Selenic acid
It is normally a colorless solution in water. It is a strong oxidizing agent. It can turn into selenium trioxide when dehydrated carefully. It is a very strong acid. It reacts with hydrochloric acid to make chlorine and selenous acid. It can dissolve gold to make gold selenate. When heated above 200°C, it releases oxygen and turns into selenous acid. It reacts with bases to make selenates.