Solids, and liquids (pure or nearly pure). They are not really left out. They are assigned the value of "1" (one). Example:
CaCO3(s) <=> CaO(s) + CO2(g)
Keq=[CO2](1) / (1)
Keq = [CO2]
Which species are left out from the Keq expression, and why?
3 answers
Solids are omitted. By definition they are in their standard state and have a concentration of 1. Water is often omitted but not always.
Good. GK mentioned pure liquids.