lesson
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If you mix ammonia gas (NH3) and hydrogen chloride gas (HCl), a thick white smoke of solid ammonium chloride instantly fills the air — with zero liquid water in sight.
How can an acid-base neutralization happen without liquid water, and why does ammonia act like a base even though it has no hydroxide (OH−) ions?
The Arrhenius Model
In 1884, Swedish chemist Svante Arrhenius revolutionized chemistry by linking acidity directly to ions dissolved in water.
An Arrhenius acid is a substance that dissociates (breaks apart into charged particles) in water to produce hydrogen ions (H+), while an Arrhenius base produces hydroxide ions (OH−).
📊Create an interactive or animated side-by-side comparison of Arrhenius acids and bases in water. Left card: 'Arrhenius Acid (HCl)' showing a beaker labeled 'Aqueous Solution' where HCl splits into a glowing red H+ ion and a green Cl- ion with the text 'Increases [H+] in H2O'. Right card: 'Arrhenius Base (NaOH)' showing a beaker where NaOH splits into a purple Na+ ion and a glowing blue OH- ion with the text 'Increases [OH-] in H2O'. Below both, show the neutralization reaction: H+ + OH- -> H2O with water molecules forming. Clean minimal design, #1e2945 text, #f8f9fa background, smooth hover tooltips.
While the Arrhenius model works well for everyday solutions like stomach acid and drain cleaner, it runs into a major wall: it only applies to aqueous (water-based) solutions and completely misses bases that lack OH−.
What if we redefined acid-base chemistry around what the particles actually trade during a reaction?
The Brønsted-Lowry Model
In 1923, Danish chemist Johannes Brønsted and English chemist Thomas Lowry independently proposed a broader, more powerful definition centered on the movement of a single subatomic particle: the proton (H+).
A Brønsted-Lowry acid is a proton (H+) donor, and a Brønsted-Lowry base is a proton (H+) acceptor.
📊An interactive diagram showing a proton transfer between ammonia and water: NH3 + H2O <==> NH4+ + OH-. Visual shows an H2O molecule (red oxygen with two white hydrogens) donating one H+ proton across a curved animated dashed arrow to an NH3 molecule (blue nitrogen with three hydrogens). A toggle button allows students to switch between this reaction and the gas-phase reaction HCl(g) + NH3(g) -> NH4Cl(s), demonstrating how Brønsted-Lowry works with and without water. Labels clearly tag: 'Proton Donor (Acid)' and 'Proton Acceptor (Base)'.
Because a hydrogen atom is just one proton and one electron, losing its electron to become H+ leaves behind a bare nucleus — which is why chemists use proton and H+ ion interchangeably.