One way of knowing that oxygen was the gas removed from the volume of air and not another is to know what the volume of air is made of first. When the composition of the volume of air is already identified, then next would be the process of separating these elements from each other and as to which is to be separated first. This would usually lead to knowing their masses, their boiling and freezing points, the temperatures at which they condense, and so on. This is to identify their differences to each other and use those differences to successfully separate those elements to each other.
The grey liquid is related to the white liquids because the change in colour is the result of a chemical change/reaction between the two white liquids.
Independent and dependent variables and constants
What they have in common is that they both have the same number of atoms.
