Tool 1: Experimental Techniques — IB Chemistry SL Revision Notes & Topical Questions

Section 1 of 33Hazards and risks

Hazard

Hazard the potential of a substance to cause harm because of its properties, e.g. concentrated HCl is corrosive, mildly concentrated is an irritant and dilute is not classed hazardous.

Risk

Risk the likelihood that harm occurs in a specific situation, depending on the amount used and how it is used.

Control

• Control eye protection reduces the risk of harm without changing the hazard.

Hazard symbol	Meaning	Example precaution
Health hazard	Skin rashes, eye damage, harm if swallowed	Gloves and eye protection
Corrosive	Skin burns and permanent eye damage	Splash goggles, gloves
Flammable	Catches fire if heated or near a flame	Water bath or mantle, no flames
Acute toxicity	Life-threatening in small quantities	Fume cupboard
Hazardous to the environment	Threat to aquatic life	Collect waste, do not pour down sinks
Oxidising	Can cause fire or explosions	Store away from combustible reducing agents

• Symbol format
  • nine standard symbols, shown in a red diamond on a white background.

Section 2 of 33Risk assessment and control

1

Identify the hazards by listing the chemicals used and their dangers.

2

Assess the risks from the amount used, whether it is heated, the product given off and the experience of the user.

3

Rate each risk as low, medium or high.

4

Implement control measures.

5

Plan for emergencies such as spills or contact with skin.

Control measures

• Control measures elimination, substitution with a less hazardous substance, good practice such as capping bottles, safety equipment such as fume hoods and personal protective equipment.

Safety data sheets

• Safety data sheets give information on safe handling and use.

Justified precautions

• Justified precautions say what the hazard is and why the precaution is taken, e.g. HCl gas is toxic and corrosive so the step is done in a fume cupboard.

Handling and storage

• Handling and storage label containers with the chemical name, concentration and hazard symbols.

Section 3 of 33Environmental and ethical issues

Waste

• Waste dispose of unused reactants and products correctly.

Green chemistry

• Green chemistry reduce volumes and concentrations, use micro-scale work and reuse excess materials.

Solvents

• Solvents choose aqueous solutions or simple alcohols and alkanes where possible.

Catalysis

• Catalysis a catalyst minimises waste and energy.

Atom economy

• Atom economy determine it and look for ways to increase it.

Microscale

• Microscale cuts reagent volumes from 50–100 cm³ to 1–2 cm³, reducing toxic waste.

Ethics

• Ethics human subjects require consent and animal work must follow ethical guidelines.

Section 4 of 33Measuring mass, volume and gas volume

Mass

• Mass a digital balance, tared before use; the SI unit is the kilogram and 1 kg = 1000 g,
  • and warm objects should not be weighed because convection currents change the reading.

Measuring cylinder

• Measuring cylinder approximate volumes only.

Volumetric pipette

• Volumetric pipette the most accurate for a fixed volume, e.g. 25.00 cm³.

Burette

• Burette the most accurate for a variable volume, with the scale running from 0.00 at the top, so read from the top down.

Volumetric flask

• Volumetric flask calibrated for one precise volume when making standard solutions.

Meniscus

• Meniscus read with the eye level with the bottom of the curve to avoid parallax.

Gas volume

• Gas volume a gas syringe, or an inverted cylinder or burette over water if the gas is insoluble.

Section 5 of 33Measuring time, temperature and length

Time

• Time a stopwatch in seconds or minutes, and human reaction time matters for very short intervals.

Recording time

• Recording time do not mix seconds and minutes, e.g. 1.30 on the display is 90 s, not 1.30 min.

Liquid thermometer

• Liquid thermometer reads to the nearest 1 °C or 0.5 °C, is robust and cheap, but slower and less precise.

Digital probe

• Digital probe reads to ±0.1 °C or better, gives fast readings and suits data logging.

Length

• Length a ruler to the nearest mm, and a tape measure for larger distances.

Section 6 of 33Measuring pH, current and potential difference

pH meter

• pH meter a glass membrane electrode gives precise pH, often to two decimal places.

Indicators

• Indicators natural ones give approximate values and synthetic ones (phenolphthalein, methyl orange) have sharp colour changes for titrations.
  • (litmus, red cabbage)

Universal indicator

• Universal indicator a blend giving approximate pH 1–14 read against a colour chart.

Ammeter

• Ammeter connected in series.

Voltmeter

• Voltmeter connected in parallel across the component.

