In A-Level Chemistry, practical skills are not just assessed in the lab for your Practical Endorsement (PAGs); they are heavily tested in your written exam papers. Understanding the "how" and the "why" behind every apparatus setup, measurement, and purification method is key to scoring top marks in these questions.

What you'll learn

• How to accurately measure physical quantities and prepare standard solutions.
• The fundamental setups for organic synthesis, including heating under reflux and distillation.
• Essential purification methods for both solid and liquid products.
• How to measure pH, chromatographic separation, and reaction rates using different experimental techniques.

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1. Fundamental Measurements and Safety

Accurate quantitative data is the backbone of experimental chemistry. You must know which tool is appropriate for recording specific physical quantities and how to use them safely.

Measuring Mass, Volume, Temperature, and Time

• Mass: Measured using a 2-decimal or 3-decimal place digital analytical balance. Always tared (zeroed) with the weighing boat on the pan before adding chemical reagents.
• Volume:
  • For approximate volumes: Use a graduated measuring cylinder.
  • For highly accurate fixed volumes: Use a volumetric pipette (e.g., $25.0{cm}^3$, which has a typical uncertainty of $\pm 0.06{cm}^3$).
  • For variable added volumes: Use a burette (readings must always be recorded to two decimal places, with the second decimal place being a $0$ or a $5$).
• Temperature: Measured using a thermometer or a digital temperature probe. Always read the scale at eye level to avoid parallax error.
• Time: Measured using a digital stopwatch, usually to the nearest second or tenth of a second depending on the rate of reaction.

Safe Heating Methods

Direct heating with a Bunsen burner is not always safe. The choice of heating source depends on the physical properties of the chemicals involved:

• Water baths: Ideal for heating flammable substances or reaction mixtures below $100^{\circ }C$ without an open flame.
• Electric heating mantles: Essential for heating flammable organic solvents above $100^{\circ }C$ safely, as they eliminate the risk of igniting vapours.
• Sand baths: Provide even, high-temperature heating up to several hundred degrees Celsius.

Safe Handling of Chemicals

You must be able to recognise hazards and suggest appropriate safety precautions:

• Corrosive / Irritant substances (e.g., concentrated acids or alkalis): Wear safety goggles and chemical-resistant gloves.
• Toxic substances (e.g., chlorine gas, volatile halogenoalkanes): Handle inside a fully functioning fume cupboard to avoid inhalation of toxic vapours.
• Flammable substances (e.g., ethanol, propanone): Ensure there are no naked flames in the laboratory; use water baths or electric heaters.

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2. Quantitative Techniques: Standard Solutions and Titrations

A key quantitative skill is the preparation of a standard solution followed by its analysis via titration.

Making Up a Standard Solution

To prepare a $250.0{cm}^3$ standard solution from a solid acid or anhydrous salt:

1. Weigh the solid sample in a weighing boat using a balance.
2. Transfer the solid to a clean beaker. Re-weigh the weighing boat to determine the precise mass of solid transferred (this is called weighing by difference).
3. Add deionised water to the beaker and stir with a glass rod until the solid has dissolved completely.
4. Transfer the solution quantitatively into a $250.0{cm}^3$ volumetric flask using a clean funnel.
5. Rinse the beaker, glass rod, and funnel with deionised water, adding all washings directly into the volumetric flask to ensure no solute is lost.
6. Add deionised water dropwise using a teat pipette until the bottom of the meniscus is exactly aligned with the graduation mark on the neck of the flask at eye level.
7. Stopper the flask and invert it at least 10 times to ensure thorough mixing and a uniform concentration.

Indicators and Titration Curves

When performing an acid–base titration, you must select an indicator that changes colour rapidly within the vertical section (the pH jump) of your titration curve:

• Strong Acid & Strong Base: The pH jump is large (approx. pH 3 to 11). Phenolphthalein (colourless to pink) or methyl orange (red to yellow) can be used.
• Strong Acid & Weak Base: The pH jump is in the acidic region (approx. pH 3 to 7). Methyl orange is suitable; phenolphthalein is not, as its transition range is too high.
• Weak Acid & Strong Base: The pH jump is in the basic region (approx. pH 7 to 11). Phenolphthalein is suitable; methyl orange is not, as its transition range is too low.
• Weak Acid & Weak Base: There is no sharp vertical pH jump. Indicators are unsuitable; instead, a pH meter must be used to find the equivalence point.

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3. Organic Synthesis and Purification Techniques

Organic chemistry relies heavily on isolating and purifying intermediate and final products.

Heating Under Reflux vs. Simple Distillation

• Heating under Reflux: Continuous boiling and condensation of a reaction mixture. A vertical condenser is attached directly to the reaction flask. This allows reactions to be heated for long periods without losing volatile reactants or products through evaporation.
• Simple Distillation: Used to separate liquids with significantly different boiling points, or to separate a volatile liquid product from non-volatile impurities. The condenser is sloped downwards, allowing the condensed vapour to be collected in a separate receiving vessel.

