C2

Accuracy in experimental work in the laboratory

Apparatus used for measurement in chemistry

- Rates of reactions:
  - Instrument: accurate stopwatch (hundredth of second)
  - Unit of time: hours (h), minutes (min), seconds (s)
- Temperatures
  - Instrument: thermometer
    - Alcohol-in-glass
    - Mercury-in-glass
      - Dangerous if it breaks
    - Unit: degrees celcius
    - Accuracy: tenth of degree
    - Read: eye level of meniscus -> no parallax effects
      - Meniscus: way liquid curves at edge of container
    - Mass
      - Instrument: electronic balance
      - Unit: grams (g) or kilograms (kg)
      - Accuracy: hundreth of gram
      - Read: wait until reading = steady onbalance
    - Volume
      - Liquid: amount of space it takes up
      - Units: litres (l), cubic centimetres (cm) or mililitres (ml)

3

1 litre = 1000 cm

3

1 cm = 1ml

3

- - - - Read: apparatus needs to be verticle & on eye level with top of miniscus

Criteria for purity

- Drugs = high purity degree
  - Ensured by dissolving in suitable solvent + subjected to fractional crystallisation
- Chemical, pharmaceutical & food industry: substances must be pure
  - Impurities affect chemical, physical & biological behaviour
- Testing purity:
  - Melting point:
    - Pure = sharp melting point
    - Impure: melting happens at range of temperatures
  - Boiling point:
    - Pure: temperature = steady at boiling point
    - Impure: boil at range of temperatures
  - Chromatography
    - Pure: produce 1 well-defined spot on chromatogram
    - Impure: several spots
  - Solubility
    - Pure: all substance dissolves in suitable solvent at perfect temperature
    - Impure: undissolved -> needs to be filtered off
Mixtures
- Contains more than one substance (element/compound)
- Examples
  - Sea water
  - Air
    - Elements
      - Oxygen
      - Nitrogen
      - Neon
    - Compounds
      - Carbon dioxide
    - Alloys
      - Brass

Copper

Zinc

Separating mixtures

Separating solid/solid mixtures

- Chromatography
  - Use: separate solids that are soluble -> can be identified
    - Colored materials – ink, dyes
  - Different types
    - Simplest: paper chromatography
  - Separate black ink
    - Spot of ink on chromatography paper
    - Paper put into suitable solvent
    - Solvent moves up paper -> ink seperates
      - Explanantion:

Substances = different solubilities in solvent

Are absorbed to different degrees by paper

Seperated gradually as solvent moves up paper

- - - - Values obtained: R (reterdation factor)

F

Ratio of distance travelled by solute (different dyes): distance travelled by solvent

Seperating solid/liquid mixtures

- If solid substance (soluble & solute) dissolves in liquid (solvent) -> forms solution
  - Sugar in tea
- If solid does not dissolve -> insoluble
  - Tea leaves
- Filtration
  - Need to separate solid from liquid
  - Tea poured through tea strainer
  - Sand from water with filtering paper
    - Small holes: allows water (filtrate) to pass, but not sand (residue)
  - Crystallisation
    - Water evaporated to get salt -> leaves seperated solution (brine)
    - Saurated solution: contains as much solute as can be dissolved at particular temperature
    - Once solution is saturated -> salt crystallises
  - Simple distillation
    - Get water from salt water
    - Get solvent from solution
    - Steps
      - Solution = heated until it boils (in flask)
      - Steam goes into Liebig condenser
      - Condenses back into water
      - Salt left in flask
    - Get pure water to drink using desalination plant

Seperating liquid/liquid mixtures

- Fractional distillation
  - Liquids have different boiling points
  - Same procedure as simple distillation
  - Liquid with lower boiling point -> boils first -> steam is collected first -> condenced first
  - Other liquid stays in fractionating column