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    When hydrogen chloride is bubbled into water, the resulting solution is acidic. When it is bubbled into the non-polar liquid, hexane, it dissolves but the resulting solution is not acidic. Explain these observations. [4]

    What do I write to get the full four marks?

    Nice question!

    I don't know if I can get 4 marks on it (since I'm not mark-scheme machine!), but I'll try and write to you what I think they are looking for in the question.

    First thing is to clarify what definition of an acid to use:
    1. There are three that I know of (Arrhenius, Bronsted-lowry and Lewis) - I'll take a stab in the dark and assume that the one we are interested in for this question is to do with Bronsted-lowry acids.
    2. The definition of a Bronsted-lowry acid is a proton donor.

    So essentially, we're just looking to explain why HCl dissolved in water acts as a proton donor, whereas HCl dissolved in hexane does not.

    HCl in water:
    1. Dissociates into H+(aq) and Cl-(aq) [Interesting side fact if you didn't know is that HCl is actually a covalent gas, but it doesn't matter, since it will still form H+ and Cl- ions in solution]

    HCl in hexane:
    1. Does not dissociate - remains as individual molecules of HCl just now it is HCl (aq) i.e. dissolved

    The way I would therefore tackle this question is:
    A Bronsted-lowry acid is defined as a proton donor [1]. When HCl dissolves in water, it dissociates to form H+ ions and Cl- ions - this hence produces H+ i.e. protons as a result [1]. However, when HCl dissolves in hexane, it does not dissociate [1]. Instead, it remains as HCl (aq), so has not donated any protons/H+ ions into the solution [1]. The resulting solutions for HCl in water would be more acidic, as pH = -log[H+] hence pH has decreased [1], whereas the solution for hexane will remain at the same pH as there has been no change in the [H+] [1].

    I just put these [1] marks in where appropriate. It doesn't mean that these are at all on the mark scheme, but its my opinion of it anyway.

    I hope I've helped a tiny bit. Just remember that I'm an A level student myself so take what I write with, well... a pinch of salt.
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