Learn Portuguese (Portugal): O chão da cozinha é de cortiça.

Chão refers to the physical ground or surface you walk on inside a room. Andar refers to a level or storey of a building, such as the first floor or second floor. Since this sentence describes the physical surface material in the kitchen, chão is the correct word. You will also occasionally hear piso used for the surface, but chão is the most standard everyday term.

Why does the sentence use da cozinha instead of just de cozinha?

We use da, which is the contraction of the preposition de and the definite article a, because we are talking about a specific kitchen (the kitchen in a particular house). If you said chão de cozinha, it would sound like a general classification, like "a kitchen-type floor", rather than referring to the actual floor of the specific kitchen being discussed.

The English translation says "is made of", but the Portuguese only says é de. Is it wrong to say é feito de cortiça?

It is not grammatically wrong to say é feito de cortiça (is made of cork), but it is usually unnecessary. In Portuguese, simply using the verb ser (to be) followed by de (of) is the most natural and standard way to state what material an object is composed of.

Words ending in -ão can have unpredictable plurals. What is the plural of chão if I want to refer to the floors of multiple rooms?