Chemical Analysis

What is a pure substance in chemistry?

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

How can melting point help identify a pure substance?

A pure substance melts at a specific temperature.

How can boiling point help identify a pure substance?

A pure substance boils at a specific temperature.

How does an impurity affect melting or boiling behaviour?

An impure substance melts/boils over a range rather than at one specific temperature.

What is a formulation?

A mixture designed as a useful product with components mixed in carefully measured quantities.

Why are the quantities in a formulation carefully controlled?

To give the product its required properties.

Give examples of formulations.

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

What is chromatography used for?

Separating mixtures + helping identify substances.

What are the two phases in chromatography?

Stationary phase + mobile phase.

What is the stationary phase in paper chromatography?

The chromatography paper.

What is the mobile phase in paper chromatography?

The solvent moving through the paper.

Why do substances separate during chromatography?

They have different distributions between the stationary + mobile phases, so move at different rates.

What does one spot on a chromatogram usually indicate?

A pure substance.

What do multiple spots from one sample indicate?

A mixture.

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

Yes - Rf depends on the solvent used.

How can an unknown substance be identified using chromatography?

Compare its spots/Rf values with known substances using the same solvent and conditions.

What is the equation for Rf?

Rf = distance moved by substance / distance moved by solvent.

From where is the distance moved by a substance measured?

From the origin to the centre of the spot.

From where is the solvent distance measured?

From the origin to the solvent front.

Does Rf have a unit?

No - it is a ratio.

Why is Rf normally less than 1?

The substance cannot travel further than the solvent front.

RP6 - What is the aim of the chromatography practical?

Use paper chromatography to separate + distinguish coloured substances and calculate Rf values.

RP6 - How is a chromatography experiment set up?

Draw pencil baseline - place small sample spots on line - place paper in solvent with spots above solvent - allow solvent to rise.

RP6 - Why must the baseline be drawn in pencil?

Graphite does not dissolve in the solvent and therefore does not interfere with the chromatogram.

RP6 - Why should the sample spots start above the solvent level?

Prevents samples dissolving directly into the solvent instead of travelling up the paper.

RP6 - Why should sample spots be small?

Produces clearer, more precise separated spots.

RP6 - What should be done when the solvent has nearly reached the top?

Remove paper + immediately mark the solvent front in pencil.

RP6 - Why must the solvent front be marked quickly?

The solvent evaporates, making its final position difficult to identify.

RP6 - How do you calculate Rf after chromatography?

Measure origin to centre of spot - divide by origin to solvent front.

How do you test for hydrogen gas?

Hold a burning splint at the open end of the test tube - hydrogen burns with a squeaky pop.

How do you test for oxygen gas?

Insert a glowing splint - it relights.

How do you test for carbon dioxide gas?

Bubble/shake gas through limewater - limewater turns milky/cloudy.

What is limewater?

Aqueous calcium hydroxide.

How do you test for chlorine gas?

Expose damp litmus paper - chlorine bleaches it white.

What are flame tests used for?

Identifying certain metal ions.

What flame colour does Li+ produce?

Crimson.

What flame colour does Na+ produce?

Yellow.

What flame colour does K+ produce?

Lilac.

What flame colour does Ca2+ produce?

Orange-red.

What flame colour does Cu2+ produce?

Green.

Why can flame tests be unreliable for mixtures?

One flame colour can mask another.

What reagent is used to test some metal ions by precipitation?

Sodium hydroxide solution.

What happens when sodium hydroxide is added to Al3+?

White precipitate forms - dissolves in excess sodium hydroxide.

What happens when sodium hydroxide is added to Ca2+?

White precipitate forms.

What happens when sodium hydroxide is added to Mg2+?

White precipitate forms.

How can Al3+ be distinguished from Ca2+ and Mg2+ using sodium hydroxide?

Aluminium hydroxide dissolves in excess sodium hydroxide - calcium and magnesium hydroxides do not.

What happens when sodium hydroxide is added to Cu2+?

Blue precipitate forms.

What happens when sodium hydroxide is added to Fe2+?

Green precipitate forms.

What happens when sodium hydroxide is added to Fe3+?

Brown precipitate forms.

What causes the precipitates in metal-ion tests?

Insoluble metal hydroxides form.

What is the ionic equation for Cu2+ reacting with hydroxide ions?

Cu2+ + 2OH- - Cu(OH)2.

What is the ionic equation for Fe2+ reacting with hydroxide ions?

Fe2+ + 2OH- - Fe(OH)2.

What is the ionic equation for Fe3+ reacting with hydroxide ions?

Fe3+ + 3OH- - Fe(OH)3.

How do you test for carbonate ions, CO3 2-?

Add dilute acid - carbon dioxide is produced - confirm by turning limewater milky/cloudy.

What observations indicate carbonate ions after adding dilute acid?

Effervescence/fizzing + carbon dioxide produced.

What reagent is used to test for halide ions?

Dilute nitric acid followed by silver nitrate solution.

Why is dilute nitric acid added before silver nitrate?

Removes interfering ions without introducing halide ions.

What result shows chloride ions, Cl-?

White precipitate of silver chloride.

What result shows bromide ions, Br-?

Cream precipitate of silver bromide.

What result shows iodide ions, I-?

Yellow precipitate of silver iodide.

What is the halide precipitate colour order?

Chloride - white. Bromide - cream. Iodide - yellow.

What is the ionic equation for the chloride test?

Ag+ + Cl- - AgCl.

What is the ionic equation for the bromide test?

Ag+ + Br- - AgBr.

What is the ionic equation for the iodide test?

Ag+ + I- - AgI.

What reagents are used to test for sulfate ions, SO4 2-?

Dilute hydrochloric acid followed by barium chloride solution.

What positive result shows sulfate ions?

White precipitate of barium sulfate.

What is the ionic equation for the sulfate test?

Ba2+ + SO4 2- - BaSO4.

RP7 - What is the aim of the ion-testing practical?

Use chemical tests to identify ions in unknown single ionic compounds.

RP7 - Which cations can be identified using flame tests?

Li+, Na+, K+, Ca2+ + Cu2+.

RP7 - Which cations can be identified using sodium hydroxide?

Al3+, Ca2+, Mg2+, Cu2+, Fe2+ + Fe3+.

RP7 - How are carbonate ions identified?

Add dilute acid - test produced CO2 with limewater.

RP7 - How are halide ions identified?

Add dilute nitric acid then silver nitrate - identify precipitate colour.

RP7 - How are sulfate ions identified?

Add dilute hydrochloric acid then barium chloride - white precipitate.

What are instrumental methods used for?

Detecting, identifying + measuring elements and compounds.

What are three advantages of instrumental methods over chemical tests?

More accurate, more sensitive + faster.

What does sensitive mean for an analytical method?

It can detect very small amounts/concentrations of a substance.

What is flame emission spectroscopy used for?

Identifying metal ions in solution + measuring their concentrations.

How does flame emission spectroscopy work?

Sample enters flame - emitted light passes through a spectroscope - line spectrum is produced.

Why does each metal ion produce a characteristic line spectrum?

Different metal ions emit characteristic wavelengths of light.

How can a metal ion be identified from a flame emission spectrum?

Compare positions of its spectral lines with reference spectra.

How can flame emission spectroscopy determine concentration?

Intensity of spectral lines can be used to measure concentration.

Why is flame emission spectroscopy better than a simple flame test for mixtures?

Individual spectral lines can identify ions even when simple flame colours would be masked.