## Measurements

Measurements provide much of the information that informs the hypotheses, theories, and laws describing the behavior of matter and energy in both the macroscopic and microscopic domains of chemistry.

Every measurement provides three kinds of information:

- the size or magnitude of the measurement (a number)
- a standard of comparison for the measurement (a unit)
- and an indication of the uncertainty of the measurement

The number in the measurement can be represented in different ways, including **decimal form** (the mass of the average mosquito is about 0.0000025 kg) and **scientific notation** (the mass of an average mosquito is about 2.5 × 10^{−6} kg).

Without units, a number can be meaningless, confusing, or possibly life threatening. Suppose a doctor prescribes phenobarbital to control a patient’s seizures and states a dosage of “100” without specifying units. Not only will this be confusing to the medical professional giving the dose, but the consequences can be dire: 100 mg given three times per day can be effective as an anticonvulsant, but a single dose of 100 g is more than 10 times the lethal amount.