Acids and Bases

In Oxides and Neutralisation you dropped two oxides into water and tested what you got. One solution turned the indicator one way, the other turned it the other way, and neither oxide contained anything that looked like an acid or a base before it went in. This page says what those two words mean, where the substances come from, and — because the definition you are about to be given has limits — exactly how far you can push it.

The Arrhenius definition, and what it does not cover

The definition this course uses is Svante Arrhenius’s, and it is the one that works for the aqueous solutions you will handle.

• An acid is a substance that produces hydrogen ions, $\mathrm{H}{}^{+}$, when dissolved in water.
• A base is a substance that produces hydroxide ions, $\mathrm{OH}{}^{-}$, when dissolved in water.

So $\mathrm{HCl}(\mathrm{aq})$ is an acid and $\mathrm{NaOH}(\mathrm{aq})$ is a base, and the $\text{(aq)}$ is not decoration. Dry hydrogen chloride gas produces no ions and behaves as no kind of acid at all.

One immediate refinement. A hydrogen ion is a bare proton, and a bare proton does not drift about in water — it attaches to a water molecule, giving the hydronium ion, $\mathrm{H}{}_3\mathrm{O}{}^{+}$. You will see both written and they say the same thing about the solution. $\mathrm{H}{}^{+}$ is shorthand, and this page uses it.

Now the limits, which are worth knowing because a definition you cannot break is a definition you do not understand.

• It only describes water. Arrhenius’s definition is written in terms of what happens when something dissolves in water. Outside water it says nothing.
• It struggles with ammonia. $\mathrm{NH}{}_3$ contains no hydroxide and yet an ammonia solution is unmistakably basic. Arrhenius has to explain this indirectly, as ammonia reacting with water to release $\mathrm{OH}{}^{-}$, which is true but is stretching a definition written about producing ions to cover a substance that has none to give.
• It cannot handle a reaction with no water in it. Hydrogen chloride gas and ammonia gas react on contact to make solid ammonium chloride, and every chemist would call that an acid–base reaction. Under Arrhenius it is not one, because nothing dissolved in anything.

The repair is the Brønsted–Lowry definition — an acid donates a proton, a base accepts one — which covers all three cases and is what you would meet in Grade 12. This course stays with Arrhenius on purpose, because it is sufficient for aqueous chemistry and it is simpler. Know which tool you are holding.

Strong and weak is about how much ionises

Two pairs of words get swapped constantly and mean completely different things.

Strong or weak describes what fraction of the dissolved substance actually releases its ions. A strong acid ionises essentially completely: put hydrogen chloride in water and, for practical purposes, there are no $\mathrm{HCl}$ molecules left, only $\mathrm{H}{}^{+}$ and $\mathrm{Cl}{}^{-}$. A weak acid ionises only slightly: in ethanoic acid solution, most of the acid is sitting there as intact molecules at any given moment, with a small fraction ionised. This is a property of the substance and you cannot change it.

Concentrated or dilute describes how much solute is present in a given volume. This is a property of the mixture and you change it by adding water, as Concentration sets out.

The ones worth recognising:

Family	Strong — essentially fully ionised	Weak — only slightly ionised
Acids	$\mathrm{HCl}$, $\mathrm{HBr}$, $\mathrm{HI}$, $\mathrm{HNO}{}_3$, $\mathrm{H}{}_2\mathrm{SO}{}_4$, $\mathrm{HClO}{}_4$	ethanoic (acetic) $\mathrm{CH}{}_3\mathrm{COOH}$, carbonic $\mathrm{H}{}_2\mathrm{CO}{}_3$, citric, $\mathrm{HF}$
Bases	$\mathrm{NaOH}$, $\mathrm{KOH}$, $\mathrm{LiOH}$, $\mathrm{Ba}(\mathrm{OH}){}_2$	ammonia $\mathrm{NH}{}_3$

Two honest notes on that table. Hydrofluoric acid is weak despite fluorine being the most electronegative element there is, because the H–F bond is exceptionally strong and holds on to the hydrogen; it is also extremely hazardous, which is a separate matter from being weak. And calcium hydroxide is a strong base in the sense that whatever dissolves is fully ionised, but very little of it dissolves — strength and solubility are two more words that are not synonyms.

