TWEED RIVER HIGH SCHOOL
2006
PRELIMINARY CHEMISTRY
Unit 3
Water
Part 3
 Water is an important solvent.
 Explain changes, if any, to particles and account for those changes
when the following types of chemicals interact with water:
- a soluble ionic compound such as sodium chloride
- a soluble molecular compound such a sucrose
- a soluble or partially soluble molecular element or compound
such as iodine, oxygen or hydrogen chloride
- a covalent network structure substance such as silicon dioxide
- a substance with large molecules, such as cellulose or
polyethylene
Introduction
The polarity and hydrogen bonding ability of water have made it the most
powerful solvent on Earth. Water is the solvent in oceans, run-off, lakes
and the fluids inside and outside living cells. It is the main transporter of
gases, solids and liquids between different environments in the
atmosphere, hydrosphere, lithosphere and biosphere. Most of the mass of
life – biomass- is found in the oceans where animals depend on the
solubility of oxygen and plants depend on the solubility of carbon
dioxide.
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Multicellular organisms depend on the movement of water in forms such
as blood sap to transport chemicals into and out of cells. The ability of
water – nature’s transport medium – to dissolve many substances enables
water to supply cells with a range of nutrients and remove wastes.
Solubility and Forces of Attraction
To explain solubility of a solute in a solvent three sets of forces of
attraction must by considered:
 between solute and solvent particles
 between solute particles
 between solvent particles
For solubility to occur the interactions between the solute and solvent
particles must be stronger than those between solute particles and those
between solvent particles.
Ionic Solids and Water
 The polar water molecules are attracted to both positive sodium ions
and negative chloride ions.
The negative end of the water dipoles are closest to Na+ while the
positive end of water dipoles surrounds Cl-.
solute – solvent attractions  solute attractions + solvent attractions
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Three processes occur in the dissolving of an ionic solid in water:
1. The breaking down of the ionic lattice and separation of positive
and negative ions absorbs considerable energy. This is aided by the
ability of water molecules to reduce the electrostatic forces of
attraction between oppositely charged ions. Water molecules can
reduce the attraction between charged ions to one eightieth ( 1/80 )
of its original value.
2. Some molecules of water separate from one another. This requires
the breaking of hydrogen bonds and so absorbs energy. The hotter
the water the more energy is available to break these hydrogen
bonds. This is one reason why hot water is usually better at
dissolving an ionic solid than cold water.
3. As the polar water molecules comes closer to each ion a significant
amount of energy is released to the surroundings. These forces of
attraction between dissolved ions and polar water molecules are
called ion-dipole forces. The attachment of polar water molecules
to ions is called hydration.
Soluble Molecular Compounds
 Molecular compounds which are soluble in water are either polar in
nature or contain polar groups which attract water molecules.
 Covalent molecular solids that form hydrogen bonds with water.
 Sucrose contains polar –OH groups which can form hydrogen bonds
with water. The attractive forces between sucrose molecules and water
are greater than the attractive forces between sucrose molecules and
the attractive forces between the polar water molecules.
Hydrogen bonding brings the polar sucrose and polar water molecules
together.
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Soluble or partially soluble molecular element or compound.
 Hydrogen chloride is a covalent molecular gas.
 Hydrogen chloride is very soluble in water.
 Hydrogen chloride molecules ionise when dissolved in water:
HCl(g)  H+(aq) + Cl-(aq)
Covalent Network Structure
 Covalent network structures have very strong covalent bonds.
 The strong covalent bonds cannot be broken by the weaker
intermolecular forces that could be formed with water molecules.
Substances with large molecules.
 Large covalent molecules such as cellulose do not dissolve in water.
 The strong covalent bonds cannot be broken by the weaker
intermolecular forces that could be formed with water molecules.
 Analyse the relationship between the solubility of substances in
water and the polar nature of the water.
 Because of the polar nature of the water molecule and it attraction to
other polar molecules the general rule for solubility is:
“Like dissolves like”
That is polar compounds will be dissolved by polar solvent and nonpolar compounds will be dissolved by non-polar solvents.
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 Perform a first hand investigation to test the solubilities in water of a
range of substances what include ionic, soluble molecular, insoluble
molecular, covalent networks and large molecules.
Obtain the experiment sheets.
Your investigation must include the following points:
 Aim
 Risk Assessment
 Method
 Results
 Discussion
 You will then perform the experiment in the lab.
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 Process information from secondary sources to visualise the
dissolution in water of various types of substance and solve problems
by using models to show the changes that occur in particle
arrangement as dissolution occurs.
Ionic Substances.
See notes page 2 and 3 above.
Dig Water
P8
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Soluble Covalent Molecules
 Molecular compounds which are soluble in water are either polar or
contain polar groups which attract water molecules.
 Compounds such as ammonia, glucose, sucrose, ethanol and
methanol are polar and are very soluble in water.
 All these molecules form hydrogen bonds.
Structure of glucose
Insoluble Molecular Compounds
 These compounds are not polar and are only able to form
intermolecular dispersion forces with water.
 Examples are compounds such as hexane, kerosene (circa C 20) and
long chain carbon compounds such as polyethylene.
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Soluble Large Covalent Molecules
 Some large covalent compounds that contain polar groups are soluble
and can form hydrogen bonds with water.
 For example:
Amylose starch, is a polymer of glucose units joined together.
Water molecules can form hydrogen bonds with the accessible
hydroxy groups making amylose starch soluble.
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Insoluble Large Covalent Molecules
 Cellulose is a large covalent polymer made up of glucose units.
 Cellulose is unlike amylose starch in that its polar hydroxy groups
are not accessible for water molecules to form hydrogen bonds.
 In cellulose, most of the hydroxy groups are used to join the long
flat cellulose chains to adjoining cellulose chains.
 Cellulose is not water soluble.
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Covalent Network Solids
 Because of the strong covalent bonds in the network structure and
the non-polarity of the structure, covalent network solids are not
soluble in water.
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