How do you say superscript and subscript notation?

Superscripts indicating multiplicative power (like 10⁹) are usually said "to the" ("ten to the ninth"). 2, 3, and -1 are special cases, often pronounced "square" or "squared", "cubic" "cube" or "cubed", and "per" respectively (-2 is sometimes said as a double "per"):

• m³ : cubic meters
• c² : c squared
• g mol^-1 : grams per mole
• m s^-2 : meters per second per second

If you do say "per" it's assumed to apply to everything after that:

• g mol^-1 K^-1 : grams per mole Kelvin

Subscripts in chemistry are usually just said as numbers:

• H₂O : H two O
• H₂SO₄ : H two S O four

If you feel a need to be precise or to indicate that the subscript applies to more than the most recently given element you might actually say "subscript" or "quantity subscript" (it's usually assumed that the person you're talking to knows enough chemistry to figure out what the quantity applies to).

• Ca₃(PO₄)₂ : C a three P O four (quantity) subscript two

Superscripts for nuclides are said as numbers but moved to the end:

• ²³⁸U : U (or uranium) two thirty eight