Is an uncountable noun treated as a singular noun?

Question

For example, if a banana is mostly rotten, I can describe it just as ‘a banana is rotten.’

Water gets frozen from the top. If the water is mostly frozen (not in degree, but in amount), can I describe it just as ‘water is frozen’?

Answer 1

When something is used as an uncountable noun it can refer to that substance in general.

For example:

Water is a compound of hydrogen and oxygen.

This statement is true of all water, everywhere.

So, you can't say "water is frozen", because that would mean all water everywhere is frozen, which it isn't.

"The water" refers to a specific quantity or body of water that you must have already identified. For example, if you were standing in front of a frozen lake you could say "the water is frozen" and it would be quite clear you meant the water in the lake.

Regarding your technical point that, when a lake freezes, not all of it freezes - what you said would depend on the level of detail the conversation demanded. In everyday conversation, "a frozen lake" would be one that was 'frozen over' - ie visibly frozen on top. When we want to say that something is completely frozen, the idiomatic expression is "frozen solid".