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Thread: What happens to the pH of an acid when water is added to it?

  1. #1 What happens to the pH of an acid when water is added to it? 
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    I was wondering what happends to a pH of an acid when water is added to it....water is neutral so the acid is more neutral... :?


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    You are simply watering it down imagine tipping a litre of very strong acid into the sea and mixing it thoroughly, the sea does not become a strong acid. The acid is simply dilluted.

    so Yes the acid will move more toward nuetral, some of our chemists might be able to offer some mathematical data.


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  4. #3  
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    The pH should increase, since the acid is less concentrated.
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  5. #4 Partly right 
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    The pH will get closer to 7 but not higher. Water can only make acid more neutral, but not basic.
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    I thing that the pH would not change. There is a big difference between the concentration of the acid, and the pH of the acid. Adding water would make an acid more dilute, and hence alter its concentration..., whereas the pH would remain the same...for example the pH of H2SO4 is always 0....no matter how concentrated or dilute it is.
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    Quote Originally Posted by scientist-to-be
    I thing that the pH would not change. There is a big difference between the concentration of the acid, and the pH of the acid. Adding water would make an acid more dilute, and hence alter its concentration..., whereas the pH would remain the same...for example the pH of H2SO4 is always 0....no matter how concentrated or dilute it is.
    Actually, the pH of a substance is simply the negative log of the concentration of H+ (in solution). As you dilute the acid, there is a lower concentration of H+; thus, the pH gets higher. A mathematical perspective is concentration (M) = moles substance/litre solution. As the litre solution increases (you add more water), the Molarity gets smaller, since L is in the denominator. As the molarity gets smaller, the -log [H+] gets higher. But, like Dantak said, it won't go past 7. You can only dilute it to the point that the solution either has an equal, or less concentration of H+ than water; if it has less or equal, then the pH of water=the pH of the solution (makes sense).

    Example:
    The [H+] of water is 1.0*10^-7. The pH is -log(1E-7) = 7.
    If HA (the acid) has a [H+] of, say 1.0E-8 (it has less H+ ions/litre), then it would make sense to take the pH of water (since it has more H+ ions/litre, and is in the same container). Thus, the pH of HA is -log(1E-7) = 7.
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    Well, then I guess you are right....when considering the addition of water as a reaction with H2O, ur statement makes perfect sense, since water would act as a base with the acid ( can be considered amphoteric), and therefore a neutralisation reaction occurs. Now, increasing the number of moles of water ( by making the acid more dilute) means there is more OH- atoms to neutralize the acid, and thus the pH of the acid would approach 7. Thank you for correcting me.
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    Quote Originally Posted by scientist-to-be
    Well, then I guess you are right....when considering the addition of water as a reaction with H2O, ur statement makes perfect sense, since water would act as a base with the acid ( can be considered amphoteric), and therefore a neutralisation reaction occurs. Now, increasing the number of moles of water ( by making the acid more dilute) means there is more OH- atoms to neutralize the acid, and thus the pH of the acid would approach 7. Thank you for correcting me.
    that's another way of looking at it 8)
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    when water is added to acid its clear that its pH will increase but why not beyond 7..........?
    Is that true ionization of water occur whose proton contribute in not increasing the pH beyond 7.......please help me out ....

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    thank u ..but still there is some thing i wanna know ....since acid is present and there is slight dilution so there will be ionised water present in whole acidic solution ...then why its proton contribution is only when it is highly diluted ....since theoretically it should be like that when we go on increasing dilution ph should also go on increasing....but it is not the real case ...we never get ph more than 6.999999 of a highly diluted acidic solution ....neither like 7...
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    why didn't anyone tell him not to add the water to the acid?!?
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    Edit: PhDemon is right, whoops.
    Last edited by CLK5; June 23rd, 2013 at 09:30 AM.
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    Quote Originally Posted by Joey View Post
    I was wondering what happends to a pH of an acid when water is added to it....water is neutral so the acid is more neutral... :?
    come on, if the acid is strong (activity coefficient is high) then there won't be much of an effect as it might have already dissociated completely and the hydrogen ions you are adding is too less compared to the already existing ones. if the acid is weak (acetic etc..) they hydrolyse much more on addition of water thereby they become stronger with respect to the increase in hydrogen ion concentration with dilution......but, in their case since adding water dilutes them, the net concentration of H+ ions decreases/nearly remains same(albeit might increase for some time).....thereby their pH varies(becomes slightly more acidic then due to volume increase just returns to original state and then gets dilute)
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  15. #14  
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    The pH will asymptotically approach 7, but never touch it.
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    A.O.A when water is added in the acid it act as a base result a neutralization reaction take place and pull the pH toward 7.It should make clear that when water is added in acid it react with acid so molecule of acid are being consumed result lowering the concentration of acid per liter of solution so pH which -log of H+ concentration increase but not be 7
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    Nope. No neutralisation reaction, just dilution.
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