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I know that sodium hydroxide dissociates into sodium cation and hydroxide, so it must be an Arrhenius base. I am also aware of the rule that all Arrhenius acids and bases are considered to be Brønsted-Lowry acids and bases. However, when it comes to sodium hydroxide itself, it seems as if it does not accept proton, but hydroxide does. And someone on this StackExchange stated that it is not a Brønsted-Lowry base, but an Arrhenius base. I am confused now. Is this an exception?

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  • $\begingroup$ OH- + H2O = H2O + OH- acceptor and donator based on Bronsted and Lowry concept $\endgroup$
    – QAIS
    Jul 4, 2020 at 13:13

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All Arrhenius acids and bases are also Bronsted-Lowry acids and bases. According to this website,

The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it.

Remember, when we write $\ce{NaOH (aq)}$, what we really mean is $\ce{Na+ (aq)}$ and $\ce{OH- (aq)}$. Sodium hydroxide is a Bronsted-Lowry base because the hydroxide ions that are a part of sodium hydroxide accept protons.

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Well, let's take very simple Brønsted-Lowry base $\ce{NH3}$, as it accepts a $\ce{H+}$ in acidic medium or in aqueous medium, so we call ammonia as a Brønsted-Lowry base, whereas $\ce{NaOH}$ (sodium hydroxide) is an Arrhenius base, as $\ce{OH-}$ from $\ce{NaOH}$ will accept $\ce{H+}$, but according to Brønsted-Lowry theory, in this case hydroxide ion should be the Brønsted-Lowry base not sodium hydroxide.

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  • $\begingroup$ Sodium hydroxide dissociates into Na+ and OH- in solution - so they are effectively the same thing for aqueous medium $\endgroup$
    – TRC
    Aug 14, 2021 at 3:56
  • $\begingroup$ Well, it is a kind of wordplay, nothing much, if you follow the definition of Brønsted Lowery and Arrhenius than the answer will really make sense . $\endgroup$ Aug 15, 2021 at 5:50

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