12. Experimental techniques and chemical analysis

Exam analysis

Most tested topics

12.1 Experimental design

Choose apparatus by identifying the quantity to measure, then matching the required type and range of measurement.

Quantity	Appropriate apparatus	What it measures
time	stop-watch / stop-clock	time interval
temperature	thermometer	temperature
mass	balance	mass
one fixed liquid volume	volumetric pipette	one stated volume
variable liquid volume delivered	burette	a chosen delivered volume
variable liquid volume	measuring cylinder	a range of liquid volumes
gas volume	gas syringe	volume of gas produced or used

To measure the time for 2.0 g of magnesium to react with 50 cm³ of acid, use a stop-clock for time, a balance for mass and a measuring cylinder for the liquid volume.

Beakers and conical flasks are useful containers, but they are not the appropriate apparatus when the task requires a measured mass, time, temperature or volume.

An advantage or disadvantage must link a feature of the method or apparatus to its effect on the measurement or the experiment.

Feature being compared	Possible advantage	Possible disadvantage
smaller scale divisions	allows a more precise reading	may take longer or require more careful reading
fixed-volume apparatus	delivers one volume consistently	cannot measure a different chosen volume
variable-volume apparatus	can measure a range of volumes	suitability depends on its scale and capacity
enclosed gas collection	reduces loss of gas to the surroundings	unsuitable if the gas volume exceeds the apparatus capacity
repeated measurements	reveal consistency and support a mean	require more time and materials

For exactly 25.0 cm³, a volumetric pipette is advantageous because it delivers that fixed volume consistently; a measuring cylinder is more flexible but gives a less precise reading.

State the feature, name its consequence and relate that consequence to the aim. For example: 'The gas syringe is enclosed, so less product gas escapes and the measured volume is more reliable.'

Do not write 'more accurate' or 'better' without explaining which design feature causes the improvement and which measurement it affects.

Term	Meaning
solvent	substance that dissolves a solute
solute	substance dissolved in a solvent
solution	mixture of one or more solutes dissolved in a solvent
saturated solution	solution containing the maximum concentration of a solute in that solvent at a specified temperature
residue	substance that remains after evaporation, distillation, filtration or a similar separation process
filtrate	liquid or solution that has passed through a filter

When rock salt is stirred with water, salt is the solute and water is the solvent. After filtration, insoluble sand is the residue and salt solution is the filtrate.

A solution is saturated only under stated conditions: at the specified temperature, no more of that solute can dissolve in that amount of solvent.

Residue is what remains after a separation process; filtrate specifically means the liquid that passes through a filter. A filtrate can still contain dissolved solute.

12.2 Acid–base titrations

An acid–base titration finds the volume of one solution needed to react with a fixed volume of the other solution.

Apparatus	Role in the titration
volumetric pipette	transfers one fixed, accurate volume into a conical flask
burette	delivers a measured, variable volume of the other solution
suitable indicator	shows when the end-point has been reached

Use the volumetric pipette to place a fixed volume of acid or alkali in a conical flask.
Add a few drops of a suitable indicator.
Fill the burette with the other solution and record the initial reading.
Add solution from the burette while swirling the flask; add it dropwise near the colour change.
Record the final burette reading when the end-point is reached. The volume delivered is final reading − initial reading.

The pipette measures the fixed volume placed in the flask; the burette measures the variable volume delivered. Do not swap their roles or estimate volume from markings on the conical flask.

The end-point is identified by the indicator changing colour as the titrant is added.

Swirl after each addition and, near the end-point, add the solution from the burette one drop at a time. Stop at the first colour change that remains after swirling.

With methyl orange and alkali initially in the flask, the observed end-point can be the change from yellow to orange as acid is added.

Use a suitable acid–base indicator with a clear colour change over a narrow range. Universal indicator gives several gradual colour changes, so it does not show one sharp titration end-point clearly.

The colour direction depends on the indicator and on whether acid or alkali is added. The end-point is the observed indicator change, not simply the moment a chosen volume has been poured in.

12.3 Chromatography

Paper chromatography separates a mixture of soluble coloured substances using a suitable solvent.

Draw a pencil baseline near the bottom of the chromatography paper.
Place a small spot of the mixture on the baseline and let it dry.
Stand the paper in a suitable solvent with the solvent level below the baseline.
Let the solvent rise through the paper.
Remove the paper before the solvent reaches the top, mark the solvent front in pencil and let the chromatogram dry.

The solvent carries the soluble substances up the paper. Components that travel different distances form separate coloured spots.

Use pencil because graphite does not dissolve and move with the solvent. Keep the starting spots above the solvent level, or they may dissolve directly into the solvent reservoir.

Observation under the same conditions	Interpretation
unknown spot matches a known spot in position / Rf	the known substance may be present in the unknown
one spot from a sample	sample is pure by this chromatogram
two or more spots from a sample	sample is a mixture / impure
no known spot matches	that component is not identified by the available standards

Compare unknowns and standards made with the same paper and solvent. A matching travel position or Rf value supports the identification.