Zero errors

• Zero errors analogue and digital meters should be checked for zero error and the zero value subtracted.

Parallax

• Parallax analogue meters should be read perpendicular to the scale.

Section 7 of 33Volumetric analysis and titration protocol

1Standard solution

• Standard solution made with a three-decimal-place balance and a volumetric flask, then inverted to mix.

2Primary standard

• Primary standard high purity, high molar mass, low hygroscopicity and stable in air, e.g. anhydrous Na₂CO₃.

3Burette

• Burette read to two decimal places with uncertainty ±0.05 cm³, with the tip filled and free of air bubbles.

4Rough titre

• Rough titre an initial pilot titre is excluded from the average.

5Concordant

• Concordant titres within ±0.10 cm³, and only concordant titres are averaged.

6End point versus equivalence point

• End point versus equivalence point the equivalence point is stoichiometric equivalence and the end point is the observed colour change.

Section 8 of 33Separation, purification and other techniques

Separation

• Separation filtration, crystallisation, simple and fractional distillation, and paper or thin-layer chromatography.

Purification

• Purification recrystallisation and melting point determination.

Other techniques

• Other techniques calorimetry, electrochemical cells, drying to constant mass, reflux, colorimetry or spectrophotometry,
  • and physical or digital molecular modelling.

Reflux

• Reflux heats a mixture to boiling for a long time without losing volatile material, using a vertical condenser and anti-bumping granules.

Distillation

• Distillation simple distillation separates liquids with very different boiling points,
• while fractional distillation uses a column for close boiling points such as ethanol and water.

Section 9 of 33More from the specification

1Volumetric analysis

• Volumetric analysis titration uses a burette and pipette to find an unknown concentration from the volume of a reacting solution.

2Hazard warning symbols

• Hazard warning symbols pictograms indicate toxic, corrosive, flammable or oxidising substances and guide safe handling.

3Environmental issues

• Environmental issues green chemistry reduces waste and hazardous solvents, and waste must be disposed of correctly.

4Applying techniques

• Applying techniques choose heating methods, measure temperature accurately, and repeat to obtain concordant results.

5Results and uncertainty

• Results and uncertainty record data with appropriate precision and calculate the average of consistent trials.

6Volumetric analysis

• Volumetric analysis titration uses a burette and pipette to find an unknown concentration from the volume of a reacting solution.

7Hazard warning symbols

• Hazard warning symbols pictograms indicate toxic, corrosive, flammable or oxidising substances and guide safe handling.

8Environmental issues

• Environmental issues green chemistry reduces waste and hazardous solvents, and waste must be disposed of correctly.

9Applying techniques

• Applying techniques choose heating methods, measure temperature accurately, and repeat to obtain concordant results.

10Results and uncertainty

• Results and uncertainty record data with appropriate precision and calculate the average of consistent trials.

11Temperature

• A measure of the average kinetic energy of the particles.

12Volumetric analysis

• Quantitative analysis by measuring volumes of solutions.

Section 10 of 33Choosing measuring apparatus

Measurement	Apparatus	Typical uncertainty
Mass	Electronic balance	±0.001 g or ±0.01 g
Fixed volume	Volumetric pipette	About ±0.06 cm3 (25 cm3)
Variable volume	Burette	±0.05 cm3 per reading
Approximate volume	Measuring cylinder	±0.5 cm3 or more
Temperature	Thermometer or probe	±0.5 °C or less

Analogue instruments

• Analogue instruments have an uncertainty of half the smallest division.

Digital instruments

• Digital instruments have an uncertainty of the smallest digit shown.

Matching apparatus

• Matching apparatus choose the apparatus for the precision the task needs.

Section 11 of 33Indicators and pH measurement

1Universal indicator paper

• Universal indicator paper dipped into solution, changes colour and gives an approximate pH.

2pH meter

• pH meter gives a value to 0.01 and is calibrated with buffer solutions first.

3Titration indicators

• Titration indicators methyl orange and phenolphthalein change colour in particular pH ranges.

4Choice

• Choice the indicator must match the pH at the equivalence point.

Section 12 of 33Accuracy, precision and reliability

Accuracy

• Accuracy how close a result is to the true value, affected by systematic error.

Precision

• Precision how close repeat measurements are to each other, affected by random error.

Reliability

• Reliability repeating gives consistent results, as with concordant titres within ±0.05 cm³ of the mean.

