Learn Portuguese (Portugal): Que cheiro é este?

In Portuguese, when the word for "what" is followed immediately by a noun like cheiro, you must use Que. Qual is generally used when asking "which one" or right before the verb ser, while O que is used when "what" stands entirely alone in the sentence.

Why is the word order Que cheiro é este instead of O que é este cheiro?

It is much more natural in Portuguese to keep the question word Que attached to the noun it modifies (Que cheiro), followed by the verb (é) and then the demonstrative pronoun (este). The structure literally translates to "What smell is this?" which is the standard way to ask this in Portuguese.

Why use the verb ser (é) instead of estar (está)? Isn't a smell temporary?

Even though a smell might drift away quickly, you are asking to identify what the source of the smell actually is. In Portuguese, we always use the verb ser for identifying and defining what things are, regardless of how temporary they seem in the moment.

Would the word este change if I was asking about a different noun?