Students always ask us: el agua or la agua? …. Is agua a feminine or masculine word? The answer is very easy. In front of the feminine names that begin with tonic vowel (the one with the accent) the article is used: El agua to avoid cacophony (sound ugly;)). However, it is correct to use this, and that, even if they start with tonic vowel : this water ( este agua), that eagle ( ese águila), that axe( ese hacha), or this area (este area).
As the Panhispanic Dictionary of Doubts explains, the la and una articles normally adopt, for reasons of sonority, the forms el and un when they go immediately in front of a noun that begins with tonic vowel. So, the right thing is, for example,”all the water” (toda el agua),”very hungry” ( mucha hambre),”the great classroom”(el aula magna),”the white weapon”( el arma blanca). With these names it is also more advisable to use the masculine of the indefinite article (un) and algún (some) and ningún(none): a classroom (un aula), some classroom (algún aula), (ningún aula) no classroom.