Elements, Compounds & Mixtures C2 (Part 2)

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  • Created on: 11-04-17 19:47
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  • Elements, Compounds & Mixtures C2 (Part 2)
    • Purity
      • A substance that is completely made up of a single element or compound
      • Test for Purity using Boiling and Melting Points
        • Every pure substance has a specific melting and boiling point so you can test the purity of a substance by comparing the actual melting or boiling points to the expected value
        • If a substance is impure the melting point will be too low and the boiling point will be too high
        • You can sometimes tell if a sample of a solid or liquid is a mixture by heating it up
          • In a mixture, the different components will melt or boil at different temperatures. This means mixtures will often melt over a range of temperatures
    • Distillation
      • Simple Distillation is used to separate out solutions
        • Simple distillation is used for separating out a liquid from a solution e.g. salt water
          • 1. Pour you sample of salt water into the distillation flask
          • 2. Set up the apparatus with a thermometer in the top of the distillation flask, a condenser and a collection beaker. Connect the bottom end of the condenser to a cold tap using rubber tubing. Run cold water through the condenser to keep it cool
          • 3. Gradually heat the distillation flask. The part of the solution that has the lowest boiling point will evaporate first - in this case, water
          • 4. The water vapour passes into the condenser where it cools and condenses. It then flows into the beaker to be collected
          • 5. Eventually, salt will be left in the flask
      • Fractional Distillation is used to separate a mixture of liquids
        • 1. Put your mixture in a flask. Attach a fractionating column and condenser above the flask
        • 2. Gradually heat the flask. The different liquids will all have different boiling points - so they will evaporate at different temperatures
        • 3. The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, it will reach the top of the column
        • 4. Liquids with higher boiling points might also start to evaporate. But the column is cooler towards the top, so they will only get part of the way up before condensing and running back down towards the flask
        • 5. When the first liquid has been collected, raise the temperature until the next one reaches the top
    • Filtration and Crystallisation
      • Filtration is used to separate an insoluble solid from a liquid
        • If the product of a reaction is an insoluble solid, you can use filtration to separate it out from the liquid reaction mixture
        • It can be used in purification
        • Put filter paper into a funnel and pour the mixture into it. The liquid part of the mixture runs through the paper, leaving behind a solid residue
      • Crystallisation separates a soluble solid from a solution
        • 1. Pour the solution into an evaporating dish and gently heat it. Some of the solvent will evaporate and the solution will become more concentrated
        • 2. Once some of the solvent has evaporated, or when you see crystals start to form, remove the dish from the heat and leave the solution to cool
        • 3. The solid should start to form crystals as it becomes insoluble in the cold, highly concentrated solution
        • 4. Filter the crystals out of the solution and leave them in a warm place to dry
    • Chromatography
      • Uses two phases
        • A mobile phase - where the molecules can move (always a liquid or gas)
          • Suitable Solvent
        • A stationary phase - where the molecules can't move (can be a solid or really thick liquid)
          • Chromatography Paper
        • The components in the mixture separate out as the mobile phase moves over the stationary phase - they all end up in different places
          • This happens because each of the chemicals in a mixture will spend different amounts of time dissolved in the mobile phase and stuck to the stationary phase
            • How fast a chemical moves through the stationary phase depends on how it distributes itself between the two phases
      • Interpreting Chromatograms
        • Rf Values
          • Rf = distance travelled by the component ÷ distance travelled by solvent
    • Relative Masses
      • Relative Atomic Mass (Ar) - Mass Number
        • Number of protons + Number of neutrons
        • All atoms are compared with the mass of Carbon-12
      • Relative Formula Mass (Mr)
        • The relative atomic masses of a compound's formula all added together
    • Molecular and Empirical Formulas
      • Molecular Formula
        • The simplest whole number ratio of the atoms in a single molecule of a compound
      • Empirical Formula
        • The simplest whole number ratio of the atoms in a compound
        • Use the Empirical Formula and Mr to find Molecular Formula
          • Molecular Formula
            • The simplest whole number ratio of the atoms in a single molecule of a compound

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