There seems to be a punctuation problem.
My preference for the gap would be "Because it"
You have the following options to make the sentence correct:
It is colorless, and contains things such as protein and salts, so it is thicker than water.
It is colorless. Because it contains things such as protein and salts, it is thicker than water.
It is colorless, but because it contains things such as protein and salts, it is thicker than water.