Repeats

• Repeats reduce the effect of random error.

Control

• Control keeping other variables constant lets a change in the dependent variable be attributed to the independent variable.

Outliers

• Outliers a result far from the others may be excluded from the average and graph, but the reason must be stated.

Section 13 of 33Planning an investigation

Research question

• Research question states clearly what is investigated, e.g. how temperature affects rate.

Independent variable

• Independent variable the factor changed.

Dependent variable

• Dependent variable the quantity measured.

Controlled variables

• Controlled variables everything else that could affect the result, with a stated way of keeping each constant,
  • such as equal volumes and concentrations and the same stirring.

Range and repeats

• Range and repeats at least five values for a graph and at least three repeats.

Analysis plan

• Analysis plan state the graph to be plotted and how the conclusion will be drawn.

Evaluation

• Evaluation identify limitations and improvements.

Section 14 of 33Recording and presenting measurements

Precision

• Precision record every measurement to the full precision of the instrument, e.g. a burette reading as 23.40, not 23.4.

Units

• Units put units in the heading of each table column.

Qualitative data

• Qualitative data record colour changes, bubbles, precipitates and temperature changes of the container.

Tables

• Tables present raw results in a table before processing and show one sample calculation.

Section 15 of 33Virtual experiments and safety

Uses of simulations

• Uses of simulations model processes that are impractical, dangerous or expensive,
  • such as radioactive decay, the Haber and Contact processes, Maxwell-Boltzmann distributions, virtual titrations and spectroscopy.

Variables

• Variables students can vary titrant concentration, indicator pK_In, slit width or path length without wasting reagents.

Limitation

• Limitation they do not replace handling real equipment.

Safety in any plan

• Safety in any plan identify hazards, assess risk and state precautions: eye protection, fume cupboards for toxic gases, no naked flames near flammable liquids and correct waste disposal.

Section 16 of 33More from the specification

More from the specification

Safety risks and temperature scale calibration

• Safety risks and temperature scale calibration identifying toxic chemical risks ensures safe operation when calibrating laboratory apparatus against the Celsius temperature scale.

Volumetric Analysis and Standard Solutions

• Preparing a standard solution of accurately known concentration requires dissolving a primary standard in distilled water within a beaker, followed by quantitatively transferring it into a volumetric flask filled to the calibration line.

Safe Heating Techniques in Organic Chemistry

• Heating volatile or flammable organic compounds requires specialized laboratory techniques,
• such as using an electric heating mantle or water bath with reflux condensers to prevent dangerous vapor escape.

Determination of Purity via Melting Point Analysis

• The purity of a solid organic product is assessed using melting point apparatus: a pure substance melts sharply over a narrow range (±1 °C),
• whereas impurities lower the melting temperature and broaden the range.

Environmental Considerations and Waste Disposal

• Experimental laboratory methods must incorporate environmental safeguards,
• such as collecting heavy metal wastes in dedicated waste containers rather than flushing them into municipal water drains.

Standard solution

• A solution whose concentration is known accurately.

Green chemistry

• The design of processes that reduce waste and hazards.

Hazard symbol

• Warning sign showing a danger.

Volumetric flask

• A flask calibrated to contain one exact volume, used to make standard solutions.

Melting

• Solid to liquid at the melting point, absorbing energy.

Substance

• A material with only one type of particle.

Section 17 of 33Apparatus for volumetric analysis

Key items

• Key items beaker, burette, volumetric pipette, conical flask and standard (volumetric) flask.

Burette

• Burette the key piece in a titration, marked to 0.10 cm³, with the uncertainty of one reading ±0.05 cm³, half the smallest marking.

Pipette

• Pipette measures a fixed volume of the solution of unknown concentration.

Section 18 of 33Making a standard solution

• Definition
  • a solution of precisely known concentration, made from a measured mass of solid and a measured final volume.

1

Weigh the required mass of solid by difference in a weighing bottle on a three-decimal-place balance.

2

Dissolve it in a beaker with a little deionised water, stirring.

3

Transfer to a volumetric flask using a funnel, rinsing the beaker and funnel into the flask.

4

Make up to the mark with deionised water, with the bottom of the meniscus on the line.

5

Stopper and invert several times to mix.

