International A Level Biology / Practical Skills in Biology I
Significant figures, units and results tables revision notes
Edexcel International A Level Biology (YBI11), chapter 13: Practical Skills in Biology I, specification point U3.2, U3.3, A.1.1. Free lesson.
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Use significant figures to show the precision of measurements, and round calculated answers only to the precision of the least accurate measurement. Clear headings, units and decimal places make results tables accurate and easy to read.
How to count significant figures
Significant figures are the digits that carry information about a measured value. Start counting at the first digit that is not zero. Leading zeros only show where the decimal point is, so they do not count. 0.0487 has 3 significant figures. The zeros before 4 do not count. 9.06 has 3 significant figures. A zero between non-zero digits counts. 25.0 has 3 significant figures. The zero after the decimal point shows measured precision, so it counts. To round, look at the next digit. If it is 5 or more, round up: 0.98888 to 2 significant figures is 0.99, not 0.98.
How to round a calculated result
Round a final result to the same number of significant figures as the least accurate measurement used. Keep extra digits during the calculation, then round at the end. If nothing asks you to round a value, do not change it. For a fibre with diameter 0.52 mm and breaking force 14.7 N, the tensile strength is force divided by cross-sectional area. The diameter has 2 significant figures, fewer than the force, so the final answer needs 2 significant figures. For a dilution, 20 cm³ of 0.25 mol dm⁻³ solution needs 12.5 cm³ of 0.40 mol dm⁻³ stock. Use 12.5 cm³ of stock and 7.5 cm³ of distilled…
How to set out a results table
A good table makes clear what was measured and how accurately it was measured. Put the unit in the heading, not beside each result, and use the same number of decimal places all the way down a column. Use a specific heading, such as Time for milk to clear / s, rather than just “Time”. Brackets around a unit are also acceptable. Write values as 45, 52 and 61 under a heading with the unit, not as 45 s, 52 s and 61 s in the cells. Match decimal places to the instrument. If readings are to 2 decimal places, write 2.1 as 2.10. A mean should have the same decimal places as its readings.…
How to write a small length in standard form
Standard form writes a number as a value from 1 up to, but not including, 10 multiplied by a power of 10. For a number below 1, the power is negative. The number before the power should keep the significant figures in the measurement. A cheek cell about 45 µm across is 0.000045 m. Move the decimal point so there is one non-zero digit before it: 0.000045 becomes 4.5. The point moves five places to the right, so the power is −5: 4.5 × 10⁻⁵ m. 45 × 10⁻⁶ has the same size, but it is not standard form because 45 is not between 1 and 10.
How to calculate a mean to the right precision
For a mean, add the readings and divide by how many readings there are. Then match the decimal places in the readings, because the mean should not suggest greater precision than the measurements. The beetroot-disc absorbance readings at 60 °C are 0.42, 0.45 and 0.47. Each has 2 decimal places. The mean is (0.42 + 0.45 + 0.47) ÷ 3 = 0.4467, which rounds to 0.45 to 2 decimal places. Absorbance has no unit.