Chemistry: Pure Substances, Mixtures, Chromatography, and Gas Tests

What is a pure substance?

A pure substance contains only one element or one compound, with no other substances mixed in.

How can purity be tested?

A pure substance has a sharp, fixed melting point and boiling point.

What is a mixture?

A mixture consists of two or more elements or compounds that are not chemically bonded together.

What is a formulation?

A formulation is a mixture designed as a useful product, with components present in precise quantities to give specific properties.

Give examples of formulations.

Paints, fuels, medicines, cleaning products, alloys.

What is chromatography used for?

Chromatography is used to separate mixtures and identify substances.

What type of analysis is chromatography?

Qualitative analysis, because it identifies substances but does not measure their quantity.

What is the stationary phase in paper chromatography?

The chromatography paper.

What is the mobile phase?

The solvent.

Why is a pencil used to draw the baseline?

Pencil is insoluble and will not dissolve or move with the solvent.

Why must the baseline be above the solvent level?

To prevent the substances dissolving directly into the solvent instead of moving up the paper.

Why do different substances separate in chromatography?

Because they have different solubilities in the solvent and different attractions to the stationary phase.

What does it mean if a substance travels further up the paper?

It is more soluble in the solvent and less attracted to the paper.

What is an Rf value?

A ratio used to identify substances in chromatography.

State the Rf equation.

Rf = distance moved by substance ÷ distance moved by solvent front.

Why is an Rf value always less than 1?

The substance cannot travel further than the solvent front.

How can an unknown substance be identified using chromatography?

By comparing its Rf value with known substances under identical conditions.

What does it mean if two substances have the same Rf value?

They may be the same substance, but the results must be obtained under identical conditions.

State two limitations of chromatography.

Different substances can have the same Rf value, and Rf values change if conditions such as solvent or temperature change.

State one hazard in the chromatography practical.

The solvent may be flammable.

State a suitable risk reduction.

Keep solvents away from naked flames and work in a well-ventilated area.

State two control variables in chromatography.

The type of solvent used and the type of chromatography paper.

How can reliability be improved in chromatography?

Repeat the experiment and calculate a mean Rf value.

How can accuracy be improved in chromatography?

Measure distances carefully using a ruler and mark the solvent front immediately.

How do you test for hydrogen gas?

A lighted splint produces a squeaky pop due to rapid combustion.

How do you test for oxygen gas?

A glowing splint relights because oxygen supports combustion.

How do you test for carbon dioxide gas?

Bubble the gas through limewater; it turns cloudy.

State the word equation for the limewater test.

Calcium hydroxide + carbon dioxide → calcium carbonate + water.

How do you test for chlorine gas?

Chlorine bleaches damp blue litmus paper, turning it red then white.

How do you test for ammonia gas?

Damp red litmus paper turns blue because ammonia is alkaline.

What are flame tests used for?

Identifying metal ions.

What flame colour is produced by lithium ions?

Crimson red.

What flame colour is produced by sodium ions?

Yellow.

What flame colour is produced by potassium ions?

Lilac.

What flame colour is produced by calcium ions?

Orange-red (brick red).

What flame colour is produced by copper ions?

Blue-green.

What reagent is used to test for metal ions in solution?

Sodium hydroxide solution.

What result indicates aluminium ions?

A white precipitate that dissolves in excess sodium hydroxide.

What result indicates magnesium ions?

A white precipitate that does not dissolve in excess sodium hydroxide.

What result indicates calcium ions?

A white precipitate that does not dissolve in excess sodium hydroxide.

What result indicates copper(II) ions?

A blue precipitate.

What result indicates iron(II) ions?

A green precipitate.

What result indicates iron(III) ions?

A brown precipitate.

How do you test for ammonium ions?

Add sodium hydroxide and gently warm.

What confirms the presence of ammonium ions?

Ammonia gas is released, which turns damp red litmus paper blue.

How do you test for carbonate ions?

Add dilute acid to produce carbon dioxide gas.

How is the gas confirmed in the carbonate test?

Limewater turns cloudy.

What reagents are used to test for halide ions?

