A amônia aparece antes do nitrito.

A is the feminine singular definite article, equivalent to the in English.

So:

• a amônia = the ammonia
• o nitrito = the nitrite

Portuguese uses articles more often than English does, especially with nouns in normal statements. In technical or scientific Portuguese, using the article here sounds very natural.

In Portuguese, every noun has a grammatical gender, either masculine or feminine. That does not necessarily mean the thing itself is biologically male or female.

Here:

• amônia is feminine, so it takes a
• nitrito is masculine, so it takes o

This is something you usually have to learn with each noun:

• a amônia
• o nitrito
• o nitrato
• a água

Aparece comes from the verb aparecer.

Its basic meaning is to appear, but depending on context it can also mean:

• to show up
• to become present
• to be detected
• to occur

In a scientific or process-related sentence like this one, aparece often means something like shows up or is detected rather than literally appears in a visual sense.

Because aparece is the conjugated verb form, while aparecer is the infinitive.

The subject is a amônia, which is third-person singular, so the verb must match it:

• a amônia aparece = the ammonia appears

Compare:

• eu apareço = I appear
• você aparece = you appear
• a amônia aparece = the ammonia appears
• as substâncias aparecem = the substances appear

So aparece is the correct present-tense form.

Because antes is followed by de when it introduces a noun.

So the structure is:

• antes de + noun

Since nitrito normally takes the masculine article o, de + o contracts to do:

• antes de o nitrito → antes do nitrito

This contraction is very common in Portuguese.

Other examples:

• antes da aula = before the class
• antes do almoço = before lunch
• antes dos testes = before the tests

Usually, antes do nitrito sounds more natural here.

Portuguese often uses an article with nouns in ordinary sentences, including many technical nouns. So:

• antes do nitrito = the most natural form here

You may sometimes see article omission in very technical, list-like, or headline-style language, but for a normal sentence, do nitrito is what a learner should use.

Because antes de is used before a noun or an infinitive, while antes que is used before a conjugated verb clause.

Here you have a noun:

• antes do nitrito = before the nitrite

Compare:

• A amônia aparece antes do nitrito.
• A amônia aparece antes de subir o nitrito.
  = The ammonia appears before the nitrite rises.
• A amônia aparece antes que o nitrito suba.
  = The ammonia appears before the nitrite rises.

So in your sentence, antes de is the right pattern.

The accent mark shows where the stress goes.

Amônia is pronounced with the stress on mô:

• a-MÔ-nia

Without the accent, the stress pattern would be unclear or wrong by normal Portuguese spelling rules.

For a Brazilian Portuguese learner, a rough pronunciation guide would be:

• a amônia ≈ ah mo-nee-ah
• aparece ≈ ah-pah-REH-see
• antes do nitrito ≈ AHN-cheez do nee-TREE-too
  (The exact pronunciation varies by region.)

Because a amônia is the thing doing the action of appearing.

The sentence structure is:

• A amônia = subject
• aparece = verb
• antes do nitrito = time/order expression

So literally the structure is:

• The ammonia / appears / before the nitrite

This is similar to normal English word order.

Yes. Portuguese allows some flexibility, especially for emphasis.

For example:

• A amônia aparece antes do nitrito.
  Neutral, standard order.
• Antes do nitrito, a amônia aparece.
  Emphasizes the timing/order.

Both are grammatical, but the original version is the most straightforward and natural for a simple statement.

Yes. It sounds natural, especially in an explanatory or technical context.

Depending on the exact nuance, a scientist or teacher might also say:

• A amônia surge antes do nitrito.
• A amônia é detectada antes do nitrito.
• A amônia aparece antes que o nitrito se forme.

But your original sentence is perfectly normal and clear. It is a simple way to say that ammonia comes first in the process or is noticed before nitrite.