Page 47 - ICSE Chemistry 8

PURE SUBSTANCES

A substance that has a fi xed composi on and fi xed chemical proper es is a pure substance. Elements
and compounds are pure substances. Pure substance are homogeneous in nature and have a uniform
composi on. They have a defi nite set of proper es. The cons tuents of a pure substance cannot be
separated by physical methods.

Characteristics of Pure Substances

A pure substance has the following characteris cs.

• A pure substance has a defi nite composi on. For example, a molecule of carbon dioxide consists of
one atom of carbon and two atoms of oxygen. We cannot have a molecule of carbon dioxide that
consists of a diff erent number of carbon and oxygen atoms other than this.
• A pure substance has defi nite mel ng and boiling points. It has a characteris c taste, colour or
smell.

Elements

Elements are pure substances consis ng of only one kind of atom. An element is a substance that cannot
be broken down into simpler substances. An atom is defi ned as the smallest possible unit of an element.
Each element is made up of only one kind of atom. At present, 118 elements are known in nature. Element
occur naturally or are ar fi cially prepared. They occur as solid, liquid as well as gases. All elements are
denoted by their unique symbols. Elements are further subdivided into metals, non-metals, metalloids
and noble gases based on their physical and chemical proper es.

Metals are lustrous, conduct heat and electricity, are malleable and duc le, and are sonorous. Gold, silver,
iron and aluminium are some familiar examples of metals.
Non-metals on the other hand, are not lustrous, do not conduct heat and electricity, are non-sonorous
and bri le. Oxygen, chlorine and nitrogen are some non-metals.

Metalloids possess proper es of both metals and non-metals. Germanium, arsenic, silicon, boron,
an mony, tellurium and bismuth are metalloids.

Noble gases are the inert gases present in gaseous state and are reluctant to react chemically with other
elements or compounds. Helium, neon, argon, xenon, krypton and radon are noble gases.

Compounds

A compound is a substance composed of two or more elements chemically combined in a defi nite
propor on by mass. The forma on of a compound occurs when atoms or molecules of diff erent elements
combine. A compound can be broken down into its cons tuent elements by chemical reac ons. The
proper es of a compound formed are totally diff erent from its cons tuent elements.

For example, water is a compound which is formed by hydrogen and oxygen but the proper es of water
are diff erent from both these elements.

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