Data can be obtained by taking measurements, and can lead to an explanation of why materials have different properties. During an experiment other factors, which could affect the outcome, need to be controlled.
Accuracy and reliability of data
Many people may have ideas about why materials have different properties, but these opinions are not very useful if they are not supported by data. To justify an explanation, you need to have data to support it. This data may be obtained by taking measurements.
The table below shows the length that equal-sized samples of one type of rubber can be stretched before they break:
Length of rubber before breaking (mm)
The accuracy of each measurement depends on the quality of the measuring apparatus and the skill of the scientists taking the measurement. If the apparatus is faulty, or the scientists make a mistake, the measurement may be inaccurate.
For the data to be reliable, the variation within the values must be small. There is always some variation in any set of measurements, whatever is being measured. There may be small differences in the composition of the rubber or the way the measuring apparatus is used.
In this set of data, each measurement is only slightly different from the others. The results are repeatable, meaning that each time a measurement is taken it has approximately the same value. We can say that this set of data is reliable.
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Evaluating titration results
When evaluating experimental results, it is important to consider the accuracy, precision and validity of the measurements.
Accuracy describes how close a result is to the true value.
Precision is a measure of the spread of the measured values. If there is a big spread, the uncertainty is large.
If an experiment has a good degree of accuracy and the results have a small uncertainty, and the experiment is not flawed in any other way (eg an unfair test), then the results are valid.
Working out the uncertainty
We can work out the uncertainty using a series of titration [titration: A quantitative procedure in which two solutions react in a known ratio, so if the concentration of one solution is known and the volumes of both are measured, the concentration of the other solution can be determined.] results: 26.5 g/dm3, 25.9 g/dm3, 26.2 g/dm3, 26.8 g/dm3, 27.1 g/dm3.
The average is calculated by adding the values together and dividing by the number of values (five in this case).
Average = 26.5 + 25.9 + 26.2 + 26.8 + 27.1/5
Average = 26.5 g/dm3
The range is between 25.9 g/dm3 and 27.1 g/dm3. Therefore, the uncertainty is 1.2 g/dm3.
The percentage error is calculated by dividing the uncertainty by the average and then multiplying by 100:
percentage error = 1.226.5 x 100
percentage error = 4.5%
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