What must be part of a quantitative observation? a change of state a change in size a number a color

The answer is: a number.

Observation is the active purchase of information from a primary source.

There are two types of data collecting: quantitative and qualitative observations.  

1) Quantitative observations are made with instruments.  

This data are measurable and can be expresed with numbers.  

2) Qualitative observations are information about the quality of substances.  

In qualitative observation, informations are collected using senses (sight, hearing, taste, smell and touch).  

Qualitative observations are not measurable and cannot be recorded in terms of numbers.