What are the steps in purifying an impure organic solid by recrystallisation?

Dissolve it in the minimum volume of hot solvent, filter while hot if insoluble impurities are present, cool so crystals form, filter the crystals under reduced pressure, wash them with a little ice-cold solvent, and dry them.

Why is the minimum volume of hot solvent used in recrystallisation?

So the hot solution is saturated and the product crystallises on cooling, leaving as little product dissolved as possible and giving the maximum yield.

What properties should a solvent have for recrystallising an organic solid?

The product should be very soluble in the hot solvent but only slightly soluble in the cold solvent, and the solvent must not react with the product.

How does recrystallisation remove soluble impurities from an organic solid?

The impurities are present in small amounts, so they stay dissolved in the cold solvent when the product crystallises and are removed with the filtrate.

In recrystallisation, why is the hot solution filtered before it is cooled?

To remove insoluble impurities while the product is still dissolved. A hot funnel and fluted filter paper stop the product crystallising in the funnel.

Why are crystals from a recrystallisation collected by filtration under reduced pressure using a Büchner funnel?

Suction draws the solvent through quickly, separating the crystals from the solution that holds the impurities and leaving them drier.

AQA A-Level Chemistry 7405

Atomic Structure

Topic overview Fundamental Particles Mass Number and Isotopes Time of Flight Mass Spectrometry: Ionisation and Acceleration Time of Flight Mass Spectrometry: Drift, Detection and Calculating m/z Electron Configuration: Sub-shells and Orbitals Electron Configuration: Writing Configurations for Atoms and Ions Ionisation Energies and Periodic Trends

Amount of Substance

Topic overview Relative Atomic and Molecular Mass The Mole and the Avogadro Constant The Ideal Gas Equation Empirical and Molecular Formula Balanced Equations and Reaction Calculations Atom Economy and Percentage Yield Acid-Base Titrations: Principles, Indicators and Apparatus Acid-Base Titrations: Standard Solutions and Calculations

Bonding

Topic overview Ionic Bonding Covalent, Dative Covalent and Metallic Bonding Crystal Structures: Giant Ionic Lattices and Diamond Crystal Structures: Graphite and Simple Molecular Solids Shapes of Molecules and Ions: VSEPR and Electron Pair Repulsion Shapes of Molecules and Ions: Predicting Shapes and Bond Angles Electronegativity and Bond Polarity Intermolecular Forces

Energetics and Thermodynamics

Topic overview Enthalpy Change and Standard Conditions Calorimetry and Heat Change Calculations Hess's Law and Enthalpy Cycles Mean Bond Enthalpies Lattice Enthalpy and Born-Haber Cycles Enthalpies of Solution and Hydration Entropy Change Gibbs Free Energy and Reaction Feasibility

Kinetics

Topic overview Collision Theory and Activation Energy Maxwell-Boltzmann Distribution and the Effect of Temperature Effect of Concentration and Pressure, and Catalysts Rate Equations: The Rate Equation and Orders of Reaction Rate Equations: The Effect of Concentration on Rate The Rate Constant and the Arrhenius Equation Determining Rate Equations: The Initial Rates Method Determining Rate Equations: Continuous Monitoring Rate Equations and Reaction Mechanisms

Chemical Equilibria

Topic overview Dynamic Equilibrium and Le Chatelier's Principle Industrial Compromise Conditions The Equilibrium Constant Kc Partial Pressures and Mole Fractions The Equilibrium Constant Kp

Redox and Electrochemistry

Topic overview Redox: Definitions and the Rules for Oxidation States Redox: Identifying Oxidation and Reduction in a Reaction Half-Equations and Combined Redox Equations Standard Electrode Potentials and the Hydrogen Electrode Electrochemical Cells and EMF Calculations Predicting Redox Reactions from Electrode Potentials Commercial Cells and Fuel Cells

