Page 273 - Physical chemistry understanding our chemical world

240 ACIDS AND BASES

go further by defining hydroxide as a base because it reacts with
A basic chemical con-
sumes protons. (i.e. consumes) labile protons. And any chemical capable of remov-
ing protons is said to be basic.

Aside
Saponification

Hydroxide ions react to split (‘hydrolyse’) natural esters
The word ‘saponify’ in the skin to form glycerol (II) and palmitic or stearic
comes from the Latin
sapo, meaning soap. acid – a reaction called saponification. Palmitic and stea-
ric acids subsequently react with the base to form the
respectivelong-chaincarboxylateanions – whichis soap.

H
C
H 2 C CH 2
OH
OH OH
(II)

These cleansing properties of bases were appreciated in antiquity. For example, in a
portion of the Bible probably written in about 1200 BC, a character called Job declares his
desire to be clean, saying, ‘If I washed myself with soap and snow, and my hands with
washing soda ...’ (snow was thought to be especially pure and soda (Na 2 CO 3 · 10H 2 O)
is alkaline and has long been used as a soap). This quote may be found in full in the
Bible, see Job 9:30.
The Jewish Prophet Jeremiah writing in about 700 BC says much the same thing:
look at Jeremiah 2:21–23 in the Hebrew Bible.

Why is aqueous ammonia alkaline?

Lowry–Brønsted bases

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-
protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers
of solvated protons and solvated hydroxide ions unless we add an acid or base to
it. A solution containing more solvated protons than hydroxide ions is said to be an
‘acid’ within the Lowry–Brønsted theory, and a solution comprising more hydroxide
ions than solvated protons is said to be a base.