Question

The compound word "water tank "
is made of two words 'water' and 'tank'. What parts of speech are both the words.

Answer 1

Both the words are 'nouns'.

Nouns are words that refer to a person, place, thing, quality etc.

The word 'water' is an uncountable noun. 

Uncountable noun is a word that refers to a thing or quality that can’t be counted. For example the word ‘water’ is an uncountable noun. Similarly ‘honesty’ is uncountable also. Uncountable nouns don’t have singular or plural form. It has only one form. We can use determiners such as a little, much, etc. with uncountable nouns. 
 

The word 'tank' is countable noun. A countable noun is a noun that can be counted; it has singular and plural forms. We can use determiners such as a, an, the, a few, many, more, etc. with countable nouns. Examples of countable nouns are: apple, carrot, chair, boy, man, etc.