Chemical Measurements (AQA GCSE Chemistry: Combined Science)

Revision Note

Test yourself
Stewart

Author

Stewart

Last updated

Uncertainty & error

  • An error is the difference between a value or quantity obtained in an experiment and an accepted or literature value for an experiment
  • There are two types of errors in experiments, random errors and systematic errors
  • Uncertainties are the same as random errors
  • Uncertainties express the confidence to which the measurement can be taken

Random Errors

  • When you are reading an instrument and estimate the final digit, there is an equal chance that you may read it slightly too high or slightly too low
    • This is a random error

  • Random errors are can be affected by:
    • How easily the instrument or scale is to read
    • The person reading the scale poorly
    • Changes in the environment, for example
      • fluctuations in the temperature of the lab
      • air currents in the room

  • Random errors will pull a result away from an accepted value in either direction (either too high or too low)

Systematic Errors

  • Systematic errors are errors that occur as a result of a faulty or poorly designed experimental procedure
  • Systematic errors will always pull the result away from the accepted value in the same direction (always too high or always too low)
  • For example,
    • If you forget to zero an electronic balance (using the tare button) the mass weighings will always be higher than they should be
    • If you don’t read the volume in a burette at eye level, the volumes will always be smaller than they should be due 
    • If you fail to keep a cap on a spirit burner in a calorimetry experiment, the alcohol will evaporate and give you a larger mass loss

Diagram of systematic errors Further diagram of systematic errors

Systematic errors always pull the result away from the accepted value in the same direction: either too high or too low

How to calculate uncertainty

  • Treatment of uncertainties depends on the type of instrument used

Using analogue instruments

  • Any instruments that have an analogue scale, the uncertainty is taken as half the smallest division on the scale
  • For example,
    • A thermometer that reads to 1oC, the uncertainty would be +0.5 C
    • A burette that reads to 0.10 mL, the uncertainty would be +0.05 mL

Using digital instruments

  • Any instruments that have a digital scale , the uncertainty is taken as the smallest division on the scale
  • For example,
    • An electronic balance that reads to 0.01 g, the uncertainty would be +0.01 g

Uncertainty in results

  • For results that are obtained from a series of repeated experiments, the uncertainty is ± half of the range of results 
  • This can be estimated by:
    • Calculating the mean average and then determining the deviation of the highest and lowest results from the mean value
    • An alternative method is to calculate the range of the results and then divide this value by 2

Other uncertainties

  • Other sources of uncertainty can arise where the judgement of the experimenter is needed to determine a changing property
  • For example,
    • Judging the end point of a titration by looking at the colour of the indicator
    • Controlling a stopwatch in a rate of reaction experiment
    • Deciding when to extinguish the flame in an  experiment

  • These uncertainties are very difficult to quantify, but they should be commented on as a source of error in an evaluation

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.