Two different substances can sometimes have the same Rf value, so their spots may overlap and appear as one. A chromatogram may therefore fail to reveal every component.

A spot is evidence for a component, not an element count or a molecular formula. More than one spot shows a mixture; it does not mean the sample is one compound made from several elements.

Colourless soluble substances can be separated by paper chromatography, but their spots must be revealed after separation.

Place the colourless mixture on a pencil baseline.
Develop the chromatogram in a suitable solvent with the baseline above the solvent.
Remove the paper, mark the solvent front and let it dry.
Apply a locating agent so the separated colourless substances form visible spots.

The solvent moves and separates the substances; the locating agent reveals their final positions. The locating agent is applied after the solvent has separated the mixture.

The locating agent does not perform the separation and is not the solvent. The syllabus does not require the name of a specific locating agent.

An Rf value compares how far a substance travels with how far the solvent front travels from the same baseline.

R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}

Measure both distances from the pencil baseline in the same direction. For a spot, measure to its centre; for the solvent, measure to the marked solvent front.

If a spot travels 13.0 cm and the solvent front travels 15.1 cm: Rf = 13.0 ÷ 15.1 = 0.86. The units cancel, so Rf has no unit.

Under the same experimental conditions, compare an unknown Rf with known values to help identify the substance.

The substance distance is the numerator and the solvent-front distance is the denominator. A spot cannot travel beyond the solvent front, so a valid Rf is less than 1.

12.4 Separation and purification

Method	Property used	Essential process	Product obtained
suitable solvent	difference in solubility	dissolve the soluble component, then separate the undissolved material	selected soluble or insoluble component
filtration	insoluble solid particles do not pass through filter paper	pour mixture through filter	residue = insoluble solid; filtrate = liquid/solution
crystallisation	solubility usually decreases as a hot saturated solution cools	concentrate to near saturation, cool, filter and dry crystals	dissolved solid as crystals
simple distillation	solvent has a lower boiling point than dissolved solute	boil, condense vapour and collect distillate	solvent/purer liquid from a solution
fractional distillation	miscible liquids have different boiling points	heat through a fractionating column and collect fractions at their boiling ranges	separated liquids

In crystallisation, cooling reduces the amount of solute that can remain dissolved, so excess solute forms crystals. Filter, wash with a little cold solvent and dry the crystals.

Distillation separates by vaporising and condensing. Simple distillation is used for a solvent from a solution; fractional distillation repeatedly separates vapours in the column when several liquids are present.

Evaporation alone may recover a dissolved solid but does not collect the solvent. Filtration cannot remove a dissolved solute because dissolved particles pass through the filter paper.

Choose the technique from the substances' physical states, solubilities and boiling points, and first decide which component must be collected.

Information about the mixture	Suitable choice
insoluble solid + liquid/solution	filtration
dissolved solid wanted from solution	crystallisation
solvent wanted from a solution	simple distillation
two or more miscible liquids with different boiling points	fractional distillation
one solid dissolves in a chosen solvent and another does not	add suitable solvent → filter → recover dissolved solid from filtrate
precipitate formed in an aqueous reaction	filter, wash residue and dry

Nickel(II) sulfate dissolves in water but sand does not: warm with water, filter off sand, then concentrate and cool the filtrate to crystallise nickel(II) sulfate.

A complete choice states both the property difference and the sequence. 'Use filtration because the desired precipitate is insoluble in the liquid' is stronger than naming filtration alone.

Do not choose a technique from the substance names alone. The same technique may be right or wrong depending on solubility, boiling point, physical state and which fraction is wanted.

A pure substance has characteristic melting and boiling points. Compare measured values with reliable reference data to support identification and assess purity.

Measurement	Pure sample	Impure sample
melting	sharp melting point at the characteristic value	usually melts lower and over a range
boiling	sharp boiling point at the characteristic value	usually boils higher and over a range

If a liquid boils sharply at the reference boiling point for substance X, the result supports the identity and purity of X. A different value or a boiling range suggests that the sample is not pure X.

A solid with a reference melting point of 80 °C that melts sharply at 80 °C is consistent with the pure substance. Melting gradually from 74–78 °C indicates impurity.

A temperature match is evidence, not proof by itself. Use the correct property for the sample and compare under the same stated conditions; boiling point also depends on pressure.

12.5 Identification of ions and gases

An anion test is identified by the complete sequence of reagents and its positive observation. Use a fresh portion of the unknown for each test so one reagent does not contaminate the next.

Anion	Ordered test	Positive result
carbonate, $\mathrm{CO_3^{2-}}$	add dilute acid; test the gas with limewater	effervescence; the carbon dioxide turns limewater milky
chloride, $\mathrm{Cl^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	white precipitate
bromide, $\mathrm{Br^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	cream precipitate
iodide, $\mathrm{I^-}$	acidify with dilute nitric acid; add aqueous silver nitrate	yellow precipitate
nitrate, $\mathrm{NO_3^-}$	add aqueous sodium hydroxide, then aluminium foil; warm carefully; test the gas with damp red litmus	ammonia is produced and turns the paper blue
sulfate, $\mathrm{SO_4^{2-}}$	acidify with dilute nitric acid; add aqueous barium nitrate	white precipitate
sulfite, $\mathrm{SO_3^{2-}}$	add a small volume of acidified aqueous potassium manganate(VII)	purple solution turns colourless

The three halides use the same reagents, so the precipitate colour makes the identification: chloride white, bromide cream, iodide yellow.

