Learn Swedish: Kallbadet är bitande kallt men uppfriskande. - The cold bath is bitingly cold but refreshing.

In this sentence, bitande is a present participle being used as an intensifying adverb to mean 'bitingly.' It modifies the adjective kallt. Present participles in Swedish end in -ande or -ende and are completely invariable. This means they never take a -t ending, even when acting as adverbs.

Why does kallt have a -t ending while uppfriskande does not, even though both describe Kallbadet?

kallt is a standard adjective, so it must agree with the neuter noun Kallbadet (an ett word), hence the -t ending. uppfriskande, however, is a present participle acting as an adjective. Because present participles are invariable, they never change for gender or number, remaining exactly the same for en words, ett words, and plurals.

Are intensifying participles like bitande interchangeable with words like mycket, or do they only go with specific adjectives?

They usually pair with very specific adjectives to create strong, visual expressions. Just as we say 'bitingly cold' in English, Swedish heavily associates bitande with kallt. You wouldn't use bitande as a generic intensifier like mycket (very) for other random adjectives. Other classic fixed pairings include brännande het (burning hot) and skinande ren (shining clean).