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OCR Gateway GCSE Chemistry

Revision Notes

Home / GCSE / Chemistry / OCR Gateway / Revision Notes / 2. Elements, compounds & mixtures / 2.1 Purity & Separating Mixtures / 2.1.1 Pure & Impure


2.1.1 Pure & Impure


Pure & Impure

  • In everyday language we use the word pure to describe when something is natural or clean and to which nothing else has been added
  • In chemistry a pure substance may consist of a single element or compound which contains no other substances
  • For example a beaker of a sample of pure water contains only H2O molecules and nothing else
  • If salt were added to the beaker then a mixture is produced
  • A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined
  • The chemical properties of the substances in a mixture remain unchanged
  • Substances in mixtures can be separated by physical means
  • Air for example is a mixture of nitrogen, oxygen and some other gases such as carbon dioxide and argon

Elements, compounds & mixtures, IGCSE & GCSE Chemistry revision notes

Diagram showing how to represent elements, compounds and mixtures using particle diagrams

Melting Point Data

  • Pure substances melt and boil at specific and sharp temperatures e.g. pure water has a boiling point of 100°C and a melting point of 0°C
  • Mixtures have a range of melting and boiling points as they consist of different substances that tend to lower the melting point and broaden the melting point range
  • Melting and boiling points data can therefore be used to distinguish pure substances from mixtures
  • Melting point analysis is routinely used to assess the purity of drugs
  • This is done using a melting point apparatus which allows you to slowly heat up a small amount of the sample, making it easier to observe the exact melting point
  • This is then compared to data tables
  • The closer the measured value is to the actual melting or boiling point then the purer the sample is

Cooling Curves

  • The influence of impurities can be more clearly seen on a heating / cooling curve
  • If the temperature of a liquid is measured as it cools and freezes the data can be used to produce a graph
  • The following graph shows the cooling curve for a sample of a compound
  • The horizontal part of the graph shows that the compound has a sharp melting point, so the compound is pure
Purity - Pure Substance Cooling Curve, IGCSE & GCSE Chemistry revision notes
Cooling curve for a pure substance
  • An impure sample of the compound would produce a gradual decrease in temperature as it freezes as shown in the graph below
Purity - ImPure Substance Cooling CurvePurity - ImPure Substance Cooling Curve                                                                   
 Cooling curve for an impure substance


