A measurement is a process by which we assign a quantity to a feature. The output of a measurement is a number paired with a unit, and the pair takes its place as the value of a property.
Observation is the broader skill; measurement is the part of observation that yields numbers. We can observe that a stone is heavier than a feather. We can measure the stone at 240 grams. The first is qualitative; the second is quantitative. A property whose value is measurable is called a property quantity. Saying a stone has mass is observation; saying it has 240 grams of mass is measurement.
Measurement requires three things: a feature to measure, a procedure for measuring it, and a unit in which to express the result. Without all three, there is no measurement. The unit matters as much as the number. To say that the stone has 240 grams is one claim; to say it has 240 ounces is a very different one. The number alone is meaningless without the unit attached, just as the property is meaningless without its value.
Measurement is the way macroscopic chemistry becomes quantitative. The five foundational assumptions, in particular conservation, only deliver useful answers when the totals can be compared. Comparison requires numbers; numbers come from measurement. Without it, we are left with description that we can speak but not check. With it, the field has its arithmetic.
Question
Why is the unit as important as the number?
Reveal Merrill's answer →Hide answer
Because without the unit, the number does not refer to anything. To say a stone has 240 is meaningless; to say it has 240 grams is a measurement. The number names how much; the unit names what kind. A property requires both, just as it requires both a name and a value.