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Q:
Why does the PH-scales neutral point 7, and not 0??
- jacob ottosen (age 17)
lind skole, denmark
A:
Jacob,
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 = -log
10[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
Q:
For the pH question-
pH is dependent on temperature. pH 7 is considered neutral at room temprature (25+273 K).
- Nimish (age 17)
Mumbai, India
A:
Yes.
Mike W,
(published on 05/04/09)
Follow-Up #2: pH scale
Q:
Why PH7 Neutral and not PH10?
- mary (age 18)
jersey city,nj
A:
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.
Mike W.
(published on 09/17/09)
Follow-Up #3: ion balance at neutral pH
Q:
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
A:
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
+ or Cl
-) there's no such constraint, so the H
+ and OH
- concentrations no longer equal. Thus NaOH forms a base, with lots of OH
-, and HCl forms an acid, with lots of H
+.
Mike W.
(published on 05/27/12)
Follow-Up #4: ions in water at neutral pH
Q:
Could you say that a solution with s pH = 7, neither contains H+ ions or OH– ions? Only H2O?
- William (age 18)
Norway
A:
Nope. Some of the water molecules fall apart into ions. At concentrations of 10
-7M H
+ and OH
-, the rates of water falling apart and ions recombining just balance, so that's the equilibrium concentration of those ions.
Mike W.
(published on 05/30/12)
Follow-up on this answer.
