Measuring skills, SI units and Uncertainties in Biology
There are some important new skills to learn in IB Biology when it comes to the simple skill of measuring. Professional biologists and IB students are expected to know how precise their measurements are.
This series of short experiments illustrates SI units and the skills a student needs to achieve high grades in data collection and processing.
What is the "uncertainty" of a measurement?
How do biologists know the precision of their equipment in any experiment?
Activity 1 Measuring the volume of a Test Tube
This experiment appears simple and illustrates the complexities in the precision of a range of measuring equipment.
Carry out the Measuring the volume of a test tube experiment and answer the questions on the worksheet.
Activity 2: Measuring Mass of Rice Grains
Carry out the experiment on the worksheet below measuring rice grains to estimate the mass of a rice grain in four different ways. This illustrates how the method can influence the precision of experimental results.
Activity 3 Making a starch solution and using a colorimeter
In this activity you test your fine measuring skills and produce a coloured solution which can be tested in the colorimeter. Only a biology guru can get close to the text book value. This is a procedure which illustrates the precision of balances, and the colorimeter as another digital device. Soluble starch is not easy to dissolve. Beware if you see any black sediment! Experiment 3 - Understanding concentrations and their SI units
This is an introductory activity to practical work in the IB Biology course. It is an opportunity for new students to get to know their way around the lab and to use some new equipment. Starting the Biology course with a few simple practicals is a good beginning and the skills of working out the uncertainties in apparatus are new to most students and well worth learning.
The three activities lead from one to another, and there are some helpful guidance about SI units and Uncertainties in the first experiment sheet.
Usually a discussion around the apparatus itself is really helpful to students.
There are aspects of calculations of uncertainties which have been left out in these experiments for the sake of not overcomplicating things. It is worth revisiting these ideas later in the course when practical work allows.
Presentation of data recorded in IB tables of results need to follow some IB conventions with regard to decimal points.
- For uncertainties, the simplest rule is that the uncertainty is plus or minus (±) the smallest division on the instrument (the least count). This is true for rulers and instruments with digital displays.
- The instrument limit of error is often a fraction of the least count value. For example, a burette or a mercury thermometer is often read to half of the smallest division of the scale.
- Results in a table must have a decimal points which correspond to the uncertainty.
E.g. A measuring cylinder volume of 4.1 cm3 becomes 4.10 cm3 (± 0.05 cm3).
The different types of experimental errors other than measurement uncertainties are not covered in this lesson. Nor are qualitative observations mentioned. In the third experiment this could be included.
Note: these experiments cannot be used for IA work as there is too much guidance for data collection, little processing and really no hypothesis to write about in the conclusions.
Model answers can be found here: Measuring equipment - model answers