Nitritul și nitratul apar în apă.

Because -ul is the Romanian definite article attached to the end of a masculine/neuter singular noun.

• nitrit = nitrite
• nitritul = the nitrite
• nitrat = nitrate
• nitratul = the nitrate

Unlike English, Romanian usually puts the at the end of the noun instead of before it.

They are neuter nouns.

In Romanian, neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular definite form you get nitritul and nitratul, which look like masculine forms.

Because the subject is made of two nouns joined by și:

nitritul și nitratul = the nitrite and the nitrate

Even though each noun is singular, together they form a plural subject, so Romanian uses the plural verb apar.

Compare:

• Nitritul apare în apă. = The nitrite appears/occurs in water.
• Nitritul și nitratul apar în apă. = The nitrite and the nitrate appear/occur in water.

It comes from the infinitive a apărea, which usually means to appear.

Here is the present tense:

• eu apar
• tu apari
• el/ea apare
• noi apărem
• voi apăreți
• ei/ele apar

So apar can mean they appear or they occur, depending on context.

Not necessarily in the everyday visual sense of suddenly become visible.

In a scientific or factual sentence like this, a apărea often means something more like:

• to occur
• to be present
• to show up
• to be found

So the verb is broader than everyday English appear.

Because apă here means water in general, not a specific, identified body of water.

• în apă = in water
• în apa = in the water

If you said în apa, it would normally suggest some specific water already known from context, such as în apa lacului (in the lake water) or în apa din pahar (in the water in the glass).

This is a very common thing in Romanian.

Romanian often uses the definite article with singular nouns when talking about a substance, class, or concept in a general way, especially in formal or scientific language.

So:

• nitritul
• nitratul

can refer to the substances generally, not only to one specific sample.

But after a preposition like în, Romanian often leaves an uncountable noun like apă without the article when it means the substance in general.

So the mix of definite and non-definite forms is normal here.

Și means and.

It connects the two nouns:

• nitritul și nitratul

Romanian uses și very much like English and.

Also, the letter ș is pronounced like English sh.

Yes. Romanian word order is fairly flexible.

The sentence as given:

• Nitritul și nitratul apar în apă.

A very natural alternative is:

• În apă apar nitritul și nitratul.

Both are grammatical. The difference is mostly emphasis:

• starting with Nitritul și nitratul focuses first on the substances
• starting with În apă focuses first on the location or medium

A rough pronunciation guide is:

nee-TREE-tool sh ee nee-TRA-tool ah-PAR uhn AH-puh

A few important sound notes:

• ș in și = sh
• ă in apă is a short neutral vowel, similar to the a in sofa
• î in în has a sound that English does not really have; uhn is only a rough approximation

If you want something closer to IPA, it is approximately:

/niˈtritul ʃi niˈratul aˈpar ɨn ˈapə/

In some contexts, Romanian can omit articles, especially in compressed technical style, headings, labels, or very shorthand scientific writing.

But in a normal full sentence, Nitritul și nitratul apar în apă sounds more natural and complete.

So for a learner, the version with -ul on both nouns is the safer and more standard choice.