Observing & Measuring
What you must understand
An observation is information gathered with the senses or with instruments; an inference is the explanation you build on top of it. Quantitative observations carry a number and a unit, while qualitative ones describe qualities such as colour or texture. You should know the common SI units and prefixes (mm, cm, m, km; mL, L; g, kg; s, min, h; °C), how to select a sensible instrument for a task, and how to read linear scales on rulers, thermometers, measuring cylinders, stopwatches and electronic balances. Accuracy is closeness to the true value; precision is how finely divided and how repeatable a measurement is. Random error shrinks when you repeat and average; systematic error, such as a zero error on a balance, does not.
Traps to watch
- Reading the top of a meniscus instead of the bottom, or viewing it from an angle (parallax).
- Quoting a number without a unit, or comparing quantities given in different units.
- Assuming a more finely divided instrument is automatically more accurate.
- Counting the marks on a scale rather than the intervals between them.
- Writing a conclusion as if it were an observation ("the acid was strong" instead of "the pH was 2").
Worked example
A 50 mL measuring cylinder is marked every 1 mL. The bottom of the meniscus sits halfway between the 23 mL and 24 mL marks, so record 23.5 mL. The rule of thumb: read at eye level at the bottom of the meniscus, and record one digit beyond the smallest marked division, quoting an uncertainty of about half that division (±0.5 mL).