Final answer:
A quantitative observation in chemistry must include a number and a unit to provide exact information, unlike qualitative observations that describe characteristics without using numbers.
Step-by-step explanation:
Quantitative observations are essential in science, especially in chemistry, where they provide exact information about the properties and changes of matter.
In contrast to qualitative observations, which describe characteristics without the use of numbers such as color or state, quantitative observations incorporate both a number and a unit.
For example, stating that 'the melting point of crystalline sulfur is 115.21° Celsius' or '35.9 grams of table salt dissolve in 100 grams of water at 20° Celsius' gives us a precise understanding that can be universally communicated and verified. Likewise, the discovery that iridium concentrations in sediments from 66 million years ago were significantly higher provided a quantitative clue to the extinction of dinosaurs. Therefore, the correct answer to the question 'What must be part of a quantitative observation?' is a number, as it provides the specific amount or measurement associated with the observation.