Chemistry REVISION OF PHYSICAL AND CHEMICAL CHANGES & ELEMENTS, MIXTURES AND COMPOUNDS FOR NDA Part-2
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Pure Substances :

`=>` These substances have fixed composition and non-variable properties.
`=>` These can not be separated into simple substances by physical methods.
`=>` Elements and compounds are pure substances.

Elements :

An element is a substance which cannot be broken up into two or more simpler substances by the usual chemical methods of applying heat, light or electric energy. There are 118 elements known at present, out of which 92 elements occur in nature, while remaining 26 elements have been prepared artificially except one. Elements may be found in all the three forms as solid, liquid or gas.

Solid : Sodium (Na), carbon (C), aluminium (Al) etc.

Liquid : Mercury (Hg), bromine (Br) etc.

Gas : Oxygen (`O_2` ), hydrogen ( `H_2`) etc.

Elements are divided in the following forms :

(i) Metals : The elements which have a tendency to form positive ions by loss of electrons are called metals, e.g, silver, tin, zinc etc.

(a) They have lustre, ductility, malleability, conductivity etc.

(b) All metals except mercury exists as solid at room temperature.

(c) Oxides of metals are basic or amphoteric in nature.

(d)Aluminium is the most abundant metal in the earth's crust.

(ii) Non-metals : The elements which have a tendency to form negative ion by gain of electrons are called non-metals. e. g. hydrogen, oxygen, sulphur etc. They are in solid or gaseous state under normal conditions except bromine which exits as liquid. There oxides are netural (in low oxidation state) or acidic (in higher oxidation state) in nature.

(iii) Metalloids : They are the elements whose properties fall between those of metals and non-metals. e.g, boron (B), silicon (Si) germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po) etc.

Symbols Derived from Latin Name of the Elements

English name of the element Latin name of the element Symbol
Copper Cuprum Cu
Gold Aurum Au
Iron Ferrum Fe
Lead Plumbum Pb
Mercury Hydrargyrum Hg
Potassium Kalium K
silver Argentum Ag
Sodium Natrium Na
Tin Stannium Sn
Tungsten Woltram W

Compounds :

A compound is a substance made up of two or more elements chemically combined in a fixed proportion by weight, e.g. water, `NaCl, NH_4 OH` and `CO_2` etc. Silicon (`Si`) is an element and silica (`SiO_2`) is a compound.

Properties of Compounds :

Following are the main properties of compounds :
• A compound cannot be separated into its components by physical methods.
• The composition of a compound is fixed, i.e. the constituents are present in fixed proportion by weight.
• The properties of a compound are entirely different from those of its constituent elements.
• A compound is a homogeneous substance.
• Compounds have fixed melting point, boiling point etc.

 
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