Water and p h

1. By
Dr.Khalida Bano
Water and pH

2. Water
Water constitutes a principal end product of oxidative
metabolism of foods and most abundant substance of the
body,making about 65% to 70% of body mass.
It solvate a wide range of organic molecules.
Water has a slightly ability to dissociate into hydroxide ions
and protons.

3. Structure of water:
Water is a dipole molecule with electrical charge distributed
asymmetrically about its molecule.
Hydrogen bond exists between water molecules and by
providing great internal cohesive forces make water a liquid
at room temperature and give it solvent properties.
Hydrogen bond is responsible for viscosity and surface tension
properties of water.

4. Shape of water is v-shaped at an angle of 104.5’.
Polar in nature due to high electro negativity of oxygen
relative to that of hydrogen.

6. Water is a compound that consists of two hydrogen atoms
and one oxygen atom attached together by two sigma bonds
and with two lone pairs of electrons around the oxygen. This
attachment of the hydrogen nucleus to the central oxygen
atom by electrons is called a covalent chemical bond. These
components of a water molecule generally form a tetrahedral
arrangement around the oxygen atom. 109.5° is the
expected bond angle between each component; however,
this is not the case due to the repulsive forces of the lone
electron pairs..

7. . As a result, the electrons push the hydrogen atoms closer
together, resulting in a bond angle between the hydrogen
atoms of 104.5°. The geometry of water molecules is
typically referred to as "bent" or "angular". Since this is a
bent structure and not a linear structure the distribution of
charges is not symmetrical. Therefore giving a polar trait to
water molecules

8. Intermolecular Structure
The partial charges on the oxygen and the hydrogen allow for
water to participate in hydrogen bonding. The partial
negative charge on the oxygen is attracted to the partial
positive charge on the hydrogen of another water molecule.
The oxygen atom is partial negative because the oxygen
nucleus draws away the electrons from the two hydrogen
atoms.

9. The net charge of the hydrogen atoms becomes partial
positive.
A water molecule exhibits an attractive force to other water
molecules and has ability of ionization due to this hydrogen
bonding characteristic.
Hydrogen bonding is one of the strongest electrostatic
interactions. Hydrogen bonding in water is responsible for
many of water's unique properties, such as high melting and
boiling points, high heat of vaporization, and high surface
tension.
.

10. Chemical and Physical Properties of
Water
Hydrogen Bonding
High melting point, boiling point, and heat of vaporization
are unique qualities of water caused by the attractions
between adjacent water molecules. Each hydrogen of a water
molecule shares an electron pair of the central oxygen atom.
Because oxygen is more electronegative, its nucleus attracts
electrons more strongly than the hydrogen nucleus. The
result gives each hydrogen a partial positive charge, and the
oxygen a partial negative charge. This allows for
hydrogen bonding the electrostatic attraction between the
hydrogen of one water molecule to that of another

11. High Heat Capacity
Specific heat is a measure of heat required to raise the
temperature of 1 gram of water 1°C. Water has a high heat
capacity, meaning that it changes temperature slowly after
gaining or losing energy.
The heat capacity of water is a property directly resulting
from hydrogen bonding.
.

12. When heat is absorbed, hydrogen bonds are broken and
water molecules can move freely. When the temperature of
water decreases, the hydrogen bonds are formed and release
a considerable amount of energy.

13. High Melting Point
Water has an unusually high melting point due to the
hydrogen bonding between water molecules.
High Boiling Point
water molecules also have a higher boiling point
reason for this is the hydrogen bonding between neighboring
water molecules. Because hydrogen bonding is a relatively
strong intermolecular force, high heat energy is required to
break up the force.

14. High Heat of Vaporization
Heat of vaporization is the quantity of heat required to
transform 1 gram of liquid water into its gaseous form.
Similarly to the high specific heat of water, the high heat of
vaporization is also due the hydrogen bondings.

15. Cohesion of Water Molecules
Hydrogen bonds are very fragile in the liquid form of water.
The property of cohesion is seen when hydrogen bonds,
collectively, hold water molecules together.
Surface tension is related to cohesion, Water has higher
surface tension .
Water is known to be amphoteric, which means it can act as
a base or an acid.

16. Water as solvent
Better solvent.many crystalline salts and other ionic
compounds readily dissolve in water.water dissolves sodium
chloride because of the strong electrostatic attraction
between water NaCl ions.
Water also dissolves some non ionic compounds such as
sugars,simple alcohols,aldehydes and ketones.

17. pH
pH is a measure of the acidity or basicity of a solution. Pure
water is said to be neutral, with a pH close to 7.0 at 25 °C .
Solutions with a pH less than 7 are said to be acidic and
solutions with a pH greater than 7 are basic or alkaline.
low pH indicates a high concentration of hydronium ions,
while a high pH indicates a low concentration

18. Mathematical definition
pH is defined as a negative decimal logarithm of the hydrogen ion
activity in a solution.

pH+=-log(H+)

19. Significance of pH
pH of pure water is 7 at 25’c.i.e neutral.
pH falls below 7 means solution is acidic.
pH rises above 7 means solution is alkaline.
It is important that pH involves taking the negative log of
hydrogen ion(H+)concentration.therefore each degree
decrease or increase in Ph value denotes 10 times more or
less number of H+ respectively.

20. PH detection
Acidity or alkanity can be detected by LITMUS PAPER.If it
turns red,it means solution is acidic and if it turn blue,it
means the solution is alkaline.

21. Ph scale

22. pH measurement
A pH meter is an electronic instrument used to measure
the pH (acidity or alkalinity) of a liquid (though special
probes are sometimes used to measure the pH of semi-solid
substances). A typical pH meter consists of a special
measuring probe (a glass electrode) connected to an
electronic meter that measures and displays the pH reading.

23. Ph meter

24. pH indicator
A pH indicator is a halochromic chemical compound that
is added in small amounts to a solution so that the pH (acidity
or basicity) of the solution can be determined visually. Hence
a pH indicator is a chemical detector for hydronium ions
(H
3
O
+
) or hydrogen ions (H
+
).e.g congo
red,phenolphthalein,methyline blue.

25. The indicator causes the color of the solution to change
depending on the pH. At 25° Celsius, considered the
standard temperature, the pH value of a neutral solution is
7.0. Solutions with a pH value below 7.0 are considered
acidic, whereas solutions with pH value above 7.0 are basic.

26. Application
pH indicators are frequently employed in titrations in
analytic chemistry and biology experiments to determine the
extent of a chemical reaction.

27. Henderson–Hasselbalch’s
equation
Henderson–Hasselbalch equation describes the
derivation of pH as a measure of acidity (using pK
a
, the acid
dissociation constant) in biological and chemical systems.
The equation is also useful for estimating the pH of a buffer
solution and finding the equilibrium pH in acid-base
reactions.

29. Significance
If the pH of a buffer solution is known then the ratio of
salt/acid can be calculated.
For research purposes the buffer solution of known pH can
be prepared by adjusting the ratio of salt/acid.