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In chemistry, we recently did a lab where we observed the different ways of reading pH of several different substances. We used things such as red cabbage juice, litmus paper, pH paper and a pH probe. One of the substances we tested was water. When we used the pH probe, it came back with a reading around 8.0. When we tested the distilled water, it came back with a reading very close to 7.0. So what is different between distilled and tap water that causes the tap water to be basic?

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  • $\begingroup$ This is a good question, but it is hard to answer, because the pH of tap water strongly varies with location. Moreover, water treatment centers will typically add certain chemicals to control the pH to some extent, but this too depends on the location your water came from. $\endgroup$ – Michiel May 27 '13 at 5:47
  • $\begingroup$ Following from @michielm's comment, you should be able to call your water provider and ask how they treat your water and get an idea of what goes into it. $\endgroup$ – Ben Norris May 27 '13 at 13:17
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    $\begingroup$ This is most likely a problem of masurement. pH with a glass electrode in weakly buffered systems like water mirrors a lot of the pH of the solution the glass electrode was stored before. One must rinse the electrode several times in the water before one con make a meaningful reading. $\endgroup$ – Georg May 31 '13 at 21:16
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The tap water is likely "hard," i.e., contains some dissolved mineral salts, most likely (primarily) calcium and magnesium carbonates and bicarbonates. The anions of these salts are slightly basic, consuming $\ce{H^+}$ ions and thereby raising the pH. Carbonates (with the obvious exception of alkali metal salts) are only sparingly soluble in water, while bicarbonates are typically very soluble. A couple of other subtle factors may affect the pH:

  1. Lower atmospheric pressure encourages the evaporation of dissolved carbon dioxide from water. Since dissolved carbon dioxide exists in equilibrium with carbonic acid, it would tend to lower water pH. If you live in an area with comparatively low atmospheric pressure, your tap water might have a lower concentration of $\ce{CO_2}$, which would favor higher pH.

  2. At higher temperatures, bicarbonate decomposes to water, carbonate, and carbon dioxide. Carbonates are relatively insoluble, and the consumption of bicarbonate should drive forward the dissociation of any dissolved carbonic acid, so a higher temperature should have the effect of lowering pH.

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  • $\begingroup$ So what happens when the same water is distilled? $\endgroup$ – Cody Guldner May 27 '13 at 6:18
  • $\begingroup$ @CodyGuldner, the evaporated water doesn't carry the mineral salts with it. The water that you collect from the condensing flask should be relatively pure, while the majority of impurities (namely, the mineral salts responsible for the basic pH) will remain behind as a residue in the boiling flask. $\endgroup$ – Greg E. May 27 '13 at 6:27
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    $\begingroup$ Wrong! So called "hard" water, containing calcium/magnesium hydrogen-carbonate is slightly acid! Guess why! $\endgroup$ – Georg May 31 '13 at 21:13
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Water treatment plants typically buffer the pH of the water supply to be slightly alakaline, with a pH of ~7.5-8 normally (the standard is that it must be between 6.5 and 9.5) with various chemicals including Sodium Hydroxide (lye) or Sodium Carbonate (soda ash).

Take a look at this photo from Wikipedia which shows all of the chemicals added at this particular water purification plant.water purification chemicals From left to right: sodium hypochlorite for disinfection, zinc orthophosphate as a corrosion inhibitor, sodium hydroxide for pH adjustment, and fluoride for tooth decay prevention.)

For treatment systems which add fluoride to tap water, a buffer is further necessary - the most commonly used additive for fluoride is fluorosilicic acid.

The reason tap water is kept slighly alkaline is because acidic water will leach chemicals from the pipes which it passes through. This can be especially problematic for homes with older plumbing (i.e. lead/copper/zinc). It also increases the corrosion and wear on the pipes/fixtures it travels through.

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