Section B

Measuring mass

• Mass is typically measured with a digital balance accurate to two decimal places

• Always tare (zero) the balance before weighing

• Usually recorded in grams although the SI unit is kg.

Measuring the volume of liquids

• Depends on how accurate the measurement needs to be

• Measuring cylinders → used for approximate volumes

• Volumetric pipettes → most accurate for measuring a fixed volume (used to deliver an exact volume to something else)

• A calibration mark indicates the volume; align the bottom of the meniscus 

• Burettes → Most accurate for measuring a variable volume 

• The scale runs top to bottom 

Measuring temperature

Temperature is measured using a thermometer or a digital temperature probe

Thermometer → use the thermal expansion of a liquid (eg mercury) in a capillary tube

Commonly give reading to the nearest degree

May take longer to equilibrate and are less precise 

Digital temperature probe → use electronic sensors to detect temperature 

Higher precision 

Often used in data logging and continuous monitoring 

Can reduce human error 

Measuring length

• Rulers can be used 

• Standard unit is meters 

Measuring the pH of a solution

Can be measured using an indicator or a digital pH meter

Digital pH meter → use an electrode with a thin glass membrane that allows hydrogen ions to pass through

These ions affect the voltage, which is converted into a pH value

Provide precise and quantitative results

Indicators → change colour depending on the pH of the solution

Measuring electric current

Current is measured using an ammeter

Ammeters should always be connected in series with the part of the circuit you wish to measure the current through.

Ammeters can be either:

Digital → with an electronic display

Analogue → with a needle and scale

Measuring electric current: Analogue ammeters 

• Should be checked for zero errors before using 

• They are also subject to parallax error, so you must always read the meter from a position directly perpendicular to the scale. 

Measuring electric current: Digital ammeters 

• Can measure very small currents 

• Display more precise values 

• Digital ammeters should be checked for zero errors

• Digital displays may flicker back and forth between values, and a judgment must be made as to which to write down. 

Measuring the electric potential difference 

Potential difference (voltage) is measured using a voltmeter 

Can be analogue (scale and needle display) or digital (electronic)

Voltmeters are always connected in parallel with the component being tested. 

They measure the difference in electrical potential between two points in a circuit. 

How to prepare a standard solution

• Weigh out a precise quantity of the solid (solute) using an electronic scale

• Add the solute to a small volume of distilled water and pre-dissolve the solid

• Transfer the solution using a funnel to a volumetric flask

• Rinse a beaker with distilled water and transfer the washing to the flask

• Fill up to the mark

• Add the stopper and turn the flask over several times to homogenise the solution.

Titration: method

1. Measure a fixed volume of one solution using a volumetric pipette and transfer it into a conical flask

2. Fill the burette with the second solution and record the starting volume

3. Add a few drops of an appropriate indicator to the solution in the conical flask if needed

4. Place a white tile under the flask to make the colour change easier to see

5. Begin the titration by slowly opening the burette tap, adding the titrant to the flask in small portions 

6. Swirl the flask after each addition to mix the solutions thoroughly

7. As you approach the endpoint, slow the addition to dropwise

8. Close the tap as soon as it causes a permanent colour change

9. Repeat the titration until concordant results are obtained.

Separating a mixture of solids: solvation

Example: separating sand from water

Steps:

1. Add the mixture to a suitable solvent (eg water)

2. Stir to help the soluble substances dissolve

3. Insoluble substances will remain undissolved

4. The soluble substances form a solution with the solvent

In water, this process is called hydration

Essential for separation techniques like filtration and crystallisation

Filtration

Used to separate an undissolved solid from a mixture of solid and liquid or solution using filter paper.

Examples → separating sand from water, collecting crystals after crystallisation

Steps:

1. Place a filter paper in a funnel above a clean beaker

2. Pour the mixture into the funnel

3. The liquid (filtrate) passes through the filter paper

4. The solid (residue) remains on the paper

Vacuum filtration can be used for very fine solids that clog filter paper

Centrifugation can also be used to separate solid-liquid mixtures, especially when particles are too small or dense for standard filtration.

Filtration is often used after solvation to remove undissolved solids 

Crystallisation

Used to separate a dissolved solid from a solution when the solid is more soluble in a hot solvent than in a cold one

Example → recovering copper(II) sulphate crystals from solution

Steps:

1. Heat the solution gently to evaporate some solvent

2. Allow the solution to cool until saturated

3. Test saturation by using a cold glass rod (crystals form on the rod)

4. Leave the saturated solution to cool slowly

5. Crystals will form as the solubility decreases

6. Filter to collect the crystals

7. Wash the crystals with distilled water and dry on filter paper.

Do not boil to dryness - this may prevent crystal formation

Cool slowly, allowing larger crystals to grow

Recrystallisation 

Used to purify an impure solid by dissolving it in a hot solvent and allowing it to crystallise as the solution cools

Examples → purifying benzoic acid, removing solid and soluble impurities from a sample 

Steps:

1. Add a minimum amount of hot solvent to the impure solid until it dissolves

2. If the solid impurities remain, perform hot filtration to remove them

3. Allow the solution to cool slowly to room temperature

4. Crystals of the purified product will begin to form as solubility decreases

5. Collect the crystals by filtration (Buchner flask)
   Wash the crystals with fresh cold solvent to remove soluble impurities 

6. Dry the crystals on filter paper

Simple distillation

Used to separate a solvent from a solute or a pure liquid from a mixture based on differences in boiling point

Examples → separating water from a salt solution, recovering pure solvent from a reaction mixture

Steps:

1. Heat the solution in a round-bottomed flask

2. The liquid with the lowest boiling point evaporates first

3. Vapour passes through the condenser, where it cools and condenses

4. The distillate (purified liquid) is collected in a clean beaker

5. The solute or other components are left behind in the flask

This method is suitable when the liquid being collected has a significantly lower boiling point than other compounds

A more effective separation of liquids with similar boiling points (ie ethanol and water) is fractional distillation.

