 In this lesson, we will learn how to test for the presence of water. There are two main tests for the presence of water. Note that these tests do not confirm the purity of water, only its presence. The first test involves the usage of anhydrous copper-2 sulfate, which is an off-white solid. The term anhydrous means without water. In the presence of water, anhydrous copper-2 sulfate becomes hydrated copper-2 sulfate, which is bright blue. The term hydrated means with water. Another method to test for water is by using cobalt-2 chloride paper, which turns from light blue to pink in the presence of water. The same concept seen in the copper-2 sulfate test applies to the cobalt-2 chloride test. The anhydrous form of cobalt-2 chloride is light blue, whereas the hydrated form is pink. Did you know that cobalt-2 chloride has been used as invisible ink? How do you think this is possible, keeping in mind what you already know about it? Let's pause the lesson to think about it and press play when you are done. A dilute aqueous solution of cobalt-2 chloride would be very light pink. When written on paper, it will not show, hence the writing is invisible. Heating the paper will dehydrate the cobalt-2 chloride turning it blue. This has been used throughout history to relay secret messages. In summary, anhydrous copper-2 sulfate and cobalt-2 chloride can be used to detect the presence of water. The color changes observed are off-white to bright blue for copper-2 sulfate and light blue to pink for cobalt-2 chloride.