Even non-Latin students can understand this phrase as meaning “pure water" since the word “pura" is similar to English’s “pure" and aqua (similar to Spanish’s agua) is related to the English word “aquarium." Simple as it may be, this phrase does illustrate two important grammatical constructions.
First, notice that the adjective “pura" follows the noun it modifies, “aqua." This is common in Latin because the noun is considered more important that any word that may modify it. Essentially, the Romans would say “water pure" rather than English’s “pure water." This is common throughout Latin so always be on the lookout for adjectives that follow nouns, a practice unfamiliar to native English speakers. As a side note, speakers of Romance languages do not have this problem as English speakers have. Many of the languages derived from Latin continue to place the adjective after the noun it modifies. Therefore, it is especially more difficult for English speakers to remember this common Latin construction.
Second, notice that the adjective “pura" agrees with the noun it modifies, “aqua", in case (nominative), number (singular), and gender (feminine). Since “aqua" is a feminine noun, “pura" is also in its feminine form. As an adjective, “pura" has masculine, feminine, and neuter forms. For example:
Vir purus (the pure man)
Femina pura (the pure woman)
Flumen purum (the pure river)
In this case, “aqua" and “pura" have similar forms but this is not always the case with nouns and adjectives as illustrated in the three examples above. Notice that “vir purus" and “flumen purum" do not have similar forms even though the adjective “purus –a –um" agrees with the nouns it modifies in case, number, and gender.