• Mass needed
  • n = cV,
  • then mass = n × M, e.g. 250 cm³ of 0.200 mol dm⁻³ NaOH needs 0.0500 mol,
    • or 2.00 g.
      What the symbols mean: c = concentration (mol dm⁻³) · n = amount (mol) · V = volume (dm³; divide cm³ by 1000)

Section 19 of 33Carrying out dilutions

Concentration

• Concentration the amount of solute per dm³ of solution, in mol dm⁻³, g dm⁻³ or ppm.

Single dilution

• Single dilution adding deionised water to a measured volume of stock solution, since the amount of solute is unchanged (c₁V₁ = c₂V₂).

Serial dilution

• Serial dilution stepwise dilutions each reduce the concentration by a constant factor, such as 100 cm³ of stock in 900 cm³ of water for 1 : 10.

Repeat

• Repeat use each diluted solution to make the next.

Section 20 of 33Carrying out a titration

Equivalence point

Equivalence point the point at which stoichiometrically equivalent amounts have been mixed.

End point

• End point the observed colour change, and a good indicator makes the two coincide.

1

Rinse the burette with the solution to be used, fill it above the zero mark, run some through the jet and remove air bubbles.

2

Pipette a fixed volume (20.0 or 25.0 cm³) into a conical flask and add a few drops of a suitable indicator.

3

Record the starting burette volume.

4

Run in the titrant quickly while swirling to get a rough titre.

5

Repeat, adding drop by drop near the end point until one drop causes a permanent colour change.

6

Record the final volume and repeat until two titres agree within 0.10 cm³.

White tile

• White tile placed under the flask to make the colour change easier to see.

Indicator

• Indicator only a few drops are used because indicators are weak acids and can affect the result.

Redox titrations

• Redox titrations in some, such as MnO₄⁻ as titrant, no indicator is needed.

Section 21 of 33Recording and processing results

1Readings

• Readings record initial and final burette readings to ±0.05 cm³.

2Titre

• Titre the volume delivered, with an uncertainty of ±0.10 cm³ because two readings are combined.

3Averaging

• Averaging average the concordant titres and discard non-concordant and rough titres.

4Percentage uncertainty

• Percentage uncertainty falls when a larger titre is used.

Section 22 of 33Calculating from titration data

1

Calculate the moles of the standard solution from its concentration and the mean titre in dm³.

2

Use the equation ratio to find the moles of the unknown in the pipetted volume.

3

Scale up by the dilution factor if the sample came from a larger solution.

4

Calculate the concentration or mass.

Section 23 of 33Evaluating the result

Sources of error

• Sources of error air bubbles in the jet, overshooting the end point, misreading the meniscus, a wet flask containing solution and an impure primary standard.

Precision

• Precision three concordant results show the data are precise.

Accuracy

• Accuracy a systematic error, such as an inaccurate standard concentration, would still affect accuracy.

Section 24 of 33More from the specification

1Acid-base indicator transition range

• Acid-base indicator transition range visual chemical indicators change colour across a distinct interval of approximately two pH units, defining their working range.

2Analogue versus digital instruments

• Analogue versus digital instruments digital balances minimize parallax reading uncertainties compared to traditional analogue scale divisions.

3Chemical hazard evaluation

• Chemical hazard evaluation reviewing safety data sheets provides vital information to implement suitable ventilation and personal protective equipment.

4Melting point determination

• Melting point determination heating purified organic crystals slowly inside capillary tubes confirms substance identity and purity.

5Laboratory Hazards vs Risks

• A chemical hazard is the inherent property of a substance or procedure that can cause harm,
  • (e.g. toxicity, flammability)
• whereas risk evaluates the likelihood and severity of harm arising during use.

6Safety Control Measures in Chemistry

• Implementing the hierarchy of control measures involves elimination of hazardous reagents, substitution with safer alternatives, using fume cupboards,
• and wearing personal protective equipment.

7Digital vs Analogue Instruments

• Digital measuring devices minimize parallax errors and display clear numerical values,
  • (e.g. digital balances and pH meters)
• whereas analogue thermometers and burettes require careful meniscus alignment.

8Controlled Heating Techniques

• Flammable organic liquids must never be heated directly over a Bunsen flame;
  • instead, controlled heating techniques using electrical heating mantles or water baths ensure safety.

9Volumetric analysis

• Quantitative analysis by measuring volumes of solutions.

10Green chemistry

• The design of processes that reduce waste and hazards.

11Hazard

• A property of a substance that can cause harm.

12Melting

• Solid to liquid at the melting point, absorbing energy.

13Temperature

• A measure of the average kinetic energy of the particles.

14Substance

• A material with only one type of particle.

Section 25 of 33Heating under reflux

Purpose

• Purpose many organic reactions are slow at room temperature,
• so the mixture is boiled while a condenser prevents loss of volatile reactants, solvents or products.

Contrast

• Contrast unlike distillation, reflux keeps all substances in the flask.

Examples

• Examples oxidation of a primary alcohol to a carboxylic acid with acidified dichromate, and esterification.
  • (ethyl ethanoate for about 15 minutes)

1

Put the reactants and a few anti-bumping granules in a round-bottomed or pear-shaped flask.

2

Fit a vertical condenser with greased Quickfit joints, water entering at the bottom and leaving at the top.

3

Heat gently with a water bath or electric mantle, never a naked flame for flammables.

4

Boil gently so vapours condense and return to the flask.

5

After the required time, let the mixture cool to room temperature before dismantling.

• Never seal
  • the top of the condenser must stay open to prevent pressure build-up.

Section 26 of 33Separation and purification techniques

Separation and purification techniques

Simple distillation

• Simple distillation separates a liquid from a solid or from a liquid with a very different boiling point.

Fractional distillation

• Fractional distillation separates liquids with close boiling points using a fractionating column.

Filtration, crystallisation, solvent extraction and chromatography

• Filtration, crystallisation, solvent extraction and chromatography the other required separation techniques.

Paper chromatography and TLC

• Paper chromatography and TLC the stationary phase is paper or a silica or alumina layer, the mobile phase is a solvent,
  • and colourless spots are revealed by UV light or a locating agent such as ninhydrin.

Purification

• Purification recrystallisation by dissolving in hot solvent and cooling, followed by a melting point or boiling point check of purity.

Section 27 of 33Melting point determination

Purity

• Purity a pure substance has a sharp, well-defined melting point, whereas an impure one melts over a broad range at a lower temperature.

Identification

• Identification compare with a data book value.

Apparatus

• Thiele tube, oil bath or melt station.

Technique

• Technique use a dry, finely powdered sample, heat very slowly and repeat three times, using the first tube for an approximate range.

Section 28 of 33Heating to constant mass

• Purpose
  • determines the amount of water or volatile substances in a sample, such as water of crystallisation in hydrated copper(II) sulfate.

1

Record the mass of an empty crucible, add the sample and record the initial mass.

2

Heat strongly, then cool in a desiccator and reweigh.

3

Repeat heating, cooling and weighing until two successive masses agree.

4

Calculate the mass lost and the moles of water.

5

Divide the moles of water by the moles of anhydrous salt to deduce the formula.

Handling

• Handling use tongs for the hot crucible and cool in a desiccator so it does not absorb moisture before weighing.

Warm weighing

• Warm weighing never weigh the crucible hot.

Section 29 of 33Colorimetry and spectrophotometry

Principle

• Principle both measure how much light a solution absorbs at specific wavelengths to find its concentration.

Colorimeter

• Colorimeter the detector measures light intensity related to concentration, quick but less precise at low concentrations.

Spectrophotometer

• Spectrophotometer measures absorbance at each wavelength, giving an absorption spectrum, and is highly sensitive and accurate.

Colorimetry and spectrophotometry

Measure the absorbance of solutions of known concentration.

Plot absorbance against concentration to make a calibration curve.

Read the unknown concentration from the line using its absorbance.

Filter choice

• Filter choice choose the filter or wavelength complementary to the colour of the solution, so absorbance is greatest.

Good practice

• Good practice use matched cuvettes, wipe the clear faces, zero with a blank and use the same wavelength throughout.

Uses

• Uses research, quality control, drug analysis, environmental monitoring and food testing.

Section 30 of 33Molecular modelling

Physical models

• Physical models plastic balls and sticks show geometry, bond angles and spatial arrangement.

Digital models

• Digital models software builds accurate 3D models of molecules, including electronic structure, from data such as bond lengths and angles.

Advantages

• Advantages handle complex molecules and show molecular motion and reactions.

Predictions

• Predictions molecular properties, behaviour in different environments and interactions, used in drug design and materials science.

Section 31 of 33More from the specification

1Risk assessment on volumetric scale

• Risk assessment on volumetric scale evaluating safety hazards and chemical risks provides guidance when scaling laboratory reactions up to larger volumetric scale.

2Volumetric Flasks and Standard Solutions

• A volumetric flask is designed to contain an accurately known volume of liquid at a calibrated temperature when preparing a standard chemical solution.

