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Pure substances.

posted Jul 20, 2017, 2:27 PM by Upali Salpadoru   [ updated Jul 3, 2019, 12:52 PM ]

   There are two forms of pure substances. They are elements and compounds.

   Elements are made up of atoms or clusters of similar atoms. as shown on left.
   Compounds are made up of molecules as shown on right.

Fig 1.

A pure substance.

All substances which are not mixed but exist in a pure state belong to this category.

  • They are made up of one kind of atoms or molecules.
  • They should have a definite boiling point and a melting point.
  • Composition is the same in all samples irrespective of the source

There are 2 kinds of pure substances.

They are:-  


The total number of elements, including the synthesized ones is 118.

·       These have only one kind of atoms.

( Mass of atoms can vary in the case of isotopes)

·       All the elements are listed in the Periodic table.



There is an un-countable number of compounds.

·       Almost all have more than one kind of atoms combined together. ( Ozone, O3, is an exception that only has oxygen atoms)

·       The properties of a compound are not the properties of its constituent elements.


Hydrogen, Helium, Lithium, Beryllium, Boron, Carbon, Nitrogen, Oxygen, Fluorine.



Water, Salt, Carbon dioxide, Sugar, Glass, Polythene, Sand, rubber, Caustic soda,Alcohol.

 The Periodic Table of Chemical Elements.



Atoms of an element has a definite number of protons.

Protons are the positively charged particles present in the nucleus of atoms. The number in each shell gives the number of protons present in an atom of that element.

The nucleus will also contain some neutral particles called neutrons. Every atom will have an equal number of electrons orbiting to neutralize the charge in protons.

Here are some common pure compounds.




Elements present

Relative density.


Table salt

1,413 °C

801 °C

Sodium     Na,

Chlorine     Cl,


White crystaline


100° C

0 ° C

Hydrogen       H

Oxygen          O


Colourless liquid.

Hydrogen peroxide

150.2 °C

-0.43 °C

Hydrogen       H

Oxygen          O


Colourless liquid.


Sugar does not melt or boil, but decomposes.


White crystalline.

Carbon dioxide

−56.6 °C;



(1562 kg/m3)

Colourless gas

Classifying Compounds.

There are several ways to classify compound. Here are two common ways.

1.There is no clearly defining wall for these two, yet we may consider them as follows:-

 Organic (Carbon based)
All most all the chemicals coming from living things.
Protein, Fat, Fatty acids. starch. cellulose.
Chemicals that have been synthesized without the help of living things.
Water, Iron oxide. Carbon dioxide.


Compounds that have an excess of hydrogen ions.
1.Hydrochloric acid.  HCl.

Oxide or a hydroxide of a metal.
Magnesium oxide. MgO.

When an acid combines with a base , water and a salt will be formed.
2HCl + MgO ----> MgCl2 +H2O