[manikantan]

Why the Ph of water isa coined as seven, why can't it to be zero or some other number?

[Capuchin]

Because the pH scale goes from 0 to 14, with 7 being neutral.

Lower than 7 indicates acidity (presence of H+) and higher than 7 indicates basicity (presence of OH-).

Please read PH - Wikipedia, the free encyclopedia for more information on the pH scale.

[Capuchin]

The exact reason is because that's the scale we use.

It's a logarithmic scale of the concentration of H+ ions.

It really is that simple.

You didn't state you were a graduate and so I couldn't have known that.

The link I gave you gives a good explanation of the exact formulation of pH which you should know, being a graduate. I certainly know it and I am not a graduate.

Your question is typical of a 12 year old who is curious about the scale, having just learned about it, and so I took that judgement and explained it simply and accurately.

[Capuchin]

Out of curiosity, how did you get your degree?

[labman]

We defined the pH scale. Under the definition used, water happens to be 7. It is not like the Celsius temperature scale where the freezing and boiling points of water were used to define 0 and 100. That definition resulted in absolute zero being -273 C. So the definition of pH results in water being pH 7.

[Capuchin]

I think this is mine and labman's statements distilled (hopefully clearly):

Labman: Why should water be 0? Water is like any other chemical and doesn't receive any special treatment. We chose a scale based on H+ ion concentration and water falls wherever it falls.

Me: The scale is a logarithmic expression based around the concentration of H+ ions, and water is 7 (at the centre of the scale) because it dissociates into both H+ and OH- in equal amounts (and hence is neutral)

Labman, feel free to dispute me if I have interpreted you wrongly

[manikantan]

Out of curiosity, how did you get your degree?

No body wants to saw the exact reasons and telling me I am fool... and it is like socarates on those days...

[labman]

Let me try again. I can't say why ''p'' is used to mean the negative of the logarithm of something. There may or may not be a good reason, I just don't know. It can be used for other things too. So pH means the negative log of the H+ ion, or in water actually the H3O+. Water is a covalent compound but a small fraction breaks down into ions, 2H2O -> H3O+ + OH-. H+ alone is non existent in water. The product of concentrations of the 2 always equals 10^-14. In equal quantities as in pure water, each will will be -7. So as pH was defined, water has a pH of 7. Water has a pH of 7 because it only disassociates enough to produce 10^-7 H+, really H3O+ ions.

The concept that the value some things have depends on whether we have used it to define the term or not can be confusing. In many cases we have used water to define a property. It has a specific gravity of 1 because we have defined specific gravity as being the ratio of something's density to that of water. I don't think you are a fool because you are confused about this. I don't think my answer should have suggested that. That is against the rules here. If you feel anybody's answer disparaged you, click on the ''Report inappropriate answer'' link at the top right corner of the answer.

[Capuchin]

PH stands for "pouvoir d'Hydrogen" or "power of hydrogen" :)

[manikantan]

Thanks for your answer, but the wordings make me little bit confusion... can u give little more simple english

Let me try again. I can't say why ''p'' is used to mean the negative of the logarithm of something. There may or may not be a good reason, I just don't know. It can be used for other things too. So pH means the negative log of the H+ ion, or in water actually the H3O+. Water is a covalent compound but a small fraction breaks down into ions, 2H2O -> H3O+ + OH-. H+ alone is non existent in water. The product of concentrations of the 2 always equals 10^-14. In equal quantities as in pure water, each will will be -7. So as pH was defined, water has a pH of 7. Water has a pH of 7 because it only disassociates enough to produce 10^-7 H+, really H3O+ ions.

The concept that the value some things have depends on whether we have used it to define the term or not can be confusing. In many cases we have used water to define a property. It has a specific gravity of 1 because we have defined specific gravity as being the ratio of something's density to that of water. I don't think you are a fool because you are confused about this. I don't think my answer should have suggested that. That is against the rules here. If you feel anybody's answer disparaged you, click on the ''Report inappropriate answer'' link at the top right corner of the answer.

[worthbeads]

no body wants to saw the exact reasons and telling me i am fool....and it is like socarates on those days.....

manikantan disagrees: "i want the exact reason becaz i am a graduate in chemistry and i konw little better in chemistry"

Obviously your major wasn't english. Well, anyway, the point is you can't go yelling at people who treat you like you're stupid if you act stupid.

[u4212128]

Here is another explanation.

Water dissociating into ions is an equilibrium. Which has an equilibrium constant

K = [H3O+][OH-]/[H20]

Water is considered a constant because it's a really large concentration so it can be represented as

Kw = [H3O+][OH-] = 1E-14 The water concentration being absorbed into the K term (you should be familiar with this, 1E-14 is a literature value)

If you rearrange to find the concentration of either H3O or OH you get

[H3O+] = [OH-] = (1E-14)^1/2 = 1E-7

pH is the negative log scale of concentration so the log of this is 7 and hence this is why pH of water is 7 not 0.

Hope this helps

If not any chemistry text will give a good explanation, try Silberberg Chemistry 3rd edition.