3Burette Precision in Titrations

• An analogue burette allows delivered liquid volume to be measured to the nearest 0.05 cm³, reading from the bottom of the liquid meniscus.

4Volumetric Titration Analysis

• Quantitative titrations use a primary standard and suitable indicator to determine the unknown concentration of an acid or base with high precision.

5Laboratory Hazard Management

• Evaluating chemical hazards allows implementation of precautions to minimize potential experimental risk.
  • (such as corrosive solutions or toxic fumes)

6Volumetric Flasks and Standard Solutions

• A volumetric flask is designed to contain an accurately known volume of liquid at a calibrated temperature when preparing a standard chemical solution.

7Burette Precision in Titrations

• An analogue burette allows delivered liquid volume to be measured to the nearest 0.05 cm³, reading from the bottom of the liquid meniscus.

8Volumetric Titration Analysis

• Quantitative titrations use a primary standard and suitable indicator to determine the unknown concentration of an acid or base with high precision.

9Laboratory Hazard Management

• Evaluating chemical hazards allows implementation of precautions to minimize potential experimental risk.
  • (such as corrosive solutions or toxic fumes)

10Standard solution

• A solution whose concentration is known accurately.

11Volumetric analysis

• Quantitative analysis by measuring volumes of solutions.

12Green chemistry

• The design of processes that reduce waste and hazards.

13Volumetric pipette

• Apparatus measuring one fixed volume accurately.

14Hazard symbol

• Warning sign showing a danger.

15Volumetric flask

• A flask calibrated to contain one exact volume, used to make standard solutions.

16Volume

• The space taken up by an object.

17Titration

• Method for finding the volume of one solution that reacts exactly with another.

18Hazard

• A property of a substance that can cause harm.

19Indicator

• Substance whose colour depends on pH.

20Burette

• Apparatus delivering a variable volume accurately, 0-50 cm³.

Section 32 of 33Key terms

Term	Meaning
Hazard	The potential of a substance to cause harm.
Risk	The likelihood that harm will occur in a specific situation.
Primary standard	A pure, stable, high-molar-mass substance used to make a standard solution.
Concordant titres	Titres that agree within 0.10 cm3.
Reflux	Heating with a vertical condenser so vapour returns to the flask.
Green chemistry	The design of processes that reduce waste and hazards.

Term	Meaning
Uncertainty	The range within which the true value is expected to lie.
Accuracy	Closeness of a result to the true value.
Precision	Closeness of repeated measurements to each other.
Outlier	A result that differs greatly from the others.
Concordant	Results that agree within 0.10 cm3.

Term	Meaning
Standard solution	A solution whose concentration is accurately known.
Burette	A graduated tube for delivering variable volumes.
Meniscus	The curved surface of a liquid in a tube.
End point	The point at which the indicator changes colour.
Volumetric flask	A flask calibrated to hold one precise volume.

Term	Meaning
Reflux	Heating with continual condensation and return of vapour to the flask.
Distillation	Separation by boiling and condensing.
Constant mass	A mass that no longer changes after repeated heating, cooling and weighing.
Calibration curve	A graph of absorbance against known concentrations.
Spectrophotometry	Measurement of absorbance at specific wavelengths.

Section 33 of 33Exam technique

1

Saying wearing eye protection removes the hazard — it only reduces the risk.

2

Giving a vague precaution like wear a lab coat — name the hazard and justify the precaution.

3

Including the rough titre in the mean — average only concordant titres.

4

Mixing seconds and minutes in a table — use one unit throughout.

1

Confusing accuracy with precision — accuracy is closeness to the true value and precision is closeness of repeats.

2

Quoting the uncertainty of one reading as that of a difference — the uncertainties of both readings add.

3

Rinsing the conical flask with the solution before a titration — rinse with deionised water only.

4

Excluding results without justification — state the reason for any outlier.

1

Adding water above the mark in the volumetric flask — make up exactly to the mark.

2

Reading the top of the meniscus — read the bottom of the curve for colourless solutions.

3

Leaving the funnel in the burette — remove it before reading.

4

Including rough titres in the mean — average only concordant titres.

1

Sealing the top of the reflux condenser — leave it open.

2

Heating flammable liquids with a naked flame — use a water bath or electric mantle.

3

Weighing the crucible hot — cool it in a desiccator first.

4

Choosing a colorimeter filter of the same colour as the solution — use the complementary colour.