Errors vs Mistakes

In practical chemistry the words error and mistake mean different things, and exam questions expect you to keep them separate.An error is the difference between a measured value and the true value. Every measurement has some error, even when the experiment is done perfectly, because of the limitations of the apparatus and the method. A balance can only display a certain number of decimal places, a burette scale has to be judged by eye, and some heat always escapes from a calorimeter. Errors can be estimated and reduced, but never removed completely.A mistake is an avoidable slip by the person doing the experiment. Examples include:• misreading a scale or writing down the wrong number• spilling some of a solid or solution• using the wrong solution or the wrong volume• overshooting the end point in a titration• an arithmetic slip in the calculationMistakes are not part of the uncertainty of an experiment. A result affected by a mistake is usually an obvious anomaly: it should be discarded and the measurement repeated, not averaged in with the good results.So when you evaluate an experiment, blaming "human error" is not a good answer. Either name a specific source of error in the apparatus or method, or identify a real mistake and say how it would be avoided.