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why should distilled water be used in chemical tests

Distillation is a process of heating some fluid and collecting the vapor, in order to leave impurities behind in the original fluid. Distilled water is a very
pure form of water, with nothing added or dissolved in it. Most people don't really need to use distilled water, but for some types of chemical processes, the added purity is helpful. Around the house, there are appliances that can work better and last longer, if they're operated with distilled water instead of tap water. These are all of the things that you fill with cold water, and the appliance heats the water before you take the water out. For example: teakettlebedroom vaporizercoffee percolatorelectric steam iron Some water always evaporates from inside these appliances, and when it does, it leaves behind any minerals that were dissolved in it. Over a period of time, the minerals build up to create that chalky white coating on the inside of the pot, and they can even clog the outlets of the percolator or steam iron.

Using distilled water avoids this inconvenience, since the water has no minerals dissolved in it. While distilled water will conduct electricity, it is usually not nearly as good a conductor as tap water, well water, lake water, river water, stream water, ra in water, melted snow, or even normal bottled water. The feature of water that makes it usually a good conductor is that it is such a good solvent for ionic compounds; if it comes in contact with something that can be dissolved - some of it WILL be dissolved, adding ions to the water. Distilled water usually has a lot less residual salts and other ionic compounds than the other previously mentioned types of water. When water is distilled, it is first vaporized - leaving behind pretty much all those dissolved ionic solutes - and then condensed again and collected.

It can still pick up small amounts of ionic solutes from some of the surfaces it may come into contact with. It also still has the ability to dissolve gasses that it comes into contact with. Some of those gasses will also form ionic compounds. If it comes into contact with air, some carbon dioxide will dissolve into it - rendering it slightly acidic and providing ions to promote conduction of electricity. Likewise - air usually contains other gasses that will readily dissolve into distilled water to enhance conductivity such as SOx and NOx, two classes of gasses that are associated with "acid rain". Any ammonia in the air can also disolve into the distilled water to create ions capable of enhancing the conductivity of the water.

Water that has been subjected to distillation under carefully controlled conditions where it doesn't come into contact with air and where it doesn't come into contact with materials that will dissolve into it in any appreciable ammounts will have very low conductivity. If it is further subjected to reverse osmosis, the conductivity of the water can be reduced to a level so low that it is considered an electical insulator rather than conductor. Some reverse osmosis equipment will use the conductivity of the water to indicate its purity - the lower the conductivity, the more pure the water is, i. e. the fewer solutes it contains other than straight H2O. Note that even pure water will still conduct SOME electricity since the dissociation constant for water [H+][OH-]/[H2O] (or [H3O+][OH-]/[H2O])is 10-14 which is low, but not zero.

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