Acids and Bases

Topic overview Brønsted-Lowry Acid-Base Equilibria The pH Scale and Strong Acids The Ionic Product of Water and Strong Bases Weak Acids and Ka pH Curves and Acid-Base Titrations Indicators Buffer Solutions

Periodicity and Group Chemistry

Topic overview s, p, d and f Block Classification Trends in Period 3 Group 2 Trends in Atomic Radius, Ionisation Energy and Melting Point Reactions of Group 2 with Water Solubility Trends of Group 2 Hydroxides Solubility Trends of Group 2 Sulfates Uses of Group 2 Compounds Test for Sulfate Ions Trends in Electronegativity and Boiling Point of Halogens Halogens as Oxidising Agents and Displacement Reactions Halide Ions as Reducing Agents: Chloride and Bromide Halide Ions as Reducing Agents: Iodide, and the Equations Test for Halide Ions Using Silver Nitrate Tests for Ammonium, Group 2, Hydroxide and Carbonate Ions Reactions of Chlorine with Water Reactions of Chlorine with Cold Dilute NaOH and Water Treatment

Period 3 Elements and Their Oxides

Topic overview Reactions of Na and Mg with Water Period 3 Elements with Oxygen: The Oxides Formed and Their Bonding Period 3 Elements with Oxygen: Structures and Oxidation States Melting Points of Period 3 Oxides Period 3 Oxides with Water: Sodium and Magnesium Oxides Period 3 Oxides with Water: The Acidic Oxides and the pH Trend Period 3 Oxides with Acids and Bases: The Reactions Period 3 Oxides with Acids and Bases: Acidic, Basic and Amphoteric

Transition Metals

Topic overview General Properties of Transition Metals Complex Ions: Ligands, Coordinate Bonds and Denticity Complex Ions: Why a Species Can Act as a Ligand, and Coordination Number Substitution Reactions with Monodentate Ligands Substitution Reactions with Bidentate and Multidentate Ligands Haemoglobin and the Chelate Effect Shapes of Complex Ions and Stereoisomerism Formation of Coloured Ions and the d-d Transition Colorimetry and Concentration Determination Variable Oxidation States and Vanadium Chemistry Tollens' Reagent and Redox Titrations Heterogeneous Catalysis Vanadium(V) Oxide and the Contact Process Homogeneous Catalysis and Autocatalysis Metal-Aqua Ions and Acidity Metal Aqua Ions with Bases: Reactions with Hydroxide and Ammonia Metal Aqua Ions with Bases: Why They React, and Their Equations Amphoteric Hydroxides Identifying Transition Metal Ions by Test-Tube Reactions

Introduction to Organic Chemistry

Topic overview Nomenclature and Formula Representations Homologous Series and General Formulae IUPAC Naming: Stems, Prefixes and Suffixes Free Radical and Curly Arrow Mechanisms Structural Isomerism E-Z Stereoisomerism and CIP Priority Rules Chirality and Enantiomers Racemic Mixtures and Optical Activity

Hydrocarbons

Topic overview Alkanes, Fractional Distillation and Cracking Combustion of Alkanes and Pollutants Free Radical Chlorination of Alkanes Alkenes: Structure, Bonding and Electrophilic Addition Markovnikov's Rule and Carbocation Stability Addition Polymers and Repeating Units Properties and Uses of Polyalkenes

Halogenoalkanes, Alcohols and Organic Analysis

Topic overview Nucleophilic Substitution of Halogenoalkanes Effect of Bond Enthalpy on Reaction Rate Elimination Reactions and Competing Mechanisms CFCs and Ozone Depletion Industrial Production of Alcohols Biofuels and Carbon Neutrality Classification and Oxidation of Alcohols Distinguishing Aldehydes and Ketones Elimination of Water from Alcohols Test-Tube Reactions for Functional Groups Mass Spectrometry and Molecular Formula Infrared Spectroscopy, Functional Groups and Global Warming

