Page 277 - Physical chemistry understanding our chemical world

244 ACIDS AND BASES

to form a salt and water:

−
HOCl (aq) + OH (aq) −−→ ClO (aq) + H 2 O (6.17)
−
−
where ClO is the hypochlorite ion.
Equation (6.17) is an example of a neutralization reaction, a topic we discuss in
more depth in Section 6.3.

How is sherbet made?
Effervescence and reactions of acids

Sherbet and sweets yielding a fizzy sensation in the mouth gen-
Malic acid (IV) occurs erally contain two components, an acid and a simple carbonate
naturally, and is the or bicarbonate. A typical reaction of an acid with a carbonate is
cause of the sharp taste effervescence: the generation of gaseous carbon dioxide. In a well-
in over-ripe apples. known brand of British ‘fizzy lolly’, the base is sodium bicarbonate
and the acid is malic acid (IV). Ascorbic acid (vitamin C) is another
common acid included within sherbet.

HO
H
CH 2 COOH
HOOC
(IV)
The acid and the bicarbonate dissolve in saliva as soon as the ‘fizzy lolly’ is placed
in the mouth. If we abbreviate the malic acid to HM (M being the maliate anion),
the ‘fizzing’ reaction in the mouth is described by

HM (aq) + HCO − − (6.18)
3(aq) −−→ M (aq) + CO 2(g) ↑+H 2 O (l)
where in this case the subscript ‘aq’ means aqueous saliva. The subjective sensation
of ‘fizz’ derives from the evolution of gaseous carbon dioxide.
This reaction between an acid and a carbonate is one of the oldest
Soda is an old fash- chemical reactions known to man. For example, it says in a portion
ioned name for sodium of the Hebrew Bible written about 1000 BC,‘... like vinegar poured
carbonate, Na 2 CO 3 · on soda, is one who sings songs to a heavy heart’, i.e. inappropriate
10H 2 O. and lighthearted singing can lead to a dramatic response! The quote
may be found in full in the book of Proverbs 25:20.

Why do steps made of limestone
Ossified means ‘to sometimes feel slippery?
make rigid and hard.’
The word comes from The typical reactions of an alkali
the Latin ossis,mean-
ing bone. Limestone is an ossified form of calcium carbonate, CaCO 3 .Lime-
stone surfaces soon become slippery and can be quite dangerous if