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ELEMENTS AND COMPOUNDS
Elements are substances that are made of a single
kind of atom. When different elements combine,
their atoms join to produce a new substance, which is
called a compound. For example, common salt is a
compound called sodium chloride. It is made by
combining the element sodium and the element
chlorine. When the two elements combine, they form
a compound that is entirely different from either of
the elements used to produce it.HAVE YOU EVER WONDERED why cooking
changes raw, tough food into a tasty meal? Cooking is
just one example of a chemical reaction that converts
raw materials into new substances. Chemists use
chemical reactions to make plastics, medicines, dyes, and
many other materials that are important in everyday life.
They also study what substances are made of and how
they can be combined to make new materials.
Chemicals are the raw materials used by a chemist.
More than four million different chemicals have
been made by chemists; there are about 35,000
chemicals in common use. These chemicals can be
made by combining simple substances, called
elements, into more complicated
substances called compounds. Early chemists considered four
elements—fire, water, air, and earth. Today we know there are 92
that occur in nature and a few others that can be made in
laboratories. The most common element in the universe
is hydrogen, which is the main component of stars.CHEMICAL
REACTIONS
When different substances
combine to form new
materials, a chemical
reaction occurs. Some
reactions need heat to
start them off; others
produce heat as the
reaction proceeds.ALCHEMY
Early chemistry, called alchemy, was
a mixture of magic and guesswork.
From about 300 ce, alchemists tried
to make gold from lead, mercury, and
other cheap metals. They also tried
to find an elixir, or preparation, to
prolong life. Although the alchemists
did not succeed in these aims, they
found ways of separating substances
and making them pure. They also
discovered many new substances.Chemists use a shorthand to
describe chemicals. H 2 O is the
symbol for water, and shows
that each water molecule
contains two hydrogen atoms
(H) and one oxygen atom (O).Sodium is a
soft, silvery
metal.Chemistry
Chlorine is
a poisonous
yellow-
green gas.Sodium chloride is
a non-poisonous
powder.HISTORY OF CHEMISTRY
The Egyptians were the first
chemists. The word chemistry
comes from Chem, the name for
Ancient Egypt. Modern chemistry
began around 1790, when a
Frenchman, Antoine Lavoisier,
explained how chemical reactions
work. In 1808, an English scientist,
John Dalton, showed that
substances were made from
atoms. By 1871, a Russian teacher,
Dimitri Mendeleyev, had
produced the periodic table,
which classifies elements
according to their properties and
is the cornerstone of chemistry.
Find out more
Atoms and molecules
Egypt, ancient
Heat
Physics
Science, history ofAPPARATUS
Chemists use special flasks
and jars to mix chemicals,
together with equipment that
is electronic and automated.