An acid is a substance that produces positively charged
particles of hydrogen, called hydrogen ions, when
dissolved in water. The more hydrogen ions an acid
contains, the stronger the acid is. A base is the chemical
opposite of an acid. Bases produce negatively
charged particles in water, called hydroxide
ions. The more hydroxide ions a base
contains, the stronger it is. Bases that
dissolve in water are called alkalis.Acids and bases
INDICATOR PAPER ▼
When a strip of indicator
paper is dipped into a liquid,
the paper changes colour. The
colour can be compared to a pH
scale to find out the acidity of the
solution. pH stands for “potential of
hydrogen”, and measures how many
hydrogen ions the solution contains.94
◀ CITRIC ACID
The sharp taste in citrus fruit such
as lemons and grapefruit is due to
the citric acid they contain. Citric
acid is often artificially added to
manufactured foods and drinks to give
a tangy sensation that tastes refreshing.◀ HYDROCHLORIC ACID
The lower the pH value, the
stronger the acid. Hydrochloric
acid, created when hydrogen
chloride gas dissolves in water,
has a pH of about 1. It is highly
corrosive, capable of eating
through metals.◀ CHEMICAL HAZARD
Strong acids and bases have to
be stored in containers that will
not be corroded by the chemical
within. These containers are
labelled with chemical hazard
symbols that show the
potential dangers.◀ VINEGAR
The sour taste of vinegar
comes from the acetic
acid it contains. Every step
on the pH scale is 10 times
less acidic than the previous
step, so acetic acid with a pH
of 4 is 1,000 times less acidic
than hydrochloric acid.◀ STINGER
When a bee stings,
it injects a mild acid
into a person’s flesh,
which causes a stinging
sensation. Washing the
sting with alkaline soap
may relieve the pain by
neutralizing the acid.Vinegar is made when
bacteria convert the ethanol
in alcohol into acetic acid094_095_AcidsBases.indd 94 03/01/19 12:10 PM