Man's physical universe

xanabras

UNIT VIII

SECTION 7

ACIDS ARE PROTON-DONORS AND BASES ARE PROTON-

ACCEPTORS

In all the reactions studied up to this point the reactions were either

of the double decomposition type, which simply involved the removing

of ions from a solution, or of the electron transfer type. This Section

will describe two types of compounds, acids and bases, which will

enter into double decomposition reactions and electron transfer reactions,

but which are fundamentally different from all other compounds

in that they lose and gain protons.

Acids Owe Their Properties to Hydronium Ions.

Before the discovery of the electrical nature of matter, the class of

substances known as acids was recognized by the properties common

to all acids.

Thus an acid is a substance whose water solution (1) has a

sour taste, (2) changes certain substances called indicators to develop a

color different from that shown with bases, (3) neutralizes bases (i.e.,

reacts with bases in such a way that the properties of both the acid and

base disappear)

,

series to produce hydrogen gas.

(4) reacts with metals above hydrogen in the displacement

It has been noted that hydrogen is the one substance that is common

to all acids, so it is only natural to attribute the acid properties to

hydrogen.

Acids are familiar in everyday life.

Citrus fruits taste sour because of

the presence of citric acid in these fruits; tartaric acid imparts tartness

to grapes; acetic acid is the acid in vinegar; muriatic acid (hydrochloric

acid) is used to clean metal surfaces for soldering and to dissolve cement

and mortar spots on brick and tile; hydrochloric acid is present in the

gastric juice and accounts for the sour taste of regurgitated stomach

contents; oxalic acid dissolves iron rust stains in clothes.

According to the theory of ionization, an acid is a solution containing

hydronium ions.^ The hydrogen compounds which react with water to

' The properties of acids were formerly ascribed to the presence of hydrogen ions

(protons, H"*") in their solutions. The modern theory presented in this text differs from the

above theory in that it ascribes the properties of acids to hydronium ions, which, as already

pointed out, are considered to be formed by the combining of hydrogen ions with water

molecules.

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