Page 74 - Chemistry ICSE Class IX
P. 74
62 ICSE Chemistry – 9
Now, rub a glass rod with a piece of silk cloth and bring the rubbed
end of the glass rod nearer to the stream of water. The water stream gets
attracted towards the glass rod. This can be explained as follows:
When a glass rod is rubbed with silk, it develops a positive charge on
it. When this positively charged glass rod is brought nearer to the stream of
water, the water stream gets attracted towards it. Opposite charges attract
each other. This shows that water molecules have some negative charge on
them.
Thus, we see that water molecules have both negative and positive
charges on them. Therefore, water molecule (H O) is a polar molecule.
2
What is the structure of water molecule
Water is made up of hydrogen and oxygen combined in the ratio of 2 : 1
by volume. Thus, the molecular formula of water is H O. In a molecule
2
of water, one oxygen atom is bonded to two hydrogen atoms and the two
O – H bonds are inclined to each other at 104.5° (Fig. 3.3). Thus, H O has a
2
Fig. 3.3 Structure of water molecule bent structure.
What makes the water molecule polar
In a water molecule, one oxygen atom forms bonds with two hydrogen
atoms by mutual sharing of electrons, as shown in Fig. 3.3. Thus, oxygen
atom shares one electron pair with each hydrogen atom. Oxygen is more
electronegative than hydrogen. So, oxygen atom pulls the shared pairs of
electrons towards it. This leads to a small negative charge on the oxygen
atom and small positive charges on the two hydrogen atoms. As a result,
the O – H bonds in water molecule are polar covalent bonds. The two O – H
bonds in H O molecule are inclined to each other at 104.5°. Therefore, the
2
water molecule develops a net polarity — hydrogen atoms lying at the
positive end and the oxygen atom at the negative end. As a result, water
molecule (H O) is a polar molecule.
2
Water as a Solvent
Why is water considered a universal solvent
One of the most important properties of water is that it can dissolve a
large number of substances. Water can dissolve both ionic as well as polar
covalent compounds. For example, water dissolves sodium chloride which
is an ionic salt. It also dissolves covalent compounds, such as sugar, urea
and ethanol.
Since water can dissolve a large number of substances in it, it is
considered a universal solvent.
What makes water a universal solvent
The following properties of water make it a universal solvent:
Water is a polar molecule.
Water has a high dielectric constant.
Water can form hydrogen bonds with other polar compounds.
Water dissolves many substances either by reacting with them
chemically or by forming hydrogen bonds with them.
Water is cheap and easily available everywhere.