Purification of a Solid: Recrystallisation

When preparing a solid product (e.g., aspirin or benzoic acid), the solid will contain impurities. Recrystallisation is used to remove these impurities based on their differing solubilities.

1. Dissolve the impure solid in the minimum volume of hot solvent. The minimum volume ensures that the solution is saturated, which maximises the yield of crystals when cooled.
2. Perform a hot gravity filtration using a pre-heated funnel and fluted filter paper. This step removes any insoluble impurities while keeping the desired product dissolved in the hot solvent.
3. Cool the filtrate slowly to room temperature, then place it in an ice bath. As the temperature drops, the solubility of the product decreases rapidly, and pure crystals precipitate out. Soluble impurities remain dissolved in the cold solvent.
4. Filter the mixture under reduced pressure using a Buchner funnel and a vacuum pump. This is much faster than gravity filtration and helps dry the crystals by drawing air through them.
5. Wash the crystals with a small volume of ice-cold solvent to rinse away any remaining soluble impurities from the surface without dissolving the product.
6. Dry the crystals in a low-temperature oven or desiccator.

Purification of a Liquid: Separating Funnel

When an organic liquid product is synthesised, it is often mixed with aqueous reagents and impurities.

1. Pour the mixture into a separating funnel and add water or an aqueous washing solution (e.g., sodium hydrogencarbonate to neutralise acid impurities).
2. Stopper the funnel, invert it gently several times, and open the tap periodically to release any built-up gas pressure.
3. Allow the mixture to settle into two distinct layers (aqueous and organic) based on density.
4. Remove the stopper and open the tap to run off the lower layer into a beaker, then collect the upper layer in a separate beaker.
5. Add an anhydrous inorganic salt (e.g., anhydrous ${MgSO}_4$ or ${CaCl}_2$) to the organic liquid. These act as drying agents by binding to trace water.
6. Filter or decant the dry organic liquid from the solid drying agent. The liquid should appear completely clear (no longer cloudy).
7. Perform a final redistillation, collecting only the fraction that distills at the boiling point of your pure target compound.

Assessing Purity: Melting Point Analysis

Pure organic solids have sharp, well-defined melting points that match literature values. Impurities lower the melting point and cause the solid to melt over a wider temperature range.

• Place a tiny sample of dry solid inside a sealed glass capillary tube.
• Insert the capillary into a melting point apparatus and heat slowly.
• Record the temperature when melting starts and when it is fully complete.

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4. Analytical and Physical Techniques

Thin-Layer Chromatography (TLC)

TLC is used to separate and identify components in a mixture based on their relative affinity for a stationary phase and a mobile phase.

• Stationary phase: A thin layer of silica or alumina on a rigid plastic or glass sheet. Polar substances adsorb strongly to this phase.
• Mobile phase: A liquid solvent that moves up the plate by capillary action. Less polar substances dissolve better in the mobile phase and travel further.
• Visualisation: Non-coloured spots can be visualised using a UV lamp or by exposing the plate to iodine vapours or ninhydrin spray.

Electrochemical Cells

To measure standard electrode potentials ($E^{\circ }$):

1. Set up two half-cells containing the metal electrodes in contact with solutions of their ions at standard concentration ($1.00{mol\; dm}^{-3}$).
2. Connect the two solutions using a salt bridge (usually filter paper soaked in saturated potassium nitrate, ${KNO}_3$). This completes the circuit by allowing ions to flow, without letting the two solutions mix.
3. Connect the two electrodes using a high-resistance voltmeter to measure the cell potential ($E_{cell}^{\circ }$).

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5. Measuring Rates of Reaction

You must be familiar with monitoring the progress of a chemical reaction over time.

1. Continuous Monitoring Method

This involves measuring how a physical property changes continuously as the reaction proceeds. Examples include:

• Gas syringe / Gas volume collection: Measuring the volume of gas produced over time.
• Mass loss: Measuring the mass of a flask containing a gas-evolving reaction on a digital balance over time.
• Colorimetry: Measuring the change in light absorption over time if a coloured reactant or product changes concentration.

2. Initial Rates Method (Clock Reactions)

In a clock reaction, you measure the time ($t$) taken for a small, fixed amount of a product to form, marked by a sudden, easily visible change (such as a colour change from clear to blue-black in the iodine-clock reaction).

• The average rate during this initial stage is assumed to be constant and proportional to:

$$Initial\; Rate\propto \frac{1}{t}$$

• By repeating the experiment at varying concentrations, you can determine the order of reaction with respect to each reactant.

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