What is chemical bonding???I mean I dont understand how can two liquid react to form a gas or a solid or the other way round?? I need key words...

Chemical bonds results primarily because of the number of
electrons around a given element. Scientists have found that electrons are arranged in
shells or energy levels around the nucleus of the atom. Every atom is most stable when
its outermost energy level is full of electrons.  However, only the elements in group 18
- the noble gases - exist naturally with a full outer energy
level.

In general, you can think of every element, except
the first five, as having room for 8 electrons in the outermost energy level. These are
called "valence" electrons.

Atoms will do one of three
things to satisfy their need to have 8 valence electrons.  Some atoms will give up
electrons to other atoms - in general these are the metals and metalloids on the
periodic table. Other atoms will take or accept electrons - these are the nonmetals on
the periodic table.

The third possibility is that two atoms
will share  valence electrons so that both are satisfied with 8 valence
electrons.

When two different atoms come together and one
loses valence electrons and the other gains valence electrons a chemical bond is formed
which is called an "ionic" bond. It is ionic because the element that loses valence
electrons forms a positive ion (a cation) while the element that gains valence electrons
forms a negative ion (an anion).  Ionic bonds are very strong but will usually dissolve
easily in water.  Ionic compounds usually contain a metal and nonmetal in some
ratio.

When two different atoms combine and share
electrons, a "covalent" bond is formed.  Metalloids and non-metals generally form
covalent bonds with each other. The most numerous type of covalently bonded compounds
are carbon compounds. In addition, two nonmetals can form a compound with covalent
bonds. Examples include water, sulfur oxide, nitrogen oxides,
etc.

A general rule is that the farther apart two elements
are on the periodic table the stronger the ionic bond between the two when they form a
compound.  Thus group 1 and group 17 compounds react very violently forming ionic
compounds, whereas carbon - group 14 and oxygen - group 16, form a covalent compound. 
Likewise, group 1 elements will react vigorously with oxygen but the transition metals
react slowly (iron rusting) or not at all (gold does not react with
oxygen).