GCSE Chemistry

Bases have pHs of over 7. Why are metal carbonated metal bases when they don't have OH- ions in them. Do they somehow form OH- ions?

Reply 1

bl0bf1sh

Original post by VoiidDev

Bases have pHs of over 7. Why are metal carbonated metal bases when they don't have OH- ions in them. Do they somehow form OH- ions?

Not all bases release OH- ions.

Base = a proton (H+) acceptor (Brønsted-Lowry definition).

Metal carbonates are basic because they can accept (gain) H+ ions. Have a think about the structure of a carbonate ion (CO₃^2-). There are 2 x O- parts, which can essentially each "accept" a proton (H+). A bit of reshuffling of some atoms to make things more stable, and you end up with H₂O and CO₂. (It's a bit more complicated than that but that's well beyond GCSE content :smile:

https://www.chemguide.co.uk/14to16/acids/carbonates.html

Another example of a base is ammonia – NH₃. It can gain a proton (H+) to form an ammonium ion, NH₄⁺.

Reply 2

Scanjo63

Bases dont have pHs over 7, alkalis have pH over 7. Alkalis are solutions of bases dissolved in water. A base is a substance that will neutralise an acid, some bases will dissolve in water to form alkalis, most bases are insoluble solids. A lot of bases such as calcium carbonate are insoluble solids, you cannot measure the pH of a solid. pH is a property of water. Both alkalis and bases will neutralise acids. To neutalise an acid all you have to do is remove the H+ ions present, both alkalis and bases will do this but in different ways.

For example if you have an upset stomach caused by having too much acid in your stomach you would take an indigestion tablet. The indigestion tablet contains 0.5g of calcium carbonate (chalk), this is a base and it will neutralise some of the excess acid in your tummy. You cannot measure the pH of calcium carbonate because it is insoluble in water. You should not drink an alkali to relieve an upset stomach as they are corrosive (kill living cells)- so not the best idea but it would cure your indigestion!

https://www.science-revision.co.uk/alkalis%20and%20bases.html