What steps would be necessary to figure out if something is an acid or base?

One of the following properties was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version. which property fits this description?

How many atoms are in 1.4 mil of phosphorus trifluoride (pf3)

Energy is released during which phase changes? check all that apply. boiling condensing depositing freezing melting subliming