This is the 1877th most frequent Japanese word.
熱く
"熱く" means "hotly" or "passionately."
Here, "熱く" is used as an adverb to describe the temperature of the water.
湯が熱くて触れない。
The water is hot and I cannot touch it.
In this sentence, "熱く" expresses enthusiasm or fervor in how he spoke.
彼は熱く語った。
He spoke passionately.
Here, "熱く" refers to a state of high temperature in general.