laboratory measurement
/ LAB-ruh-tor-ee MEZH-er-ment /
When you weigh flour for a recipe, you put it on a scale and read a number. A laboratory measurement is the same act done with discipline: you compare an unknown quantity against a known standard and write down the result, but you also ask how good that number really is and what could have nudged it off.
More precisely, a laboratory measurement is the experimental determination of the value of a physical or chemical quantity — a mass, a volume, a concentration, a temperature — using calibrated instruments under controlled conditions. A complete measurement is not just a number and a unit; it carries an estimate of its uncertainty and rests on the instrument having been calibrated against a recognized standard.
Why it matters: every experimental result, law, and constant in chemistry ultimately comes from measurements, so the care taken in making them sets the limit on what can be known. A caveat: no measurement is exact. Each one has random scatter and possible systematic bias, which is why a single reading without repetition, calibration, and an uncertainty estimate is of limited value.
To find a solution's concentration, a student measures out 25.00 mL with a calibrated pipette, weighs the dissolved solid on a balance reading to 0.0001 g, and repeats the whole thing three times. The three values agree to within a few tenths of a percent, and that spread becomes part of the reported result.
A measurement is finished only when it carries a unit, an uncertainty, and a calibration behind it.
Accuracy and precision are different qualities of a measurement. Precision is how closely repeated readings agree with each other; accuracy is how close they are to the true value. A measurement can be precise yet inaccurate (tight cluster, wrong spot) if the instrument carries a systematic bias.