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1. Water: Essential to life

2. A molecule essential to life

Water is the most abundant liquid on earth, covering over 70%
of the planet

3. The water cycle

4. The reactions of life

Photosynthesis
6H20(l) + 6CO2 (g)

Chlorophyll
Sunlight
C6H1206 (s) + 6O2 (g)
Respiration
C6H1206 (s) + 6O2 (g)
6H20 (l) + 6CO2 (g)
Without these two reactions life on earth could not be
sustained. Water is essential for both of these
reactions

5. Other functions of water

It provides a system to transport nutrients and
soluble wastes.


It provides a system to transfer heat.


It dissolves a range of substances which are transported
around the body.
Water can store large amounts of heat energy, it transports
heat energy from cells to the body surface where it can be
removed.
It cools the body.

When water evaporates from the skin, it absorbs a large
amount of heat energy from the body allowing the body to
cool down.

6. Properties of water

Water is a covalent molecular compound.

Molecular formula H2O.

The O-H bonds are polar, with the O atom
having the larger share of the bonding electrons.

The forces between water molecules are
hydrogen bonds, which are strong in comparison
to other types of intermolecular bonds

7. 
Water is unique
Water is the only substance commonly found in all three states on earth.
Looking at this graph
can you think of another
reason that water is so
unique.?
• Water exists as a liquid over
a temperature range commonly
found on earth.
• The melting and boiling temp
of water are significantly higher
than those of other molecules
of similar size.

8. Why is this so?

In ice each molecule forms hydrogen
bonds to FOUR other molecules.

A lot of energy is required to break these
four bonds.

When ice melts enough energy is added
to break some of these bonds.

When water is boiled, all the hydrogen
bonds are broken.

A significant amount of energy to
overcome these strong hydrogen bonds.

9. High latent heat values

Latent heat measures the energy needed to change the state of a
substance at its melting or boiling temperature:

Latent heat of fusion is the amount of energy needed to change a fixed
amount of water from a solid to a liquid at 0°C

Latent heat of vaporisation of water is the amount of energy needed to
change a fixed amount of water from liquid to a gas at 100°C

10. 4c

As it freezes, water expands, unlike most liquids. This is
because of hydrogen bonding. Each molecule is surrounded
by four others in what is almost a crystal-type situation. (See
graph below, which shows the variation in density of water
with temperature.) Therefore, ice is less dense than liquid
water, and it floats on liquid water. (For most liquids, the solid
is denser than the liquid.) This is good news for fish, but not
good news for travellers on the Titanic!

11. Solutes and solvents

When a solid, liquid or gas dissolves in water, an aqueous
solution is formed.

The dissolved substance is the solute and the water is the
solvent.

12. Solutions

Solutions have the following important characteristics:

They are homogenous; that is, the solute and the solvent can not
be distinguished from one another.

The dissolved particles are too small to see.

The proportion of dissolved solutes varies from one solution to
another.

13. Water as a solvent

The polar nature of water molecules enables water to dissolved a large
number of substances.

Due to this water is not found pure in nature.

Not all substances dissolve in water, however. Oils, fats and other nonpolar substances will not dissolve in water. Also many gases have low
solubility in water.

14. What dissolves and what doesn’t

When one substance dissolves in another, the
following process occur:




The particles of the solute are separated from one another.
The particles of the solvent are separated from one
another.
The solute and solvent particles attract each other.
A solute will dissolve if the attraction between the
solute and solvent particles is strong enough to
compete wit the solute-solute and solvent-solvent
forces of attraction.

15. Substances that dissolve

Polar covalent compounds that can
form hydrogen bonds with water.

Polar covalent molecular compounds
that ionise.

Ionic compounds.

16. Polar covalent compounds that can
form hydrogen bonds with water

When ethanol dissolves in water:



Hydrogen bonds between water molecules are broken
Hydrogen bonds between ethanol molecules are broken
Hydrogen bonds form between ethanol and water molecules.
C2H5OH(l)
H2O
C2H5OH(aq)

17. Non-Polar molecules

Non-polar substances are insoluble in water because
the water molecules form hydrogen bonds with each
other in preference to the weak attractions with nonpolar molecules.

The larger the non-polar molecule is the less soluble
it is in water.

18. Comparing Vitamins

19. Polar covalent molecular
compounds that ionise

Some compounds have one of more covalent bonds
that are so highly polarised that they break when the
compounds is placed in water.

Such bonds break as a result of the electrostatic
forces of attraction between the solute molecules
and the water molecules.

20. Polar covalent molecular
compounds that ionise

This attraction results in the H-Cl bond breaking,
both electrons go with the chlorine atom and a H+ ion
forms a covalent bond with water.

This process is called ionisation and can be
represented by the following equation.
HCl(aq) + H2O(l)

H3O+(aq) + Cl-(aq)
A H3O+ ion is called a hydronium ion.

21. In summary when polar covalent
molecules ionise in water.

Polar covalent bonds within molecules are broken,
producing hydrogen ions and anions.

A covalent bond forms between each H+ and a H2O
molecule giving H3O+ ions. Ion-dipole attractions
between the newly formed ions and the polar water
molecules are formed.

Other polar covalent molecular compounds that
ionise in water include the common acids nitric acid,
sulfuric acid and ethanoic acid.

22. Ionic compounds

Ionic solids contain +ve and –ve ions held in a 3D
lattice by strong electrostatic forces.

When an ionic solid such as NaCl is placed in water,
the +ve ends of the water molecule are attracted to
the –ve chloride ions. The –ve ends of the water
molecule are attracted to the +ve sodium ions.

23. Ionic Compounds

The partially charged water molecules start
pulling the sodium and chloride ions on the
outer part of the ionic lattice apart and
dragging them into the surrounding solution.

24. Dissociation

When an ionic compound dissolves in water, positive
and negative ions in an ionic lattice are separated
from one another.

This process is known as dissociation.

It can be represented by the equation:
H20
NaCl(s)
Na+(aq) + Cl-(aq)

25. Dissociation Equations

Dissociation equations must balance.

The number of elements on each side must balance.

The charges on each side must balance.

The equation breaks into its cation and anion in the
presence of water.

For example:
H20
K2CO3(s)
2K+(aq) + CO32-(aq)

26. Insoluble ionic substances

Not all ionic substances are soluble, limestone (CaCO 3) is not
soluble.

For insoluble ionic substances, the energy required to separate
the ions from the lattice is much greater than the energy
released when the ions are dissolved in water.

The ions tend to stay in the lattice.

27. So what is soluble?
All group 1
metals are
soluble.

28. POSTED BY
M.ADITHYA SRIKANTH