Experimental design

Chemistry is a practical science built on the scientific method. This is a logical process for discovering new knowledge. It starts with an observation that leads to a question. You then form a hypothesis, which is a testable prediction or explanation. To test it, you plan and carry out a controlled experiment. In any experiment, it's crucial to understand variables. The independent variable is the one factor you deliberately change to see its effect. The dependent variable is what you measure to see if it changes; its value depends on the independent variable. All other factors that could possibly affect the outcome must be kept constant; these are the controlled variables. This ensures a fair test, where only the independent variable is causing the observed changes in the dependent variable.

Choosing the correct apparatus is vital for accurate and reliable results. Different pieces of glassware are designed for different levels of precision. For approximate volumes, a beaker is suitable. A measuring cylinder offers better accuracy. For preparing solutions or for titrations where high precision is needed, volumetric flasks, burettes, and pipettes are essential. A pipette is designed to deliver a single, highly accurate fixed volume (e.g., 25.0 cm³). A burette can deliver variable, precise volumes and allows you to measure the exact volume delivered. A volumetric flask is used to make up a solution to a specific volume with high accuracy.

Chemists often need to separate mixtures into their pure components. The method used depends on the physical properties of the substances.

• Filtration: Separates an insoluble solid from a liquid (e.g., sand from water) using filter paper and a funnel. The solid left on the paper is the residue, and the liquid that passes through is the filtrate.
• Crystallisation: Separates a soluble solid from a solution (e.g., salt from water). The solution is heated to evaporate some of the solvent, creating a saturated solution. Upon cooling, the solid forms pure crystals.
• Simple Distillation: Separates a liquid from a dissolved solid (e.g., pure water from salt water). The liquid is boiled, turns into a vapour, then passes through a condenser where it cools and turns back into a pure liquid (the distillate).
• Fractional Distillation: Separates a mixture of liquids with different boiling points (e.g., ethanol and water). The mixture is heated, and the liquid with the lower boiling point vaporises first, rises up a fractionating column, cools, and condenses separately.
• Paper Chromatography: Separates mixtures of soluble substances (e.g., inks). A spot of the mixture is placed on a baseline on filter paper (the stationary phase). The bottom of the paper is dipped in a solvent (the mobile phase). As the solvent moves up the paper, it carries the different substances at different speeds, separating them based on their solubility and attraction to the paper.

Titration is a highly accurate technique used to determine the concentration of an unknown solution. It involves reacting a solution of known concentration (a standard solution) with a solution of unknown concentration. For an acid-alkali titration, a pipette is used to measure a precise volume of one solution into a conical flask. A few drops of an indicator (e.g., methyl orange or phenolphthalein) are added. The other solution is placed in a burette and added drop by drop to the flask while swirling. The point at which the indicator just changes colour is the end point, signifying that the reaction is complete. By recording the volume added from the burette (the titre), you can calculate the unknown concentration.

In the lab, specific reactions are used to prepare gases. The method of collection depends on the gas's properties (density and solubility in water). Each gas also has a unique chemical test for identification.

• Hydrogen (H₂): Prepared by reacting a metal (like zinc) with an acid (like dilute HCl). It's less dense than air, so can be collected by downward displacement of air or over water. Test: A lit splint makes a 'squeaky pop'.
• Oxygen (O₂): Prepared by the catalytic decomposition of hydrogen peroxide (H₂O₂), using manganese(IV) oxide as a catalyst. It is slightly denser than air and insoluble, so it is best collected over water. Test: Relights a glowing splint.
• Carbon Dioxide (CO₂): Prepared by reacting a carbonate (like calcium carbonate) with an acid (like dilute HCl). It's denser than air and slightly soluble, so it's collected by upward displacement of air or over water (for short periods). Test: Bubbling it through limewater (calcium hydroxide solution) turns it milky/cloudy.
• Ammonia (NH₃): Prepared by gently heating an ammonium salt (like ammonium chloride) with a strong base (like calcium hydroxide). It is less dense than air and very soluble in water, so must be collected by downward displacement of air. Test: Turns damp red litmus paper blue.

Qualitative analysis is the process of identifying the chemical substances present in a sample. You can identify unknown cations (positive ions) and anions (negative ions) using a series of simple test-tube reactions that usually produce a precipitate (an insoluble solid) or a gas. Cation Tests:

1. Add aqueous sodium hydroxide (NaOH) dropwise, then in excess. Observe any precipitate colour and whether it redissolves.
2. Add aqueous ammonia (NH₃) dropwise, then in excess. Observe any precipitate colour and whether it redissolves.

Key results: Cu²⁺ gives a blue precipitate with both. Fe²⁺ gives a green precipitate. Fe³⁺ gives a red-brown precipitate. Al³⁺, Zn²⁺, and Ca²⁺ all give a white precipitate with NaOH; in excess NaOH, Al³⁺ and Zn²⁺ precipitates redissolve but Ca²⁺ does not. To distinguish Al³⁺ and Zn²⁺, use aqueous ammonia: the Zn²⁺ precipitate redissolves in excess ammonia, but the Al³⁺ precipitate does not. Anion Tests:

• Carbonate (CO₃²⁻): Add dilute acid. Effervescence (fizzing) is seen. Bubble the gas through limewater; it turns milky.
• Sulfate (SO₄²⁻): Add dilute nitric acid, then aqueous barium nitrate. A white precipitate (barium sulfate) forms.
• Halide (Cl⁻, Br⁻, I⁻): Add dilute nitric acid, then aqueous silver nitrate. Cl⁻ gives a white precipitate. Br⁻ gives a cream precipitate. I⁻ gives a yellow precipitate.