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Practical & Investigative Skills

11woodhousea Chemistry IGCSE Inglês

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Apparatus
- Burette
- Glass Pippette
- Condenser
- Gas Syringe
- Funnel
- Beaker
- Round-bottomed Flask
- Conical Flask
- Dropping Funnel
Hazard Symbols
- Highly Flammable
- Toxic
- Corrosive
- Harmful
- Irritant
- Oxidising Agent
Separation Techniques
- Filtration
  - - Separates insoluble solid from a solid/liquid
  - - Residue remains on filter paper
  - - Filtrate runs through the filter paper
- Distillation
  - - Separates the solvent from a solution
  - - Can be used to determine the BP
  - - Pure water can be obtained from seawater
- Fractional Distillation
  - - Separates immiscible liquids
  - - Substance with lowest boiling point is collected first
- Separating Funnel
  - - Separates immiscible liquids
  - - The liquid with the greatest density separates out as the lower layer
- Magnetism
  - - Iron, Nickel and Cobalt are attracted to magnets
- Crystallisation
  - - Used to remove small quantities of an impurity from a solid
    - 1. The impure sample is dissolved in a minimum volume of hot solvent
    - 2. As the solution slowly cools, the saturated substance crystallises out
    - 3. The mixture is then filtered and the crystals washed with a small amount of cold solvent
    - 4. The crystals can be dried out between two filter papers or placed in a warm oven
- Paper Chromatography
  - - Separates mixes of different substances e.g. dyes
  - METHOD
    - 1. Small spots of coloured substances are placed in a line near the base of the paper
    - 2. The paper is placed vertically into a beaker or tank contaning a small volume of a suitable solvent
    - 3. The solvent is absorbed and rises up the paper
    - 4. The coloured chemicals in the spots dissolve in the solvent and are carried up the paper
    - 5. The resulting chromatogram is dried and analysed
  - - The greater the solubility of a compound in the solvent, the greater the distance it will travel up the paper
  - This technique can be used to...
    - - Detect whether a coloured dye is a pure compound or a mixture of pigments
      - A pure compound will produce a single spot on the chromatogram
    - Indentify a compound/substance by comparison to a known sample
      - The unknown and known compound will produce the same coloured spot at the same height on the chromatogram
An Investigation
- Planning
  - Aim
    - What is the investigation trying to find out?
  - Safety
    - A lab coat and safety glasses must be worn for all practical work
    - You need to consider specific hazards such as...
      - Toxic gases - work in a fume cupboard
      - Corrosive chemicals - wear gloves
      - Flammable organic compounds - do not heat with a Bunsen burner, use a water bath
  - Preliminary work
    - Trial and error is used to find out which practical method and apparatus will work
    - One of the reagents must be in EXCESS
    - One of the reagents LIMITS the reaction
  - Prediction
    - A prediction is made using you knowledge and the preliminary work
    - Your prediction should include...
      - - a simple statement of the expected results
      - - the predicted shape of any graph which you will draw
      - - a quantitative prediction based on the reaction equation, mole ratios or other scientific knowledge
    - Always support and explain your prediction with scientific knowledge
  - Fair test
    - Variables make and experiment Valid
      - Only one variable can be changed
        Independent Variable
        This is the condition that you decide to change
        Dependent Variable
        This is the variable that you measure as a consequence of the changes in the independent variable
        Controlled Variable
        These are the conditions that need to be kept constant to make the experiment a fair test
        keep the temperature the same
        keep the volume the same
        keep the same atmosphere
  - Method
    - Give a detailed step-by-step method of the experiment, such that another person could carry out the investigation without further help
  - Obtaining sufficient, reliable and accurate
    - Five is the minimum number of variations of the factor under investigation
    - Reliability of data
      - If the repeated results for an experiment are the same or very similar, they are said to be reliable
      - Anomalous results
        - A result which is significantly different from the rest, in a set of results from an experiment repeated several times
        - Results that do not follow the pattern formed by the other results/points on a graph or in a table
        Anomalous results must be clearly identified and not used when calculating averages or drawing lines of best fit on graphs
    - Accuracy : Is the result obtained correct?
      - Accuracy is increased by using the correct measuring Apparatus for the task
      - All apparatus possess inherent errors
- Obtaining (and recording) the evidence
  - Results should be displayed in a table
    - The independent variable must be in the left column
    - Use clear, accurate headings, which include units
    - Use SI units
    - Record all the readings to the appropriate level of accuracy
    - Record all the readings to the same number of decimal places
  - Calculations
    - An average value is usually calculated - DO NOT include anomalous
    - In rate investigations use - rate of reaction = 1/time
- Analysis
  - Graphs
    - - Use a sensible scale which uses as much of the graph paper as possible
    - - The independent variable goes on the x-axis and the dependent variable goes y-axis
    - - Use small crosses to mark the points
    - - Label the axes carefully and give the units
    - - Always add a title, indicating what the graph shows
    - - Circle (and label) any anomalous results
    - - Draw a line or curve of best fit (do not join up the points)
  - Conclusion
    - - State simply what the table of results and graph show
    - - Use scientific knowledge to explain what you have found out
    - - Refer to your prediction
- Evaluation
  - An evaluation should comment on how well the investigation went and address...
    - Reliability of the procedure and evidence
      - - How close together are your results? Are your results reliable?
      - - Identify and try to explain what caused any anomalous results
      - - Were there any problems with the procedure?
      - - Do you have sufficient evidence to support or disprove your prediction?
    - Accuracy
      - How close to the real figure are your results? (not always possible to answer)
    - Inprovements
      - Reliability
        Can be improved by doing more repeats and only using results which are very similar
      - Accuracy
        Can be improved by changing the method or equipment used
      - Limitations
        What are the flaws in the practical method and how could it be improved?
        Always give a reason with your suggestion
      - Validity
        Assess the above factors and decide how they have affected your results and hence your conclusion. Is your conclusion valid?
    - Further work
      - Further work is carried out to obtain more evidence to support the conclusion made in your investigation
      - It could involve
        - Changing one or more reactants and using same method
        - Using different method/apparatus
        - Obtaining a wider range of results