A dureza da água muda na água salgada.

In Portuguese, it is very common to use the definite article with general ideas or categories.

So A dureza da água can mean water hardness in a general sense, not necessarily one specific sample of water. English often drops the article in these cases, but Portuguese often keeps it.

That is why A dureza da água... sounds normal.

Portuguese usually does not build noun compounds the way English does.

English:

• water hardness
• car door
• coffee cup

Portuguese usually prefers:

• dureza da água
• porta do carro
• xícara de café

So dureza da água is the normal Portuguese structure: noun + de + noun.

Da is a contraction of de + a.

• de = of / from
• a = the for a feminine singular noun

So:

• de + a água → da água

That is why you get dureza da água.

Here, muda is the verb mudar in the present indicative, 3rd person singular.

It matches the subject a dureza, which is singular.

• a dureza muda
• as durezas mudam

In this sentence, it expresses a general fact or regular behavior, which is a very common use of the present tense in Portuguese.

Here it is a verb.

That matters because muda can also be:

• an adjective meaning mute (feminine form)
• a noun meaning seedling / young plant

But in A dureza da água muda na água salgada, the structure clearly shows that muda is the verb: the subject is a dureza da água, and muda is what that subject does.

So here it means changes.

Na is a contraction of em + a.

• em = in / on / at
• a = feminine singular article

So:

• em + a água salgada → na água salgada

In this sentence, na água salgada gives the setting or condition: in salt water.

Yes, sometimes you can.

The difference is mostly one of style and nuance:

• na água salgada often sounds a bit more definite or concrete
• em água salgada can sound a bit more general, scientific, or category-like

Both are possible in many contexts. In everyday Brazilian Portuguese, na água salgada sounds perfectly natural.

Because salgada is an adjective, and it must agree with água.

Água is a feminine noun, so the adjective must also be feminine:

• água salgada
• água fria
• água limpa

So even though English does not change adjectives, Portuguese does.

Yes, it is understandable and grammatically correct.

That said, some speakers might feel it is a little repetitive because água appears twice:

• A dureza da água
• na água salgada

Depending on context, a Brazilian speaker might rephrase it to sound smoother. But as a learning sentence, it is useful because it clearly shows:

• article use
• contractions like da and na
• adjective agreement in água salgada
• the verb muda

Água is pronounced roughly like AH-gwa in Brazilian Portuguese.

The accent mark in á shows that the stress is on the first syllable:

• Á-gua

Without getting too technical, the accent helps show both the stress and the vowel quality.

In the full sentence, the main stresses are roughly:

• du-RE-za
• Á-gua
• MU-da
• Á-gua sal-GA-da