?

So the substances can travel up the paper with the solvent.

Why must the solvent level be below the start line?

If the solvent covers the spots, the substances can dissolve directly into the solvent rather than travelling up the paper correctly.

What is placed on the chromatography paper?

A small spot of the mixture being investigated.

Why should only a small amount of the mixture be used?

To produce distinct spots that can be separated and identified.

What happens when the chromatography paper is placed into the solvent?

The solvent travels up the paper and carries the substances with it.

Why do different substances travel different distances during paper chromatography?

They have different affinities for the solvent and the paper.

What should be done before the solvent reaches the end of the chromatography paper?

The paper should be removed from the solvent.

Why is a finish line marked after removing the paper?

To record the position reached by the solvent.

Why is the chromatography paper dried after the experiment?

To remove the solvent and allow the separated spots to be observed clearly.

What is an origin/start line in chromatography?

The line where the mixture is initially placed.

Why should pencil be used to draw the start line?

Pencil graphite does not dissolve and travel with the solvent, unlike some inks.

What would happen if ink was used for the start line?

The ink could dissolve and separate, interfering with the chromatogram.

What is a chromatogram?

The pattern of separated spots produced by chromatography.

What does a single spot suggest about a substance?

It may be a pure substance.

What does multiple spots suggest about a substance?

It contains multiple substances and is therefore likely to be a mixture.

How can chromatography help identify a substance?

Its position or Rf value can be compared with known substances analysed under the same conditions.

Why must the same solvent be used when comparing Rf values?

Different solvents can produce different Rf values for the same compound.

Why must the chromatography conditions be kept the same when comparing Rf values?

Rf values depend on the conditions, including the solvent and stationary phase.

What is an Rf value?

The ratio of the distance travelled by a compound to the distance travelled by the solvent.

What is the formula for Rf?

Rf = distance moved by spot ÷ distance moved by solvent.

What distance is measured for the compound in an Rf calculation?

The distance from the origin to the centre of the spot.

What distance is measured for the solvent in an Rf calculation?

The distance from the origin to the solvent front.

Why is the centre of the spot used when calculating Rf?

It provides a consistent position for measuring the distance travelled by the compound.

What range of values can an Rf value have?

Between 0 and 1.

What does a high Rf value generally indicate?

The substance has a greater affinity for the mobile phase than for the stationary phase.

What does a low Rf value generally indicate?

The substance has a greater affinity for the stationary phase than for the mobile phase.

What does an Rf value of 0.85 indicate?

The compound has a relatively high affinity for the solvent compared with the paper.

Why can Rf values help identify compounds?

Different compounds have different Rf values under the same conditions.

Can the same compound have different Rf values in different solvents?

Yes. Rf values depend on the solvent and other chromatography conditions.

How can chromatography distinguish a pure substance from an impure substance?

A pure substance produces one spot, whereas a mixture can produce multiple spots.

Why might a mixture produce different numbers of spots in different solvents?

Different solvents affect the distribution of the substances between the stationary and mobile phases.

What is the purpose of Required Practical 6?

To investigate how paper chromatography can be used to separate and distinguish coloured substances.

What is the main variable that affects separation in paper chromatography?

The interactions of the substances with the stationary phase and mobile phase.

What should be measured in chromatography to determine Rf?

The distance moved by the centre of each spot and the distance moved by the solvent front.

What are the four common gases that must be identified by chemical tests?

Hydrogen, oxygen, carbon dioxide and chlorine.

How is hydrogen tested for?

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

What is the positive test for hydrogen?

The gas burns with a squeaky pop/pop sound.

Why does hydrogen produce a pop with a burning splint?

Hydrogen reacts rapidly with oxygen and burns.

How is oxygen tested for?

Insert a glowing splint into a test tube containing the gas.

What is the positive test for oxygen?

The glowing splint relights.

Why does a glowing splint relight in oxygen?

Oxygen supports combustion.

How is carbon dioxide tested for?

Bubble or shake the gas through limewater.

What is the positive test for carbon dioxide?

The limewater turns milky/cloudy.

What is limewater?

An aqueous solution of calcium hydroxide.

Why does limewater turn milky when carbon dioxide is present?

Carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate.

How is chlorine tested for?

Place damp litmus paper into the chlorine gas.

What is the positive test for chlorine?

The damp litmus paper is bleached and turns white.

What is the flame test used to identify?

Some metal ions, which are cations.

What colour flame does a lithium compound produce?

Crimson red.

What colour flame does a sodium compound produce?

Yellow.

What colour flame does a potassium compound produce?

Lilac.

What colour flame does a calcium compound produce?

Orange-red.

What colour flame does a copper compound produce?

Green.

Which metal ion produces a crimson flame?

Lithium.

Which metal ion produces a yellow flame?

Sodium.

Which metal ion produces a lilac flame?

Potassium.

Which metal ion produces an orange-red flame?

Calcium.

Which metal ion produces a green flame?

Copper.

Why can flame tests be difficult when a mixture of ions is present?

Some flame colours can mask other flame colours.

What is sodium hydroxide used for in chemical analysis?

To identify some metal ions by the coloured or white precipitates they form.

What is a precipitate?

An insoluble solid formed during a reaction in solution.

What happens when sodium hydroxide is added to copper(II) ions?

A blue precipitate of copper(II) hydroxide forms.

What is the ionic equation for the copper(II) hydroxide test?

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

What colour precipitate does copper(II) produce with sodium hydroxide?

Blue.

What happens when sodium hydroxide is added to iron(II) ions?

A green precipitate of iron(II) hydroxide forms.

What is the ionic equation for the iron(II) hydroxide test?

Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)

What colour precipitate does iron(II) produce with sodium hydroxide?

Green/dirty green.

What happens when sodium hydroxide is added to iron(III) ions?

A brown precipitate of iron(III) hydroxide forms.

What is the ionic equation for the iron(III) hydroxide test?

Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)

What colour precipitate does iron(III) produce with sodium hydroxide?

Brown.

Which ions form white precipitates with sodium hydroxide?

Aluminium ions, calcium ions and magnesium ions.

What happens when sodium hydroxide is added to aluminium ions?

A white precipitate of aluminium hydroxide forms.

What is the ionic equation for aluminium hydroxide formation?

Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)

What happens to aluminium hydroxide in excess sodium hydroxide?

The precipitate dissolves, forming a colourless solution.

Which of the white hydroxide precipitates dissolves in excess sodium hydroxide?

Aluminium hydroxide.

What happens when sodium hydroxide is added to calcium ions?

A white precipitate of calcium hydroxide forms.