# Q & A: Why is pH = 7 the neutral point?

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 = -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.

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 05/03/2009)

## 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 09/17/2009)

## 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 05/25/2012)

## 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 OHâ€“ 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 05/30/2012)

## Follow-Up #5: pH of hot and cold water

Q:
OK, I think I follow all the answers so far... start with H2O, one H+ for every OH-, at room temperature that happens to be 10-7 for pH=7.But, what about other temperatures? Does it follow that at 99C (or higher under pressure) that a "neutral" pH might not be 7 because the water might disassociate more at higher temperatures??Thanks for your answers.
- Christopher (age 16)
Greer, SC USA
A:

Exactly. Here's a table of the neutral pH values over a range of temperatures, borrowed from the link below. That site also has a nice discussion

T (°C)   pH
0   7.47
10   7.27
20   7.08
25   7.00
30   6.92
40   6.77
50   6.63
100   6.14

http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Acids_and_Bases/Aqueous_Solutions/The_pH_Scale/Temperature_Dependent_of_the_pH_of_pure_Water

Mike W.

(published on 03/16/2016)

## Follow-Up #6: acid-base equilibria

Q:
My question contains two parts:First, dissolving a strong base in water produce a lot of OH-. I'm assuming that this does not change the hydrogen ion concentration. Why is the PH of a base bigger than 7?Second,At the equivalence point of a titration between acetic acid (weak acid) and sodium hydroxide (strong base). The PH is around 9. This is because CH3COOH reacts with OH- produced water and CH3OO- which is a relatively strong conjugate base. CH3OO- react with water produce some OH- thus increase the PH at the equivalence point. I wonder why can't the OH- produce in second reaction going back to be the reactant of the first reaction? This way, fewer moles of NaOH than acetic acid will be needed to add to the system to reach the endpoint. Also, the endpoint would still be natural. I know this hypothesis is wrong but please let me know why.Thank you for taking the time to answer this long question!
- Alina Wang (age 17)
Mechanicsburg, Pennsylvania, USA
A:

The key point is that your assumption here is wrong. "...produce a lot of OH-. I'm assuming that this does not change the hydrogen ion concentration." Some of the OH- combines with H+ to make plain H2O. At room temperature in equilibrium [H+][OH-]=10-14 Molar2.

I got a little lost in your second question, but perhaps the first answer will clarify it.  For each reaction there's an equlibrium reached, where (approximately) the product of the concentrations of reactants on one side of the reaction equals some constant time the product of the concentrations of reactants on the other side. (In the products each reactant concentration is multiplied in the number of times that the reactant appears in the reaction formula.)

The reactions are maintained in dynamic equilibrium, where both directions of reaction keep happening, but in equilibrium the forward and backward rates are equal.

Mike W.

(published on 03/26/2020)