“Whose chair is this?” “It is mine.”
イス in Katakana is also written as 椅子 in Kanji or いす in Hiragana. We usually use Katakana for loan words. But イス is an exception.
According to an academic paper published by National Institute for Japanese Language and Linguistics in Y2013, 椅子 is written in Katakana (8.8%), in Hiragana (14.1%), and in Kanji (77.2%), respectively.