Most people would say that the pH of pure water is seven. However, this is not completely accurate. In order to be more precise, one most know the temperature of the water and some facts about pH.
pH as a Measurement
pH is a unit of measure, from zero to fourteen, of the acidity or base of a liquid. It is a result of the activity of dissolved hydrogen ions within the solution. Hydrogen ion coefficients cannot be measured in actuality, so they are given theoretical numbers based on a set of standard solutions whose pH is established by international agreement.
What is pH?
The concept of pH was first introduced by Soren Peter Lauritz Sorensen (1868-1939), a Danish chemist. It cannot be confirmed what exactly the "p" in "pH" stands for. Some experts claim it stands for "potential" hydrogen." Others claim it is a corruption of a similar Greek letter that was formerly used as the symbol for concentration and represents the power of the hydrogen ion concentration. It is also theorized that the "p" stands for "potenz" -- which is German for "power."
The pH Scale
Hydrochloric acid, one of the strongest acids, has a pH of zero. A strong alkali, for example, sodium hydroxide, has a pH of fourteen. pH is measured as a logarithmic scale, so a difference of one pH unit means an increase of ten times in hydrogen ions.
Pure water is simply called "neutral" or a measurement of seven. At 25 °C, water is as close as possible to the perfect measurement of 7.00. However, once the room becomes cooler or warmer, the pH will change. Studies show that at 10 °C, water will have a score of 7.27. At 20°C, it goes down to 7.08. At 50°C , water sinks to a score of 6.63, and at 100°C it will have a score of 6.14.
Why Does It Change?
Water will range between six and seven because air will bring in carbon dioxide (CO2) and cause additional H+ to form in the solution. If the water being tested is in glass, the water can increase or decrease pH by capturing metals or hydroxide ions.
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