Dilute nitric acid followed by silver nitrate solution.

Why is nitric acid used before silver nitrate?

To remove carbonate ions which could cause a false positive.

Why is hydrochloric acid not used in the halide test?

It contains chloride ions which would interfere with the results.

What result indicates chloride ions?

A white precipitate of silver chloride.

What result indicates bromide ions?

A cream precipitate of silver bromide.

What result indicates iodide ions?

A yellow precipitate of silver iodide.

How do you test for sulfate ions?

Add dilute hydrochloric acid followed by barium chloride solution.

What result confirms sulfate ions?

A white precipitate of barium sulfate.

Why are instrumental methods used instead of chemical tests?

They are more accurate, more sensitive, and faster.

What is flame emission spectroscopy used for?

Identifying metal ions and measuring their concentration.

Why is flame emission spectroscopy more accurate than flame tests?

Each element has a unique emission spectrum.

What does infrared spectroscopy identify?

Functional groups in organic compounds.

Why do different bonds absorb different infrared frequencies?

Because different bonds vibrate at different energies.

What is the fingerprint region?

A unique region of the infrared spectrum used to identify a specific compound.

What does mass spectrometry measure?

Relative atomic mass or relative molecular mass.

What does the molecular ion peak represent?

The mass of the whole molecule.

How does the melting point of an impure substance compare to a pure substance?

An impure substance melts over a range of temperatures and usually at a lower temperature than a pure substance.

How does boiling point data show a substance is impure?

An impure substance boils over a range of temperatures rather than at a fixed boiling point.

How can melting and boiling point data be used to distinguish pure and impure substances?

A pure substance has sharp, fixed melting and boiling points, whereas an impure substance melts and boils over a range of temperatures.

What does the term 'pure' mean in everyday language?

In everyday language, 'pure' means nothing harmful has been added, not that the substance contains only one element or compound.

How is a formulation made?

A formulation is made by mixing substances in carefully measured quantities which are tested and adjusted to produce the required properties.

How can a formulation be identified from given information?

A formulation can be identified if it is a mixture designed for a specific purpose with each component present in precise amounts.

How does chromatography show whether a substance is pure or impure?

A pure substance produces a single spot on the chromatogram, whereas an impure substance produces two or more spots.

How should paper chromatography be carried out?

A pencil baseline is drawn, samples are spotted onto the line, the paper is placed in solvent below the baseline, a lid is used, and the solvent front is marked when it reaches near the top.

Why is a lid used during paper chromatography?

To prevent the solvent evaporating and to allow the solvent front to rise evenly.

How can the result of a gas test be interpreted?

The observation from the test is compared to known results to identify the gas present.

How is a flame test carried out?

A nichrome wire loop is cleaned with hydrochloric acid, dipped into the sample, and placed in a blue Bunsen flame.

Why can flame tests be unreliable for mixtures?

Different metal ions produce overlapping colours and sodium produces a strong yellow flame that can mask other colours.

How are the results of flame tests interpreted?

The flame colour is compared to known flame test colours to identify the metal ion present.

How are the results of metal hydroxide tests interpreted?

The colour of the precipitate and whether it dissolves in excess sodium hydroxide are used to identify the metal ion.

How are the results of carbonate tests interpreted?

The production of carbon dioxide, confirmed by limewater turning cloudy, indicates carbonate ions are present.

How are the results of halide tests interpreted?

The colour of the precipitate formed with silver nitrate is used to identify the halide ion present.

How are the results of sulfate tests interpreted?

The formation of a white precipitate with barium chloride indicates sulfate ions are present.

How is flame emission spectroscopy carried out?

A sample is vaporised in a flame and the emitted light is passed through a spectroscope to produce a spectrum.

What does a flame emission spectrum show?

It shows specific wavelengths of light emitted by metal ions, which are unique to each element.

How is flame emission spectroscopy used to identify substances?

The wavelengths in the spectrum are compared to known reference spectra.

How can flame emission spectroscopy data be interpreted quantitatively?

The intensity of the light emitted is proportional to the concentration of the metal ion in the sample.