What does it mean when a substance has a pH greater than 7?

2 Answers
May 31, 2017

Well, at #25^@ "C"# and #"1 atm"#, the substance is expected to be basic. In other conditions, you'll have to try it and tell me.


At #25^@ "C"# and #"1 atm"#, the autoionization constant of water is:

#K_w = ["H"^(+)]["OH"^(-)] = 10^(-14)#

and thus, #["H"^(+)] = ["OH"^(-)]# would result in #"pH" = 7#, since

#"pH" = -log["H"^(+)]#

and #["H"^(+)] = sqrt(K_w) = 10^(-7) "M"# when #["H"^(+)] = ["OH"^(-)]#.

When #"pH" > 7#, it follows that #["H"^(+)] < 10^(-7) "M"#, i.e. that the solution is basic... at #25^@ "C"# and #"1 atm"#. This reflects the fact that there is less #"H"^(+)# than #"OH"^(-)# in solution, and #"OH"^(-)# influences the basicity.

How would you describe a solution with #"pH" < 7# at #25^@ "C"# and #"1 atm"#?

May 31, 2017

Answer:

It is alkali

Explanation:

A substance with a pH less than 7 is acidic, greater than 7 is alkali and equal to 7 is neutral.
As mentioned by Truong-Son N, this is assuming that the substance is at room temperature (#25^@# C) and room pressure (1 atmosphere).

pH actually refers to the concentration of #H^+# ions in a solution. Each step down the pH scale is an increase of x10 in concentration. This refers to acid strength because #H^+# ions are released into solution when an acid is dissolved in water and the stronger the acid, the higher the proportion of acid molecules that are ionised (split into ions - including #H^+#).

Hope this helped; let me know if I can do anything else:)