Carbonyls, Carboxylic Acids and Derivatives

Topic overview Oxidation of Aldehydes and Distinguishing Tests Reduction of Carbonyls with NaBH4 Nucleophilic Addition with KCN and Hydroxynitrile Formation Hazards of KCN and Formation of Enantiomers Structure and Acidity of Carboxylic Acids Esterification and the Uses of Esters Vegetable Oils, Fats and the Hydrolysis of Esters Soap and Biodiesel Production from Fats Structure of Acid Anhydrides, Acyl Chlorides and Amides Nucleophilic Addition-Elimination Reactions of Acyl Chlorides Acid Anhydrides and the Industrial Synthesis of Aspirin Purifying organic solids and liquids

Aromatic Chemistry and Amines

Topic overview Bonding and Stability of Benzene Electrophilic Substitution: Nitration Electrophilic Substitution: Friedel-Crafts Acylation Preparation of Aliphatic and Aromatic Amines Base Strength of Amines Amines as Nucleophiles, and Quaternary Ammonium Salts Amines with Acyl Chlorides and Acid Anhydrides

Polymers, Amino Acids, Proteins and DNA

Topic overview Condensation Polymers: Polyesters Condensation Polymers: Polyamides Intermolecular Forces in Condensation Polymers Biodegradability and Disposal of Polymers Amino Acids and Zwitterions Peptide Bonds and Protein Structure Hydrogen Bonding and Disulfide Bridges in Proteins Hydrolysis of Proteins and TLC Identification Enzymes, Active Sites and Inhibitors in Drug Design DNA: Nucleotides, the Double Helix and Base Pairing Cisplatin and Anticancer Drug Action

Organic Synthesis, NMR and Chromatography

Topic overview Multi-Step Synthesis Routes Green Chemistry Principles in Synthesis Design Principles of NMR and the Delta Scale Carbon-13 NMR Interpretation Proton NMR Chemical Shift and Integration Spin-Spin Splitting and the n+1 Rule TMS and Deuterated Solvents Thin-Layer Chromatography and Rf Values Column and Gas Chromatography, and GC-MS

Carbonyls, Carboxylic Acids and Derivatives

Purifying organic solids and liquids

3 things to know
Papers 2 and 3

Overview

Required practical 10: recrystallising an impure organic solid, testing its purity by melting point, and purifying an organic liquid with a separating funnel, a drying agent and redistillation.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

How is the purity of an organic solid tested?

Process

How is a separating funnel used to purify an organic liquid product?

Process

How does an impurity affect the melting point of an organic solid?

Questions from this subtopic

Each one with the model answer underneath.

1
What are the steps in purifying an impure organic solid by recrystallisation?
Model answer
Dissolve it in the minimum volume of hot solvent, filter while hot if insoluble impurities are present, cool so crystals form, filter the crystals under reduced pressure, wash them with a little ice-cold solvent, and dry them.
2
Why is the minimum volume of hot solvent used in recrystallisation?
Model answer
So the hot solution is saturated and the product crystallises on cooling, leaving as little product dissolved as possible and giving the maximum yield.
3
What properties should a solvent have for recrystallising an organic solid?
Model answer
The product should be very soluble in the hot solvent but only slightly soluble in the cold solvent, and the solvent must not react with the product.
4
How does recrystallisation remove soluble impurities from an organic solid?
Model answer
The impurities are present in small amounts, so they stay dissolved in the cold solvent when the product crystallises and are removed with the filtrate.
5
In recrystallisation, why is the hot solution filtered before it is cooled?
Model answer
To remove insoluble impurities while the product is still dissolved. A hot funnel and fluted filter paper stop the product crystallising in the funnel.
6
Why are crystals from a recrystallisation collected by filtration under reduced pressure using a Büchner funnel?
Model answer
Suction draws the solvent through quickly, separating the crystals from the solution that holds the impurities and leaving them drier.

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