Questions & Answers about ¿Qué tan opaca es esa pared de hielo?
How does qué tan work here to mean "how"?
In Latin American Spanish, qué tan followed by an adjective or adverb is the standard way to ask "how [adjective/adverb]?" to measure degree or extent. Here, qué tan opaca translates exactly to "how opaque".
Why does it say opaca instead of opaco right after qué tan?
Even though it comes right after the fixed phrase qué tan, the adjective still has to match the noun it describes in gender and number. Since it describes esa pared (that wall), which is a feminine singular noun, you must use the feminine form opaca.
Wait, how do I know pared is feminine if it doesn't end in -a?
Why use es instead of está for a wall made of ice? Couldn't the ice melt and change?
Even though an ice wall could melt, its opacity is an inherent physical characteristic of the material itself. We use ser (here as es) to describe these defining, intrinsic traits of an object, rather than temporary conditions or states.