Question

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

Answer 1

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.

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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"`?

Answer 2

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:)