Liquids and Solids
Use the Force Luke!
Learning objectives
 Describe
origins of surface tension and
meniscus
 Describe different types of cohesive force
 Identify type of cohesive force based on
molecular formula
 Describe origin of hydrogen bonding
 Explain unique properties of water
 Determine concentrations of solutions
using the common concentration scales
Liquids and solids exist because of forces
 Cohesive
forces are attractive forces
between molecules of the same substance
 Adhesive forces are attractive forces
between molecules of different substances
Surface tension results from
cohesive forces
 Molecules
on surface are drawn inwards
 Tendency to form sphere
 Surface behaves like a shrink-wrap film
 Liquids form into spherical drops
 Denser objects can “float” on the surface
tension
Consequences of surface tension:
What will these hands ne’er be clean?
 Cleaning
requires wetting
 Surface tension prevents wetting

Beading on a waxed car
 Detergent
acts as go-between
Cohesive forces and meniscus
 Adhesive
forces pull H2O molecules to
maximize coverage
 Cohesive
forces between H2O molecules
drag liquid up
 Gravity pushes liquid down
Solid: strong interactions
 Fixed
shape
 Not compressible
 Rigid
 Dense
Solids

Atoms in solids rotate and vibrate but do not
translate
 Melting occurs when the translational energy of
the atoms is sufficient to break free of the lattice
 Usually this is a very well defined point
 With amorphous solids it can be smeared out –
softening of fats
Liquid: medium interactions
 Liquid
 Not
rigid
 Assumes shape of container
 Not compressible
 Dense
Vapour pressure and boiling
 Molecules
do not all have the same
energy
 Evaporation: High energy molecules
escape the liquid – vapour pressure
 When vapour pressure = atmospheric
pressure boiling occurs – all liquid
becomes gas
 Sublimation is direct transition of solid to
gas (dry ice)
The Four Forces of the Apocalypse
Name of force
Origin
Strength
Ion-dipole
Between ions and
molecules
Quite strong
Between
Weak
permanent dipoles
Hydrogen bonds Polar bonds with H Quite strong
and (O,N)
Dipole-dipole
London
dispersion
forces
Fluctuating dipoles in
non-polar molecules
Weak
Dispersion force
 Arises
from fluctuations in electron clouds
in atoms
 Only force present in nonpolar molecules
 Increases with size of atoms/molecules
Dipole-dipole force
 Usually
stronger than dispersion forces
 Present in polar molecules
 Polar substances higher boiling point than
nonpolar substances
Molecule
Molar mass
(g/mol)
Boiling point
(ºC)
Ethane
(C2H6)
Non-polar
30.0
-88.0
Formaldehyde
(CH2O)
Polar
30.0
-19.5
Polar or nonpolar: that is the
question?
 Polar

Determine bond polarity from electronegativity
 The

molecules must contain polar bonds
polar bonds must not cancel out
Determine molecular shape
 Examples:
•
•
•
•
O2 nonpolar (no polar bond)
HCl polar (one polar bond)
CHCl3 polar (three polar bonds)
CCl4 nonpolar (four polar bonds but they all
cancel)
Hydrogen bonding




The ultimate expression
of polarity
Small positive H atom
exerts strong attraction
on O atom
Other H-bonding
molecules: HF, NH3
H2O is the supreme
example: two H atoms
and two lone pairs per
molecule
Terms in solution
 Solute:
the dissolved substance
 Solvent: the dissolving substance
Molarity
 Concentration
is usually expressed in
terms of molarity:
 Moles

of solute/liters of solution (M)
Moles of solute = molarity x volume of solution
Example
 What
is molarity of 50 ml solution
containing 2.355 g H2SO4?




Molar mass H2SO4 = 98.1 g/mol
Moles H2SO4 = .0240 mol
Volume of solution = 50/1000 = .050 L
Concentration = moles/volume
= .0240/.050 = 0.480 M
Dilution
 More
dilute solutions are prepared from
concentrated ones by addition of solvent
M1V1 = M2V2
Molarity of new solution M2 = M1V1/V2
To dilute by factor of ten, increase volume by factor
of ten
Trace quantities
 Percent
means one in a hundred
 PPM measures trace amounts – 1 in a
grams solute
ppm =
x10
million
6
grams solution


Iodized salt contains tiny amounts of KI – 7.6
x 10-5 g in 1 g of salt
7.6 x 10-5 g = 7.6 x 10-2 mg = 76 μg
7.6 x105 g KI
76 g KI
76 g KI 106 ppm
 6
 6
x
 76 ppm KI
1 g salt
1g
10 g salt 10 g salt
Milligrams per liter
 Units
for impurities in drinking water
mg/L =
milligrams solute
liters solution
 0.38
mg lead in 250 mL water
 Concentration in mg/L
mg/L =
0.38mg 1000mL 0.38mg
x

 1.5mg / L
250mL
1L
0.250 L
Something about water

High boiling point
compared with similar
compounds


Solid less dense than
liquid


Essential for life on earth
High heat capacity


Liquid at earth temperature
Modifying influence on
climate
Universal solvent
H2O has optimum combination of
lone pairs and H atoms
Compound
Number of lone Number of H
pairs
atoms
HF
3
1
H2O
2
2
NH3
1
3
H bonding generates threedimensional network
Ice floats!
 Something
so familiar we might believe all
solids float on their liquids. Not so. Water
is the exception.
Hydrogen bonding and life






hold the two strands of the DNA double helix
together
hold polypeptides together in such secondary
structures as the alpha helix and the beta
conformation
help enzymes bind to their substrate
help antibodies bind to their antigen
help transcription factors bind to each other
help transcription factors bind to DNA
Implications for life on earth
 Without
H-bonds molecules like DNA
would not exist
 H-bonds hold the two strands together
 Comparative weakness of bonding allows
for DNA replication
Unusual variation of the density of
water with T: maximum density at 4°C
Effects of density on lakes
seasonal cycling of water
replenishment of oxygen
Warm water on top
Cold water on top
Water contamination

Biological


Human and animal waste – bacteria leading to hepatitis, cholera,
typhoid, dysentery
Chemical

Organic
• Benzene
• Chlorohydrocarbons

Inorganic
•
•
•
•

Asbestos
Nitrates
Lead
Mercury
Radioactivity
• Uranium
• Tritium spills
Legislating cleanliness: The Safe
Drinking Water Act 1974
 Establish
maximum contaminant levels
(MCLs) for 84 contaminants
 All water supplies must pass
 Periodic sampling required
 Too much or too little? Water treatment
costs money
Don’t trust the EPA? Treat at home
 Active


Effective on organic contaminants
Need regular replacement
 Water


carbon filters
softeners
Specific to hard water
Ion exchange using zeolites
 Reverse

osmosis
Apply pressure to push water from saline to
pure side of membrane