固
This Chinese character means harden, solidify, hard, solid.
You can watch how to write this. - (「固」の書き順（筆順）動画 - 漢字書き順辞典)
There are 3 ways to read this, "かたい (固い)" or "かたまる (固まる)" or "こ".
"固い (かたい)" is "い" adjective which means hard.
"固まる (かたまる)" is a verb which means harden, solidify.
"固体 (こたい)" means solid.
Substances change their state depending on temperature and pressure.
They are solids, liquids, and gases.
By Higashi-Mikawa Kyōjindō Shujin (東三河狂人堂主人)