You will find an empty chemistry flask inside the van at the roadblock.
Prepare the chemical mix into the flask. There are three different chemical ingredients in the van labeled "A", "B" and "C".
This is the low-spoiler guide for "Preparing the chemical mix" puzzle in Don't Escape: 4 Days to Survive. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.