My understanding is that “chão” is used to refer to the ground outside or the ground floor in a building.
Is there a difference between “andar” and “piso” when referencing the floor of a building?
@jrlitton, chão not only refers to the ground outside, but also to any floor you’re walking over (e.g. kitchen floor = chão da cozinha; wood floor = chão de madeira). The ground floor of a building is specifically called rés-do-chão (and we don’t abbreviate this). As for other floors of a building, you can use andar and piso interchangeably - there’s no practical difference between them 
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Muito obrigado!
This had been confusing me for some time
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My teacher, who lives in Lisbon, explained that while andar and piso are mostly interchangeable, one tends to use andar to refer to the floors of an apartment building (prédio) and piso more for the floors of an office building (edificio).
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