Is a weak acid a safe acid?

No, and this is the most dangerous confusion in the topic. Glacial ethanoic acid is a weak acid and will burn skin badly. Concentrated sodium hydroxide is not an acid at all and dissolves the fats and proteins that your skin and the surface of your eye are made from — alkali burns penetrate deeper than acid burns and hurt less at first, so they are often noticed late. “Weak” describes the degree of ionisation and says nothing whatsoever about hazard. Read the label, not the category, and follow Lab Safety and WHMIS.

Oxides are where acids and bases come from

This is the part Oxides and Neutralisation was testing, and it is a rule with almost no exceptions at this level:

• A metal oxide dissolved in water gives a basic solution.
• A non-metal oxide dissolved in water gives an acidic solution.

$\mathrm{CaO}(\mathrm{s})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to \mathrm{Ca}(\mathrm{OH}){}_2(\mathrm{aq})$

$\mathrm{Na}{}_2\mathrm{O}(\mathrm{s})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{NaOH}(\mathrm{aq})$

$\mathrm{CO}{}_2(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to \mathrm{H}{}_2\mathrm{CO}{}_3(\mathrm{aq})$

$\mathrm{SO}{}_3(\mathrm{g})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\to \mathrm{H}{}_2\mathrm{SO}{}_4(\mathrm{aq})$

The pattern follows from bonding. A metal oxide is ionic and contains the oxide ion, which takes a proton from water and leaves hydroxide behind. A non-metal oxide is molecular and covalent, and water attacks it to give a molecule that can release a proton of its own.

That is not a laboratory curiosity. Sulfur oxides and nitrogen oxides released when fuels burn dissolve in atmospheric moisture and come down as acid precipitation, which is why controlling them mattered so much in the Great Lakes region — see The Air We Share. Carbon dioxide dissolving in the ocean does the same chemistry more slowly and over a far larger volume. In the other direction, lime spread on acidic farmland is a metal oxide put deliberately where its basic solution will do some good.

Neutralisation

An acid and a base in the same beaker give a salt and, usually, water:

$\mathrm{HCl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq})\to \mathrm{NaCl}(\mathrm{aq})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})$

Swap the partners and you can see it is a double displacement, exactly as in Predicting Products. What drives it is not the salt — sodium and chloride ions were dissolved before and are dissolved after, and nothing about them changed. The real reaction is

$\mathrm{H}{}^{+}(\mathrm{aq})+\mathrm{OH}{}^{-}(\mathrm{aq})\to \mathrm{H}{}_2\mathrm{O}(\mathrm{l})$

and it goes because it removes two reactive ions from solution and locks them into a molecule. The energy released as that bond forms is the temperature rise you recorded. Writing reactions in this stripped-down form is what Precipitation and Net Ionic Equations is about.

“Salt” here means any ionic compound formed this way, not just the one on the table. And usually is carrying weight in the definition above: an acid with a carbonate gives a salt, water, and carbon dioxide, while ammonia with hydrogen chloride gives a salt and no water at all.

Because neutralisation goes essentially to completion and stops abruptly, it can be used to measure an unknown concentration precisely. That is a titration, and it is what Titrating an Acid and The Water Report are built on.

Curriculum connection

C3.3

explain the chemical reactions that result in the formation of acids and bases from metal oxides and non-metal oxides (e.g., calcium oxide reacts with water to produce a basic solution; carbon dioxide reacts with water to produce an acidic solution)

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E3.5

explain the Arrhenius theory of acids and bases

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E3.6

explain the difference between strong and weak acids, and between strong and weak bases, in terms of degree of ionization

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