4A_2 – Slide 1
Topic 04A:
Atoms, Molecules, and
Seawater Chemistry
Part II
Online Lecture:
Weird Water and
Its Unusual Bonding
4A_2 – Slide 2
Weird Water
○ Water molecules make unusually strong bonds
(for a molecule) called hydrogen bonds.
Because of the “V”-shape of water molecules:
– electrons of the hydrogen atoms (H)
are attracted to the nucleus
Oxygen
of the oxygen atom (O)
H
H
– spend more time near the oxygen nucleus,
making its side of the molecule extra negative
– hydrogen side of the molecule has fewer electrons
to balance the hydrogen nuclei: becomes positive
Water molecules are said to be “polar” or
“dipolar,” meaning that they have 2 sides,
like the North and South Poles of the Earth
V-shape related to room
for electrons in orbit
around O nucleus
Adhesion, Cohesion, and Surface Tension
4A_2 – Slide 3
○ Because water molecules have both a positive side
and a negative side, they are attracted to nearly everything.
○ Water molecules form stronger bonds than alcohol molecules.
Cohesion
Water
Water molecules “hold together”
instead of being pulled apart
and down by gravity.
Alcohol
4A_2 – Slide 4
4A_2 – Slide 5
Adhesion
“Capillarity”
Water
Alcohol
Water molecules “hold onto” the glass
molecules, resisting gravity’s downward pull.
Surface Tension
4A_2 – Slide 6
Water molecules “hold together,”
and don’t let objects push them apart.
overcomes the
“density effect”!
Picture of Salt and Dissolved Salt
What is the easy way to clean
dishes with crusted-on food?
4A_2 – Slide 7
4A_2 – Slide 8
Water: The “Universal” Solvent I
○ water molecules surround and try to bond with the objects’
atoms/molecules, to pull atoms/molecules away
○ one water molecule = NOT very attractive to salt atoms
a group of water molecules = very attractive:
O
O
O
O
+
O
Cl-
O
O
Cl-
O
O
O
O
O
C
O
Na+
Na
O
O
O
+
Cl
O
O
N
Dissolved
Atoms
+
O
Cl-
Na+
Na+
O
O
Cl-
O
O
O
Cl-
O
Na+
Cl
O
O
Na
-
O
Cl
Na+
Example on
Next Slide…
+
O
O
-
O
– water molecules pull
the atoms of the object
away from one another
and bond with the atoms
(i.e., water molecules
“dissolve” the object)
Salt Atoms:
Very
Strong
Bonds
Tug of War
4A_2 – Slide 9
Water Molecules:
Fairly Strong Bonds
4A_2 – Slide 10
3 Liquids Dissolving Salt
Fresh
Water
Salt
Water
Alcohol
Water is a Better Solvent
○ Why didn’t the saltwater dissolve
as much salt as the fresh water?
Many water molecules in salt water
are already bonded to salt atoms.
They are not available to bond
with the added salt atoms.
○ Why didn’t the alcohol dissolve
as much salt as the fresh water?
4A_2 – Slide 11
In salt water, a lot of
the water molecules are
already “married” to…
can break apart the atoms
of pollutant molecules
(destroying them), or just
separate pollutant molecules
and help them spread
Why don’t salts and
nutrients fall to the
bottom of the ocean?
Water molecules form stronger bonds
(i.e., are more attractive) than alcohol molecules.
A few water molecules can pull a salt atom away from its
neighbors (it takes more alcohol molecules to do the same).
Picture: Adding Acid to Fresh & Seawater
4A_2 – Slide 12
Which became more acidic, fresh water or seawater?
Acid
Neutral
Ocean pH: Acids & Bases
4A_2 – Slide 13
○ Ocean water has a variety of stuff dissolved in it, including
the gas carbon dioxide (CO2) which tends to bond with
H2O to form carbonic acid (H2CO3).
– if the ocean becomes too basic (OH−), carbonic acid
will release acid (H+) with neutralizes the bases.
It becomes bicarbonate (HCO3−) or carbonate (CO3 2 −).
– if the ocean becomes too acidic,
The ocean is
bicarbonate & carbonate will absorb acid a little basic,
on average.
● call this “buffering”:
keeps the ocean from getting too acidic or basic
Humans have added lots of carbon dioxide to the atmosphere…