The Wright Stuff

Acids and bases are very common topics in science and chemistry courses. But what exactly are they? Just something corrosive? (No.) Here you'll find the answers. We start with the commonest definitions used in introductory science courses, then move on to more advanced definitions such as the Bronsted-Lowry definition and the Lewis definition, if your course needs them. We start with acids, then move on to bases.
There is a separate section on strong acids and bases, as many people don't know that there is a special meaning to the words strong and weak in these names, and they confuse what the words actually mean.

What is an Acid

The definition of an acid depends on the level of the course you are taking.

At its simplest, we use what is known as the Arrhenius definition. This is the definition used in almost all first courses in science or chemisty at schools.

An acid is a substance which forms H+ ions as the only positive ion in aqueous solution.


Hydrochloric acid dissolved in water forms H+ and Cl- ions
  HCl ---> H+ + Cl-

Sulphuric acid dissolved in water forms H+ and SO42- ions
  H2SO4 --->2H+ + SO42-

Nitric acid forms H+ and NO3- ions when dissolved in water
  HNO3 ---> H+ + NO3-

Ethanoic acid,also know as acetic acid, forms H+ and CH3COO- ions in water
  CH3COOH ---> H+ + CH3COO-

Acetone, also known as propanone, doesn't form any ions in water, so it isn't an acid.
CH3COCH3 just dissolves in water.
Methane, CH4, doesn't form any ions in water, so this isn't an acid either.
So just containing hydrogen doesn't make something an Arrhenius acid.


Properties of Acids


What is a Base

The definiton of a base depends on the level of the course you are taking. It also includes the definition of an alkali - an alkali is a water soluble base.

At its simplest, we use what is known as the Arrhenius definition. This is the definition used in almost all introductory courses in science or chemisty at schools.

An alkali is a substance which forms OH- ions as the only negative ion in aqueous solution.
A base is an insoluble hydroxide.


Sodium hydroxide, when dissolved in water, forms Na+ and OH- ions
  NaOH ---> Na+ + OH-

Calcium hydroxide dissolves in water to give Ca+ and OH- ions
  Ca(OH)2 ---> Ca2+ + 2 OH-

Ethanol CH3CH2OH does not form OH- ions when dissolved in water, so it isn't a base.
Ethanol just dissolves. Strictly speaking, ethanol actually forms a tiny amount of H+ ions in water and is a very weak acid.

Properties of Bases


At more advanced levels, other definitions are used instead of the Arrhenius definition. It's not that Arrhenius wasn't a good chemist - by anyone's standards he was brilliant (although a little eccentric) - he won a Nobel Prize for his work on electrolytic dissociation. He simply looked at all the substances that people called acids and alkalis and bases at that time (the late 1880s and 1890s), and worked out what they all had in common. Then he produced a definition that matched what he had discovered. The Bronsted Lowry definition that follows dates from 1923.


Advanced Definitions

The following section only applies to those doing more advanced courses. The general properties of an acid and base given above don't apply when we use the more advanced definitions that follow.


At more advanced levels, we use the Bronsted-Lowry definition of an acid and a base:-

An acid is a proton donor.
A base is a proton acceptor.

This has the advantage that we are no longer restricted to using water as a solvent, or even to using any solvent at all. Anything that was an acid or base under the first, simpler, Arrhenius definition, is also a Bronsted-Lowry acid or base, but now some other things count as acids and bases as well.

As an example, let's look at a reaction between two gases.

Hydrogen chloride gas and ammonia gas react to give the white solid ammonium chloride.
  HCl(g) + NH3(g) ---> NH4Cl(s)

As neither are dissolved in water, then according to the Arrhenius definition this is not an acid-base reaction. But if you dissolve them in water, HCl forms hydrochloric acid and H+ ions and NH3 reacts with water to form a small amount of NH4+ and OH- ions. Mix them and they from a solution of NH4Cl. Clearly it is silly not to call the gas phase reaction an acid-base reaction.

