At lower or higher atmospheric pressures the boiling point changes. That would have to be 100☌ for the water to be pure. ![]() To check its purity, you could check is boiling point at 1 atmosphere pressure. These tests say nothing about it being pure water!Īll these tests do is to show that the liquid you have contains water - it doesn't tell you that you have got pure water. If you are doing a UK-based syllabus, you can find links to the Exam Boards' websites where you can download a copy of your syllabus and other useful stuff on the about this part of Chemguide page. If it is only one of them, learn that one, and forget about the other one. You need to check your syllabus so that you know which version your examiners are going to ask you about. Warning: It is easy to confuse these two colour changes. You may need to turn the volume up - the sound level is a bit low. The next short piece of video shows this as well as the use of anhydrous copper(II) sulfate. When you add water to it, it turns pink again. The paper turns blue as anhydrous cobalt(II) chloride is formed. You can, if you are careful, dry the paper sufficiently just by holding it very close to a Bunsen flame. Note: A desiccator is a piece of glassware with a close-fitting lid containing some substance which absorbs water under a perforated tray that holds whatever you want to dry. (The (II) is almost always missed out when you are talking about cobalt chloride paper.)įilter paper is dipped into cobalt(II) chloride solution and then dried, either by heating it in an oven at less than 100☌, or by leaving it in a desiccator for a longer time. The anhydrous form is used as cobalt chloride paper. If you add water to white anhydrous copper(II) sulfate, it turns blue again as it goes back to the hydrated form.Ĭobalt(II) chloride solution forms pink crystals, CoCl 2.6H 2O, and if this is heated gently it turns into blue anhydrous cobalt(II) chloride, CoCl 2. Using anhydrous copper(II) sulfate to test for water That is what is used to test for the presence of water. If you heat copper(II) sulfate crystals, they lose this water of crystallisation and you get white anhydrous copper(II) sulfate. There are 5 molecules of water of crystallisation: CuSO 4.5H 2O. The copper(II) sulfate is said to be hydrated. The familiar blue copper(II) sulfate crystals contain molecules of water bound into the crystal structure. ![]() Testing using anhydrous copper(II) sulfate When conducting experiments, wearing gloves can ensure the substance doesn’t transfer to hands.This page looks at two common tests for the presence of water - anhydrous copper(II) sulfate and cobalt chloride paper. ![]() Named for a shade of blue, cobalt chloride in liquid or paper form is toxic when ingested. When heated, wet cobalt chloride paper can turn back from pink to blue as the water molecules are burned off and the original crystalline structure of the chemical is restored. According to the American Chemistry Council, cobalt chloride has a crystalline structure that changes as the molecules shift to make room for water molecules, leading to the color change that makes this substance ideal for detecting the presence of humidity. Cobalt chloride paper can be purchased in premade sheets, but it can also be made by dipping paper into a cobalt chloride solution and letting it dry.Īs humidity increases, cobalt chloride will shift from blue to pink, with a purplish color serving as the mid-phase hue between these two colors. Cobalt chloride is an inorganic salt that is bright blue when it is not exposed to humidity. When testing for humidity, the experimenter can simply use cobalt chloride paper to detect airborne water particles.
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