Do not replace dilute nitric acid with hydrochloric acid before the silver nitrate test: hydrochloric acid introduces chloride ions. Likewise, sulfuric acid would introduce sulfate before a sulfate test.

Add each reagent dropwise to a fresh portion of the solution, record the precipitate colour, then add the reagent in excess and record whether the precipitate dissolves. For ammonium, warm with aqueous sodium hydroxide and test the gas.

Cation	Aqueous sodium hydroxide	Aqueous ammonia
$\mathrm{Al^{3+}}$	white precipitate; soluble in excess to a colourless solution	white precipitate; insoluble in excess
$\mathrm{NH_4^+}$	on warming, ammonia is produced	—
$\mathrm{Ca^{2+}}$	white precipitate; insoluble in excess	no precipitate or a very slight white precipitate
$\mathrm{Cr^{3+}}$	green precipitate; soluble in excess	green precipitate; insoluble in excess
$\mathrm{Cu^{2+}}$	light blue precipitate; insoluble in excess	light blue precipitate; soluble in excess to a dark blue solution
$\mathrm{Fe^{2+}}$	green precipitate; insoluble in excess; turns brown near the surface on standing	green precipitate; insoluble in excess; turns brown near the surface on standing
$\mathrm{Fe^{3+}}$	red-brown precipitate; insoluble in excess	red-brown precipitate; insoluble in excess
$\mathrm{Zn^{2+}}$	white precipitate; soluble in excess to a colourless solution	white precipitate; soluble in excess to a colourless solution

A white precipitate alone is not an identification. Use behaviour in excess to distinguish aluminium from zinc with ammonia, and use the sodium-hydroxide and ammonia results together to distinguish calcium.

For $\mathrm{NH_4^+}$ , the positive result is ammonia gas on warming with aqueous sodium hydroxide; confirm it because damp red litmus paper turns blue.

State both the initial observation and the result in excess. 'Soluble' refers to the precipitate dissolving after excess reagent is added, not to the original salt solution.

A gas identification must pair the specified test with its distinctive positive result.

Gas	Test	Positive result
ammonia, $\mathrm{NH_3}$	damp red litmus paper	turns blue
carbon dioxide, $\mathrm{CO_2}$	limewater	turns milky
chlorine, $\mathrm{Cl_2}$	damp litmus paper	bleached
hydrogen, $\mathrm{H_2}$	lighted splint	a ‘pop’ sound
oxygen, $\mathrm{O_2}$	glowing splint	relights
sulfur dioxide, $\mathrm{SO_2}$	acidified aqueous potassium manganate(VII)	purple turns colourless

Keep the splint states exact: hydrogen is tested with a lighted splint; oxygen with a glowing splint. Reversing them loses the identifying observation.

Litmus must be damp for ammonia and chlorine. Potassium manganate(VII) must be acidified for sulfur dioxide; an unqualified colour change is not the complete specified test.

Place a little salt on a clean wooden splint or clean nichrome/platinum wire and introduce it into a roaring Bunsen flame. Observe the characteristic flame colour.

Metal ion	Flame colour
lithium, $\mathrm{Li^+}$	red
sodium, $\mathrm{Na^+}$	yellow
potassium, $\mathrm{K^+}$	lilac
calcium, $\mathrm{Ca^{2+}}$	orange-red
barium, $\mathrm{Ba^{2+}}$	light green
copper(II), $\mathrm{Cu^{2+}}$	blue-green

Record the observed colour first, then match it to the table. Distinguish the paired descriptions precisely: calcium is orange-red, barium light green and copper(II) blue-green.

Use clean test equipment and a fresh sample. A contaminated wire or splint can add another flame colour, especially the strong yellow from sodium, and make the identification unreliable.

12. Experimental techniques and chemical analysis

Exam analysis

Most tested topics

12.1 Experimental design

Choose apparatus for each measurement

Compare experimental methods and apparatus

Distinguish solution and separation terms

12.2 Acid–base titrations

Carry out an acid–base titration

Recognise the titration end-point

12.3 Chromatography

Separate coloured substances by paper chromatography

Interpret identities and purity on a chromatogram

Reveal colourless substances on a chromatogram

Calculate and use an Rf value

12.4 Separation and purification

Explain five separation and purification methods

Choose a separation technique from substance data

Identify substances and judge purity from melting and boiling points

12.5 Identification of ions and gases

Identify anions with ordered chemical tests

Identify aqueous cations with sodium hydroxide and ammonia

Identify six gases with their specific tests

Identify metal ions by flame colour