# 3.1.4 Chemical Measurements

### 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 to a parallax error
• 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

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

### Calculating 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

#### 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

### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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