Using 'a bit of water / wine ... ' vs 'a drop of water / wine' to describe a small quantity of liquid

Is it correct to say, "a bit of water" or should it be "a drop of water"?

More generally, can the word "bit" be used to describe a small quantity of a liquid or can it only be used for solids?

A bit is used informally to refer to:

• a small piece or amount of something: (Cambridge Dict.)

  • Would you like a bit of chocolate? The glass smashed into little bits. There were bits of paper all over the floor. She tries to do a bit of exercise every day. I don't understand this bit.

It is also used informally to refer to a small amount of a liquid:

Ngram: a bit of water, beer, whiskey.

• Some more whiskey? Just a little bit please.

A drop may figuratively convey the same meaning as a small amount but a bit or a little bit are probably more common expressions with that respect.

Both the phrases are used commonly.

For ex: I am in such a pathetic state that even a drop of water will prove to be of some use.

I need a little bit of water to quench my thirst.

If you look at the synonyms for 'bit', you'll find 'piece' and 'segment' which are commonly used for solids.

We also use 'bit' in phrases like: 'a bit of sunshine', 'a bit of luck' etc.(for intangible ideas)

'A drop' is way smaller in quantity than 'a bit'. So check the context and use the most appropriate one.

More generally, can the word "bit" ...only be used for solids?

Certainly not. Using bit or little bit with liquids is perfectly normal.

Drop is acceptable too, but you should note that more often than not, drop is used in negative sentences. As in:

I stopped drinking. I haven't tasted a drop of whiskey in six months.

In positive sentences, bit is more natural. Drop sounds old-fashioned, like Janus noted, or humouous.

PS - Bit can also be used with gases. See Google books results for 'a bit of air'