in this video we're going to look at the difference between strong and weak acids and then see how ph is related to the concentration of hydrogen ions an acid is a substance that forms aqueous solutions with a ph of less than seven and the reason for this is that they ionize in aqueous solutions to release hydrogen ions which just means they split or dissociate into their ions for example hydrochloric acid actually exists as h plus and cl minus ions while nitric acid ionizes into a hydrogen ion and a nitrate and remember a hydrogen ion is used a hydrogen atom that's lost one electron so it's actually just a proton now hydrochloric and nitric acid are both examples of strong acids just like sulfuric acid and what defines them as strong acids is that they all ionize completely so all the acid particles will dissociate to release hydrogen ions or in other words the reactants turn completely into products weak acids on the other hand which include things like ethanoic acid citric acid and carbonic acid don't fully ionize in their case only a small portion of the acid particles actually dissociate to release hydrogen ions this is because the ionization of a weak acid is reversible which means that there's an equilibrium between the undissociated and associated forms of the acid we can see this in our equation here for the dissociation of ethanoic acid because there's a double arrow which shows that the reaction is reversible we cover reversible reactions properly in another video but all it means is that as well as the reactants being able to form the products the products can react to reform the reactants so basically the reaction can go both forwards and backwards because only a few of the acid particles actually dissociate we say that the equilibrium lies to the left which just means that at equilibrium we'll have far more molecules of undissociated acid then we will molecules of dissociated acid now it's really important that you don't confuse an acid strength with this concentration being strong or weak is all about how much an acid associates so what proportion of the acid molecules ionize in water however concentration is a measure of how much acid there is in a certain volume where the more concentrated solutions containing more acids per unit of volume so it's possible to get strong acids like hydrochloric acid at both high and low concentrations just like a weak acid could also be either concentrated or dilute the last thing we need to cover is ph which is just a measure of the concentration of hydrogen ions in the solution the weird thing though is that as the concentration of hydrogen ions gets higher the ph gets lower in fact each decrease of one on the ph scale represents the concentration of hydrogen ions increasing by a factor of 10. so if we moved from ph 5 to ph 3 the hydrogen ion concentration would increase by 10 times and then 10 times again so it would be 100 times higher in total or if we moved from ph 8 to 11 the hydrogen ion concentration would decrease by a factor of 10 three times so would be one thousand times lower so putting all of this together to get a very acidic solution that has a low ph we'd need a high concentration of hydrogen ions if we had a strong acid like hydrochloric acid we could achieve this low ph at most concentrations because each particle dissociates fully and so overall we'd have loads of hydrogen ions being released on the other hand if we had a weak acid like carbonic acid then to get a low ph it would have to be really concentrated because so few of the acid particles would actually ionize and release their hydrogen ions another way of putting this concept is that at any given concentration a strong acid will always have a lower ph than a weak acid because a higher proportion of the strong acid molecules will dissociate to release their hydrogen ions and is only the concentration of hydrogen ions that determines the ph anyway that's all for this video so if you enjoyed it then please do give us a like and subscribe and we'll see you next time you