Solids can also additionally be measured by length as well.
fun fact: natural gas is measured by volume (cubic feet) but is sold based on its heating content (Btus). A cubic foot of natural gas is the amount of natural gas that can be contained in a cube one foot on a side, at a certain standard temperature and pressure. (source; oilgaslawyer blog)
because the ion are Mobil which mean they are free not combined and carry a charge but when they are combined/ fixed in position they can't carry a charge so therefore can't conduct electricity :)
