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Why does the PH-scales neutral point 7, and not 0??
- jacob ottosen (age 17)
lind skole, denmark
pH is a measure of the amount of Hydrogen ions (H+) in a solution. Ions are just atoms that have an electric charge on them, so H+ is a hydrogen atom with charge of 1. Even in pure water ions tend to form due to random processes (producing some H+ and OH- ions). The amount of H+ that is made in pure water is about equal to a pH of 7. That's why 7 is neutral.
For those who want a more complicated answer, pH is defined: pH = -log10
[H+], where [H+] is the concentration of H+ , expressed in moles/liter. In pure water near room temperature, the concentration of H+ is about 10-7
moles/liter, which gives a pH of 7.
I hope this answers your question.
math dan (w. mike w)
(published on 10/22/2007)
Follow-Up #1: pH and temperature
For the pH question-
pH is dependent on temperature. pH 7 is considered neutral at room temprature (25+273 K).
- Nimish (age 17)
(published on 05/04/09)
Follow-Up #2: pH scale
Why PH7 Neutral and not PH10?
- mary (age 18)
The pH scale actually is based on another scale. We usually keep track of the concentration of solutes in moles per liter (M). The pH is minus the log (base 10) of the H+
concentration in moles per liter. Since at room temperature in pure water, that concentration is very close to 10-7
M, pH 7 is neutral.
(published on 09/17/09)
Follow-Up #3: ion balance at neutral pH
The above explanations only explain why water has a pH of 7, but not why this NEUTRAL.
I think it is "neutral" because the concentration of hydroxide ions (OH-) is also 7 (pOH=7)and thus balances out the concentration of hydrogen ions (pH=7).
- Alan Bottomley (age 67)
Hobart, Tasmania, Australia
That's true. We implied that by describing the liquid as pure water, so that the formation of an H+
always goes along with the formation of an OH-
In solutions with other ions (say Na+
) there's no such constraint, so the H+
concentrations no longer equal. Thus NaOH forms a base, with lots of OH-
, and HCl forms an acid, with lots of H+
(published on 05/27/12)
Follow-Up #4: ions in water at neutral pH
Could you say that a solution with s pH = 7, neither contains H+ ions or OH– ions? Only H2O?
- William (age 18)
Nope. Some of the water molecules fall apart into ions. At concentrations of 10-7
, the rates of water falling apart and ions recombining just balance, so that's the equilibrium concentration of those ions.
(published on 05/30/12)
Follow-up on this answer.