Suggested practical investigations

This section outlines the suggested practical investigations that form the basis of the Pearson EdExcel Chemistry assessment. While these specific experiments are recommended, schools may substitute them with other suitable practicals that cover the same skills and chemical principles. The goal is to develop proficiency in experimental design, data collection, analysis, and evaluation. Throughout all investigations, adherence to strict safety protocols, including the use of personal protective equipment (PPE) like safety goggles, is mandatory.

### 1. Volumetric Analysis (Titration)

Volumetric analysis is a quantitative technique used to determine the concentration of a solution. The most common form is titration.

* Acid-Base Titrations: This involves determining the concentration of an acid or alkali. A standard solution (a solution of accurately known concentration) is prepared, often by dissolving a known mass of a primary standard in a volumetric flask. A pipette is used to accurately measure a fixed volume of one solution into a conical flask. The other solution is added from a burette until the reaction reaches its equivalence point, which is identified by a sharp colour change of a suitable indicator (e.g., phenolphthalein for strong alkali/weak acid, methyl orange for strong acid/weak alkali). Concordant results (titres within 0.10 cm³) are required for an accurate average. Calculations involve using the formula n = cV and stoichiometric ratios from the balanced chemical equation.

* Redox Titrations: These follow similar principles but involve redox reactions. Common examples include the titration of Fe²⁺ ions with potassium manganate(VII) (KMnO₄) or I₂ with sodium thiosulfate (Na₂S₂O₃). KMnO₄ acts as its own indicator (purple to colourless), while starch is used in iodine/thiosulfate titrations (blue-black to colourless).

### 2. Enthalpy Changes

These investigations measure the heat energy change during a chemical reaction.

* Enthalpy of Neutralisation/Solution: A simple calorimeter, often a polystyrene cup with a lid, is used to minimise heat loss. Known volumes and concentrations of reactants are mixed, and the temperature change (ΔT) is measured using a thermometer or digital probe. The heat energy change (q) is calculated using the formula q = mcΔT, where 'm' is the mass of the solution (often assumed to be the mass of water) and 'c' is the specific heat capacity of water. The molar enthalpy change (ΔH) is then calculated by dividing q by the number of moles of the limiting reactant. A key evaluation point is addressing heat loss to the surroundings as a primary source of systematic error.

### 3. Rates of Reaction

These experiments investigate factors affecting the speed of a reaction, such as concentration, temperature, surface area, and catalysts.

* Monitoring Gas Production: Reactions that produce a gas, like a metal reacting with an acid (e.g., Mg + 2HCl → MgCl₂ + H₂), can be monitored by collecting the gas in an inverted measuring cylinder over water or, more accurately, with a gas syringe. A graph of volume of gas produced against time is plotted, and the initial rate of reaction is determined from the gradient of the tangent at t=0.

* Monitoring Change in Concentration: The reaction between sodium thiosulfate and hydrochloric acid produces a sulfur precipitate, making the solution cloudy. The time taken for a cross drawn on paper beneath the conical flask to become obscured is measured. The rate is considered proportional to 1/time. By varying the initial concentration of a reactant, the order of reaction can be investigated.

### 4. Qualitative Analysis

This involves identifying unknown inorganic ions present in a sample through a series of chemical tests.

* Test for Cations: Flame tests identify Group 1 and 2 metal ions by their characteristic colours (e.g., Li⁺: red, Na⁺: yellow-orange, K⁺: lilac). For other cations, particularly transition metals, adding a precipitating agent like sodium hydroxide (NaOH) or aqueous ammonia (NH₃) produces characteristically coloured precipitates (e.g., Cu²⁺ gives a blue precipitate with NaOH).

* Test for Anions: A systematic approach is required. Carbonates (CO₃²⁻) are tested first by adding dilute acid and bubbling the gas produced through limewater (calcium hydroxide), which turns cloudy. Sulfates (SO₄²⁻) are identified by adding acidified barium chloride, which forms a white precipitate. Halides (Cl⁻, Br⁻, I⁻) are tested by adding acidified silver nitrate, forming precipitates of different colours (AgCl: white, AgBr: cream, AgI: yellow).

### 5. Synthesis and Purification

These practicals focus on preparing a pure sample of a substance and assessing its purity.

* Preparation of an Organic Compound: An example is the synthesis of an ester like ethyl ethanoate from ethanol and ethanoic acid using an acid catalyst. The process often involves heating under reflux to prevent volatile reactants from escaping. The product is then separated using distillation.

* Purification Techniques: A solid product is purified by recrystallisation, which involves dissolving the impure solid in a minimum amount of hot solvent and allowing it to cool slowly, forming pure crystals. These are separated by filtration (using a Büchner funnel) and washed with cold solvent. Purity can be assessed by measuring the melting point and comparing it to the known data book value. The efficiency of the synthesis is evaluated by calculating the percentage yield.