Verses with the word soap in the Old Testament (2 verses):

Jeremiah 2:22
For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord GOD.

Malachi 3:2
But who may abide the day of his coming? and who shall stand when he appeareth? for he is like a refiner's fire, and like fullers' soap: