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GCSE Chemistry

The pH scale measures the relative amount of free hydrogen and hydroxyl ions in water. Bases are substances that react with acids to form a salt and water. Why do bases have pH's greater than 7 when for example a metal oxide is a base and doesn't dissociate to form OH- ions. I understand why alkalis have pH's above 7 on the pH scale but not why all bases do.
Original post by VoiidDev
The pH scale measures the relative amount of free hydrogen and hydroxyl ions in water. Bases are substances that react with acids to form a salt and water. Why do bases have pH's greater than 7 when for example a metal oxide is a base and doesn't dissociate to form OH- ions. I understand why alkalis have pH's above 7 on the pH scale but not why all bases do.

Metal oxide in aqueous solution form metal hydroxide
These are your A Level definitions
An acid is a proton donor
A base is a proton acceptor
(Your brønsted lowry ones)
A salt - H+ in acid replaced by metal or ammonium ion
An base neutralises an acid to form a salt
A metal carbonate is a base because of the above

See https://www.chemguide.co.uk/physical/acidbaseeqia/acids.html
I know its A Level, but the pH scale only involves concentration of H+ as pH = -log([H+])

The pH of water is at 7 at RTP only. A neutral pH is when [H+] = [OH-] see
https://www.chemguide.co.uk/physical/acidbaseeqia/kw.html

A base by accepting protons, reduces H+ concentration, there by increasing the pH (higher pH, less H+)
(edited 4 months ago)
Reply 2
Hi,

the pH scales the concentration of hydrogen ions in a solution, you can use a pOH to measure the concentration of hydroxide ions.
It is important to realise that pH is a property of water. Acids and alkalis are solutions formed when substances dissolve in water, these substances which dissolve can change the concentration of the hydrogen ions and the hydroxide ions originally present in the water and form either acids or alkaline solutions. Bases are mostly insoluble in water and so do not have a pH value. You cannot measure the pH of a solid, bases which do dissolve in water will form alkalis, which since they are solutions in water and can have a pH value, but you really mentioned this in your question above.

Not sure if this answers your question or not!!

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