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Mistakes vs Sources of Error
Mistakes vs Sources of Error
Experiments never turn out exactly as we predict. There are different reasons for this. Sometimes, we make mistakes:
We measure wrong
We write down the wrong measurement
We calculate wrong
We spill something
We forget to zero the balance
We calibrate wrong
Mistakes can be labeled in different ways: blunders, goofs, mishaps, oversights, etc.. In general, mistakes are avoidable; they can be identified and fixed.
On the other hand, our results can be thrown off by unavoidable factors. These factors can be minimized, but never fully eliminated. They are known as sources of error. Sources of error are not the same as mistakes.
Sources of Error Include...
Subjectivity
Whenever data collection depends on judgment. Our senses are imperfect, and their imperfections are further compounded by our judgment.
Assumptions
Every experiment involves assumptions. While these assumptions never fully represent reality, we allow for them because the extent to which they are inaccurate is usually minimal.
Confounding Variables
When you carry out an experiment, you are trying to see the effect of one variable on another. Other variables that can have an undesirable effect on the outcome are known as confounding variables. Although we can account for confounding variables to an extent, we can never control them completely.
Chemical Purity
The chemicals that we use are usually not 100% pure. Even if they are pure, they are prone to degradation over time, as in the rusting of metal by oxygen in the air. The presence of impurities can lead to inaccuracies in measurements (such as mass).
Instrumental Interference
Sometimes the act of taking a measurement can affect the reading itself.
Non-Representative Sampling
When we collect a series of samples, we assume that the samples represent a population. Although taking repeated measurements is a way to combat experimental variability, there is always the uncertainty that our samples are not representative of the whole.
Instrumental Limitations
Measuring instruments have a limit to their reliability. For example, the electronic balances in the general chemistry lab are not accurate past 1/1000th of a gram (0.001 g). In addition, the reliability of an instrument’s reading can drift over time. This tendency to drift necessitates the calibration of our instruments.
Calibration is a process in which an instrument is set to a known value. For example, the electronic balances are calibrated with a standard mass, a block of metal that is known to be 100.000 grams. Some instruments need to be calibrated periodically (once a year in the case of the balances) while others need frequent calibration (pH probe, turbidity probe, conductivity meter, and spectrophotometer). When you use an instrument that needs frequent calibration, it is necessary to constantly evaluate instrumental drift and calibrate/re-calibrate as needed.
You will be asked to evaluate sources of error in every experiment. You will need to think about how some of these could apply to that specific experiment. That is, it is not enough to say that a source of error is simply "subjectivity".
Which source of error applies in the following situations?
Click the descriptions to reveal the answers
The Environmental Protection Agency (EPA) is assessing the water quality of a river into which treated wastewater is released. There is a major rainstorm the day before technicians collect samples at the site.
The Environmental Protection Agency (EPA) is assessing the water quality of a river into which treated wastewater is released. There is a major rainstorm the day before technicians collect samples at the site.
Answer - Non-Representative Sampling
It is likely that the water quality will be affected by the sudden influx of rain water. Therefore, the samples will be less representative of the actual health of the waterway.
You immerse a room temperature thermometer into a beaker of ice water.
You immerse a room temperature thermometer into a beaker of ice water.
Answer - Instrumental Interference
Since the thermometer is at a higher temperature than the ice water, it will slightly warm the mixture when it is immersed in it and possibly interfere with the reading.
You are designing an experiment to study the effect of a new type of compost on plant growth. You mix compost into the soil of three plants and set them up as shown in the image.
You are designing an experiment to study the effect of a new type of compost on plant growth. You mix compost into the soil of three plants and set them up as shown in the image.
Answer - Confounding Variables
As you can see in the picture, the plants are placed in different locations. The difference in their positions relative to the source of sunlight varies. Therefore, we can't say for certain whether this confounding variable will also have an impact on the plant growth. It will be difficult to discern whether the plant growth is due to the compost and/or the amount of sunlight that the plant receives. Can you think of other potentially confounding variables?
You are using potassium carbonate for an acid-base titration (a lab technique in which exactness is very important). The potassium carbonate comes from the bottle in the picture.
You are using potassium carbonate for an acid-base titration (a lab technique in which exactness is very important). The potassium carbonate comes from the bottle in the picture.
Answer - Chemical Purity
If you look closely at the second picture, you will see that the chemical was produced in the 1950s! Because of the age of the chemical, you can't know for certain the extent to which it has degraded or been contaminated in its decades spent sitting on a shelf.
You are carrying out an experiment on reaction rates. The reaction involves mixing two solutions and then measuring the time that it takes for the mixture to change color. In this case, the solution changes from colorless to dark blue. You are carrying out multiple trials of the reaction. Using a stopwatch, you start the timer when the two solutions are mixed and stop it at the instant that you see the dark blue color.
You are carrying out an experiment on reaction rates. The reaction involves mixing two solutions and then measuring the time that it takes for the mixture to change color. In this case, the solution changes from colorless to dark blue. You are carrying out multiple trials of the reaction. Using a stopwatch, you start the timer when the two solutions are mixed and stop it at the instant that you see the dark blue color.
Answer - Subjectivity
You are using your senses (eyes) to tell you when to stop the timer. The exact point at which you stop the timer will vary slightly between trials. As a result, there will be an inherent variability to your readings.
You are measuring the pH of a solution with a pH probe. Initially, the pH reads 2.01. Two hours later, you check the pH of the solution, and it now reads 3.44.
You are measuring the pH of a solution with a pH probe. Initially, the pH reads 2.01. Two hours later, you check the pH of the solution, and it now reads 3.44.
Answer - Instrumental Limitations
The pH probe requires frequent calibration. It is likely that the probe has drifted in the two hours since the initial reading and should have been re-calibrated before the second reading.
You are studying how the amount of air in the balloon affects its volume. To determine the volume of the balloon, you measure its diameter. Then, you calculate the volume using the equation for the volume of a sphere.
You are studying how the amount of air in the balloon affects its volume. To determine the volume of the balloon, you measure its diameter. Then, you calculate the volume using the equation for the volume of a sphere.
Answer - Assumptions and/or Subjectivity
In calculating the volume of the balloon, you are assuming that it is in the shape of a perfect sphere. As you can see in the first image. this is not quite the case. As a result, the actual volume of the balloon will differ slightly from the calculated volume.
When measuring the diameter of the balloon, it is important to take the measurement at its midpoint. This involves using your eyes to judge the position of the middle of the balloon. This subjectivity can affect your measurements.
References
West York Area High School. (2013). Sources of Experimental Error WYChem. YouTube. Retrieved December 13, 2023, from https://www.youtube.com/watch?v=ahraXWLN0ec.
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