The test paper allows the quick and easy detection of water and water vapor. On contact with water the paper changes color from light blue to dark blue. The reaction is not reversible. Even after drying the contact with water can clearly be proved.
The test paper is used to control the leak tightness of tubing or to detect condensed water. It can also be used to control, if sensitive products like electronics come into contact with water. This helps to detect unjustified complaints.
In chemical laboratories the test paper is used to control solvents. If the paper remains uncolored after evaporation of the solvent, the solvent is anhydrous. In reactions sensitive to water it can then safely be used.
For the detection of water vapor in gas streams the paper is moistened with anhydrous isopropanol. The gas stream is directed on the paper. The presence of water vapor is indicated by a blue color.