Uncertainty

Because every measurement has some error, a measured value is really a range. The uncertainty gives the size of that range: it is the amount either side of the reading within which the true value is expected to lie. It is written with a ± sign, for example 25.0 ± 0.1 cm³.The uncertainty depends on the apparatus used.• Scales read by eye (thermometer, burette, measuring cylinder): the uncertainty of a single reading is normally half the smallest division. A thermometer marked every 1 °C gives ± 0.5 °C, and a burette marked every 0.1 cm³ gives ± 0.05 cm³.• Digital instruments (balance, pH meter): the uncertainty is normally half of the last digit shown. A balance reading to 0.01 g gives ± 0.005 g; one reading to 0.001 g gives ± 0.0005 g.• Volumetric glassware (pipette, volumetric flask): the uncertainty (tolerance) is printed on the glass by the manufacturer, for example a 25.0 cm³ pipette of ± 0.06 cm³.If a question gives a different uncertainty for a piece of apparatus, always use the value given.The number of decimal places you record should match the apparatus. A burette reading is recorded to two decimal places ending in 0 or 5, such as 23.45 cm³, and a temperature from a 1 °C thermometer is recorded to the nearest 0.5 °C. Writing more figures than the instrument can support suggests a precision you do not have.