Fractional distillation

Used to separate two or more miscible liquids with similar boiling points

Examples → separating ethanol and water, separating components of crude oil on an industrial scale 

Steps: 

1. Heat the mixture in a round-bottomed flask

2. The liquid with the lowest boiling point evaporates first

3. Vapour passes up the fractionating column, allowing for better separation of components 

4. Vapour enters the condenser, cools and condenses into a liquid 

5. The distillate is collected in a beaker

6. As the temperature rises, the next component evaporates and is collected in turn 

7. Stop heating when the target components have been separated 

Use an electric heater when flammable liquids are present 

Paper chromatography

Used to separate dissolved substances in a mixture based on differences in solubility and adsorption to the paper

Examples → analysing dyes in black ink, plant pigments, testing food colourings

Steps:

1. Draw a pencil line near the bottom of the chromatography paper

2. Place small spots of the sample mixture on the line

3. Suspend the paper in a container with a shallow layer of solvent

4. Ensure the spots stay above the solvent level

5. Let the solvent more up the paper by capillary action

6. As the solvent travels, it carries components of the mixture at different rates

7. Allow the paper to dry and observe the separated spots

Uv light or chemical locating agents like ninhydrin help reveal colourless spots 

Melting point determination

• The melting point of a solid is indicative of its purity and identity

• It can be compared to a known value to identify or confirm a compound

• The proximity of a melting point to the actual data book value can express purity

• Impurities tend to lower the melting point of a solid

• Pure substances have sharp, well-defined melting points

• Impure substances have a broad melting point range, ie a large difference between when the substance first melts and when it completely melts

• The sample must be totally dry and fine-powdered

Drying to constant mass

Used to determine the amount of water or volatile substances in a sample

Steps:

1. Record the initial mass using a balance

2. Heat the sample in an oven or drying chamber at a controlled temperature

3. At regular intervals, cool the sample in a desiccator and reweigh it

4. Repeat until the mass stays the same, indicating all moisture has been removed

Commonly used to calculate the water of crystallisation in hydrated transition metal compounds

Heating under reflux

Reflux involves heating a reaction mixture so that it boils, while a condenser prevents the loss of volatile components

This ensures that reactants remain in the system, allowing the reaction to be completed without evaporation

Example → oxidation of a primary alcohol to a carboxylic acid (using acidified potassium dichromate), esterification reactions between an alcohol and a carboxylic acid using a concentrated acid catalyst.

Steps:

1. Use a pear-shaped or bound-bottomed flask

2. Add anti-bumping granules to ensure smooth boiling

3. Heat using a water bath or heating mantle for controlled temperature

4. Fit a vertical condenser using the Quickfit apparatus

5. Run cold water in at the bottom and out at the top of the condenser to maintain efficient condensation

6. The mixture boils gently while vapours condense and return to the flask

7. Once heating is complete, allow the mixture to cool to room temperature

Measuring rate of reaction: colorimetry

• If a solution changes colour during a reaction, this can be used to measure the rate

• The intensity of light reaching the detector is measured every few seconds, and the data is plotted to show how the concentration of the reactants or products changes with time.

• The light intensity is related to the concentration, so the graph represents a graph of the concentration of products or reactants against time.

• Cannot be used to monitor the formation of coloured precipitates as the light will be scattered or blocked by the precipitate.

Measuring rate of reaction: changes in mass

• When a gas is produced in a reaction, it usually escapes from the reaction vessel, so the mass of the vessel decreases

• The mass is measured every few seconds, and the change in mass over time is plotted as the carbon dioxide escapes.

• The mass loss provides a measure of the amount of reactant that remains in the vessel, so the graph is the same as a graph of the amount of reactant against time

• Limitation → the gas must be sufficiently dense or the change in mass is too small to measure on a 2 or 3 decimal balance, ie, hydrogen (mr =2) would not be suitable but carbon dioxide (mr=44) could be used

Measuring concentration changes: titrations

• The concentration of a sample can be measured by performing a titration

• However, the act of taking a sample and analysing it by titration can affect the rate of reaction, and it cannot be done continuously

• To overcome this, samples of the reaction mixture are taken at regular intervals during the course of the reaction

• The reaction in each of the samples is deliberately stopped; this is called quenching

• Quenching freezes the reaction at a specific point in time to allow the concentration to be determined by titration

• Based on the collected data, the rate of reaction can be calculated by determining the change in concentration with time.

Absolute uncertainty

The actual amount by which the quantity is uncertain

Example → if the volume is 5 ± 0.1cm³, the absolute uncertainty is 0.1cm³

Fractional uncertainty 

The absolute uncertainty divided by the quantity itself 

Example → if the volume is 5cm³ ± 0.1cm³,, the fractional uncertainty is 0.1/5.0 = 1/50

Percentage uncertainty

The ratio of the expanded uncertainty to the measured quantity on a scale relative to 100%

Calculated using the following formula: % unc = (uncertainty)/(measured value) x 100

Example → if the volume is 5 ± 0.1cm³, the percentage uncertainty is (0.1)/(5.0) x100

Propagating uncertainty: multiplying or dividing measurements

• When you multiply or divide the experimental measurements, you add together the percentage uncertainties

• You can then calculate the absolute uncertainty from the sum of the percentage uncertainties

Propagating uncertainties: adding or subtracting measurements

• Add together the absolute measurement uncertainties