  • 1. Particles
    • 1.1 The Particle Model
      • 1.1.1 The Particle Model
        • 1.1.2 Limitations of the Particle Model
        • 1.2 Atomic Structure
          • 1.2.1 Atomic Models Through Time
            • 1.2.2 Atomic Structure
              • 1.2.3 Calculating the Particles
            • 2. Elements, compounds & mixtures
              • 2.1 Purity & Separating Mixtures
                • 2.1.1 Pure & Impure
                  • 2.1.2 Relative Masses
                    • 2.1.3 Empirical Formula
                      • 2.1.4 Alloys
                        • 2.1.5 Separation Techniques
                          • 2.1.6 Chromatography
                          • 2.2 Bonding
                            • 2.2.1 Metals & Non-Metals
                              • 2.2.2 Electronic Structure
                                • 2.2.3 Ionic Bonding
                                  • 2.2.4 Covalent Bonding
                                    • 2.2.5 Giant Covalent Structures
                                      • 2.2.6 Polymers
                                        • 2.2.7 Metals
                                          • 2.2.8 Atomic Structure & the Periodic Table
                                          • 2.3 Properties of materials
                                            • 2.3.1 Carbon
                                              • 2.3.2 Changes of State
                                                • 2.3.3 Bulk Properties
                                                  • 2.3.4 Nanoparticles
                                                • 3. Chemical Reactions
                                                  • 3.1 Introducing Chemical Reactions
                                                    • 3.1.1 Chemical Symbols & Formulae
                                                      • 3.1.2 Deducing Formulae Using Common Ions
                                                        • 3.1.3 Balancing Chemical Equations
                                                          • 3.1.4 Writing Balanced Ionic Equations
                                                            • 3.1.6 Conservation of Mass
                                                              • 3.1.7 Using Moles to Balance Equations
                                                                • 3.1.8 Calculating Masses from Balanced Equations
                                                                • 3.2 Energetics
                                                                  • 3.2.1 Exothermic & Endothermic Reactions
                                                                    • 3.2.2 Calculating Energy Changes
                                                                    • 3.3 Types of Chemical Reactions
                                                                      • 3.3.1 Oxidation & Reduction
                                                                        • 3.3.2 OxidatIon & Reduction in Terms of Electrons
                                                                          • 3.3.3 Acids & Alkalis
                                                                            • 3.3.4 Neutralisation
                                                                              • 3.3.4 Strong & Weak Acids
                                                                                • 3.3.5 Effect of Hydrogen Ion Concentration on pH
                                                                                • 3.4 Electrolysis
                                                                                  • 3.4.1 Electrolysis
                                                                                    • 3.4.2 Electrolysis of Aqueous Solutions
                                                                                      • 3.4.3 Reactions at Electrodes
                                                                                        • 3.4.4 Half Equations
                                                                                      • 4. Predicting & Identifying Reactions & Products
                                                                                        • 4.1 Predicting Chemical Reactions
                                                                                          • 4.1.1 Group 1
                                                                                            • 4.1.2 Group 7
                                                                                              • 4.1.3 Group 0
                                                                                                • 4.1.4 Transition Metals
                                                                                                  • 4.1.5 The Reactivity Series
                                                                                                  • 4.2 Identifying the Products of Chemical Reactions
                                                                                                    • 4.2.1 Testing for Common Gases
                                                                                                      • 4.2.2 Testing for Aqueous Cations
                                                                                                        • 4.2.3 Testing for Aqueous Anions
                                                                                                          • 4.2.4 Flame Tests
                                                                                                            • 4.2.5 Instrumental Analysis
                                                                                                          • 5. Monitoring & Controlling Chemical Reactions
                                                                                                            • 5.1 Monitoring Chemical Reactions
                                                                                                              • 5.1.1 Concentration in g/dm3
                                                                                                                • 5.1.2 Concentration in mol/dm3
                                                                                                                  • 5.1.3 Titrations
                                                                                                                    • 5.1.5 Gas Volume Calculations
                                                                                                                      • 5.1.6 Yields
                                                                                                                        • 5.1.7 Atom Economy
                                                                                                                          • 5.1.8 Choosing a Reaction Pathway
                                                                                                                          • 5.2 Controlling Reactions
                                                                                                                            • 5.2.1 Measuring Rates
                                                                                                                              • 5.2.2 Rate Graphs
                                                                                                                                • 5.2.3 Change Rates
                                                                                                                                  • 5.2.4 Catalysts & Enzymes
                                                                                                                                  • 5.3 Equilibria
                                                                                                                                    • 5.3.1 Equilibrium
                                                                                                                                      • 5.3.2 Le Chatelier's Principle
                                                                                                                                    • 6.1 Global Challenges
                                                                                                                                      • 6.1 Improving Processes & Products
                                                                                                                                        • 6.1.1 Extracting Metals
                                                                                                                                          • 6.1.2 Biological Extraction Methods
                                                                                                                                            • 6.1.3 The Haber Process
                                                                                                                                              • 6.1.4 Making Fertilisers
                                                                                                                                                • 6.1.5 Explaining the Haber Process
                                                                                                                                                  • 6.1.6 The Contact Process
                                                                                                                                                    • 6.1.7 Life Cycle Assessment
                                                                                                                                                      • 6.1.8 Recycling Materials
                                                                                                                                                        • 6.1.9 Properties & Uses of Alloys
                                                                                                                                                          • 6.1.10 Corrosion of Iron
                                                                                                                                                          • 6.2 Organic chemistry
                                                                                                                                                            • 6.2.1 Alkanes
                                                                                                                                                              • 6.2.2 Alkenes
                                                                                                                                                                • 6.2.3 Addition Polymers
                                                                                                                                                                  • 6.2.4 Condensation Polymers
                                                                                                                                                                    • 6.2.5 DNA
                                                                                                                                                                      • 6.2.6 Alcohols
                                                                                                                                                                        • 6.2.7 Carboxylic Acids
                                                                                                                                                                          • 6.2.8 Crude Oil & Fractional Distillation
                                                                                                                                                                            • 6.2.9 Cracking
                                                                                                                                                                              • 6.2.10 Cells
                                                                                                                                                                              • 6.3 Interpreting & Interacting with Earth Systems
                                                                                                                                                                                • 6.3.1 Origins of the Atmosphere
                                                                                                                                                                                  • 6.3.2 The Greenhouse Effect
                                                                                                                                                                                    • 6.3.3 Climate Change
                                                                                                                                                                                      • 6.3.4 Atmospheric Pollutants
                                                                                                                                                                                        • 6.3.5 Potable Water
                                                                                                                                                                                      • 7. Practical Skills
                                                                                                                                                                                        • 7.1 Practical Skills
                                                                                                                                                                                          • 7.1.1 Reactivity Trends
                                                                                                                                                                                            • 7.1.2 Electrolysis
                                                                                                                                                                                              • 7.1.3 Separation Techniques
                                                                                                                                                                                                • 7.1.4 Distillation
                                                                                                                                                                                                  • 7.1.5 Identification of Species
                                                                                                                                                                                                    • 7.1.6 Titration
                                                                                                                                                                                                      • 7.1.7 Production of Salts
                                                                                                                                                                                                        • 7.1.8 Measuring Rates of Reaction


                                                                                                                                                                                                        DOWNLOAD PDF

                                                                                                                                                                                                      Author: Alexandra

                                                                                                                                                                                                      Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.


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