cidro -dro- gismu

x1 is a quantity of/contains/is made of hydrogen (H); [metaphor: light, flammable].

See also gapci, xukmi.

In notes:

gapci
x1 is gaseous/fumes/a gas/vapor of material/composition including x2, under conditions x3.
xukmi
x1 is an instance of substance/chemical/drug x2 (individual or mass) with purity x3.
cmuxu'i
x1 is an element of kind x2 with purity x3.