According to the Bronsted-Lowry definition, the HCl donates a proton to the NH3, which obviously accepts it. Therefore this is an acid-base reaction. If you look carefully at all the earlier examples on an acid-base reaction given under the Arrhenius definition, you will notice that the acid donates its H+ ion to the base's OH- ion to form water. Hence these also are acid-base reactions. All that the Bronsted-Lowry definition does is describe what happens when they react. And in doing so, it includes other very similar reactions that do not involve water as a solvent.

As another example, look at the reaction between these two familiar compounds when they are dissolved in ethanol, instead of water, and you will see that it is obvious that this is an acid-base reaction.

NaOH + HCl ---> NaCl + H2O

Yes, it is exactly the same reaction as when they are dissolved in water!

This also extends the definition of a base to substances such as carbonates, metal oxides and the like. When they react with an acid, they accept the donated proton.

2HCl + MgO ---> MgCl2 + H2O

In the reaction above, the HCl donates an H+ ion to the O2- ion. Therefore the HCl acts as an acid and the MgO acts as a base. There is no mention of a solvent here.


Lewis Acids and Bases

Another advanced definition of acids and bases is that of Lewis. This looks a bit more closely at what happens when the chemical bonds are formed in the reaction between an acid and a base, and is a very useful definition at advanced levels.

The Lewis definition of an acid and base says:-

An acid is an electron acceptor,
and a base is an electron donor.

This totally removes the concept of hydrogen ions being a pre-requisite for an acid. But like the Bronsted Lowry definiton above, it still includes every acid and base under the Arrhenius definition, and all those under the Bronsted Lowry definition. It also includes a huge number of extra reactions as well. When a Bronsted Lowry base accepts a proton from an acid, it forms a bond to it, using electrons of course. Hence Lewis is simply examining what really hapens in more detail. Then extending it to other situations including the formation of complexes (coordination compounds).

When HCl reacts with NaOH, the OH- ion donates electrons to the H+ ion, so this is a Lewis acid-base reaction.

When HCl gas reacts with NH3 gas, the nitrogen donates a pair of electrons to the hydrogen, so this in a Lewis acid-base reaction.

When HCl (gas or in solution) reacts with MgO, the O2- ion donates a pair of electrons to the hydrogen. So this is a Lewis acid-base reaction.

Many transition metal ions form complex ions with neutral or negatively charged ligands, and these are all actually Lewis acid-base reactions. For example, when you add NaOH solution to ZN(OH)2, the Zn(OH)2 dissolves, forming a complex ion. The OH- ions from the NaOH each donate a pair of electrons to the Zn2+ ion. So here the OH- is acting as a Lewis base and the Zn2+ is acting as a Lewis acid.

Zn(OH)2 + 2NaOH(aq) ---> 2Na+(aq) + [Zn(OH)4]2-(aq)

Silver chloride dissolves in concentrated solutions of ammonia, and this too is an acid base reaction. The Lewis base NH3 donates electrons to the Ag+, a Lewis acid, forming a soluble complex ion.

AgCl(s) + 2NH3(aq) ---> [Ag(NH3)2]+(aq) + Cl-(aq))

Mixing NH3(g) with BF3(g) forms a white solid as the Lewis base NH3 donates a pair of electrons to the neutral boron atom in BF3 (a Lewis acid).

BF3 + NH3 ---> F3B.NH3

In fact any substance with a lone pair of electrons is a Lewis base, and any substance which can accept electrons is a Lewis acid. Examples of Lewis bases include NH3, PF3, PCl3, H2S, H2O, HOCH2CH2CH2OH, Cl-, OH-, O2CCO22-,and any negatively charged ion. Lewis acids include H+, any positively charged metal ion e.g. Ti3+, Fe2+, Ni2+, Pt4+, Na+, K+, BH3 (actual formula is B2H6), BCl3, AlCl3 (actual formula is Al2Cl6).

We've come a long way from an acid as a substance that forms H+ ions in water. If your course doesn't cover these advanced definitions, then just ignore them.