1. The Chemistry
of H2O
Science is Organized Knowledge

2. Diffusion
• Mixing due to molecular movement is called diffusion.
– All particles of matter are constantly moving.
• gas molecules move in straight lines until colliding with other molecules
• liquid molecules slide in one direction, but are "pulled" by the bonds they
have with the molecules around them
• even in solids, atoms move by vibrating in place
– those movements allow other molecules to enter the space around them
Diffusion Animation
Equilibrium is
reached when the
two substances are
evenly mixed.

3. Diffusion Rates
• The rate of diffusion measures how fast molecules disperse.
– A slow process, because it depends on the random movement of molecules.
• Rate of diffusion is affected by:
– concentration gradient: the difference in concentration of a substance across
space
• molecules always move from an area of higher concentration to an area of lower
concentration
– temperature & pressure
• higher temperature = faster moving molecules
• higher pressure = more frequent collisions
• The size of molecules also
affects diffusion rates
–

4. Osmosis
• Osmosis is the diffusion of water across a semi-permeable
membrane.
• A membrane is
like a net.
– will not let
bigger
molecules
through
– some small
molecules will
collide and
bounce back

5. Interactions of Water
• Nonpolar substances are hydrophobic
– literally means "water-fearing"
– will repel water
– disperse into fats (oily or greasy
substances)
– i.e. oil & water do not mix

6. Hydrogen Bonds in Water
• The polar nature of
water results in
hydrogen bonding
– weak attractive forces
between the negative
Oxygen end of the
molecule and the
positive Hydrogen end
– gives water many
unique physical and
chemical properties

7. Water as a Solvent
• Water is an excellent
solvent because of its
strongly polar molecular
arrangement.
• Hydrophilic (water-loving,
polar) substances readily
dissolve in water
– because the polar ends of
the molecule are attracted
to (and pulled apart by)
the charged ends of the
H2O molecule
– i.e. salt (NaCl) dissolves
easily in water to become
Na+ and Cl-

8. Surface Tension
• Surface tension is caused by cohesion
– an attraction between H2O molecules
due to hydrogen bonding

9. H-Bonding and Changing States
Water is unlike
most substances,
which get denser
when going from
liquid to solid.
• Hydrogen bonds cause water
to become less dense as it
freezes.
• Water in its solid state (ice)
floats.
• Important for aquatic species in
cold climates.

10. Insulating Properties of Water
• Water resists temperature change.
• It absorbs a huge amount of heat as it changes into vapor.
• H2O releases a lot of heat when freezing into ice.

11. Week 4 Lab
Acids & Bases
Lab Coat Diffusion

12. The pH Scale
• pH describes how acidic or basic a solution is
– a solution is any mixture with one or more
solutes distributed evenly in a solvent
• Substances with pH less than 7 are acids
– i.e. battery acid, hydrochloric acid (HCl)
• A pH of 7 is neutral
– i.e. water, blood
• Substances with pH higher than 7 are bases
– i.e. bleach, drain cleaner

13. pH
• pH stands for potential of Hydrogen ions
– when a substance dissolves in a solution,
the molecules will break apart
– in acids and bases, the broken-apart
molecules form either OH- or H+
– the more H+, the lower the number on
the pH scale and the more acidic it is
– Log scale means 10X change per unit!
• Water has a pH of 7 (it's neutral)
– equal # of hydrogen & hydroxide ions
• Substances with pH less than 7 are acids
– molecules separate to form hydrogen ions (H+)
– more H+ than OH-
– the farther from 7, the stronger the acid
• Substances with pH higher than 7 are bases
– molecules separate to form hydroxide ions (OH-)
– more OH- than H+
– the closer to 14, the stronger the base

14. Acids
• A pH below 7 is an acid
– acids form hydrogen ions (H+) in water
• In strong acids, the H+ will dissociate (separate)
from most or all molecules
– i.e.: lemon juice, hydrochloric acid (HCl), battery acid
• With weak acids, only a few of the molecules will
dissociate into hydrogen ions
– i.e.: tomatoes, milk, vinegar (CH3COOH or acetic acid)

15. Bases
• A pH above 7 is a base
– also described as alkaline
– bases form hydroxide ions (OH-) in water
• In bases, hydroxide ions (OH-) will
form in two ways
– by the dissociation of the base
– by stripping an H+ from water
• Examples of strong bases
– bleach, oven cleaner, drain opener
– sodium hydroxide (NaOH)
• Examples of weak bases
– baking soda, eggs, ammonia, dish
soap

16. Review
The basic building blocks of matter are
atoms
The Atomic Number of an atom equals
the number of protons in the nucleus (also the number of electrons)
The Atomic Mass of an atom equals
the number of Protons + Neutrons in the nucleus (average of all isotopes)
Elements are different than compounds because
compounds have 2 or more different elements bonded together
Electrons are found where in an atom?
circling the nucleus in shells (also called orbitals or energy levels).
Each energy shell can only hold
a set number of electrons.*(octet rule)
The small (subscript) 2 in H2O tells
the number of Hydrogen atoms in each molecule of water.
What do we get if one neutron is added to the nucleus of a carbon atom?
an isotope - Carbon-13 (13C)
What do we get if one electron is added to the outer orbital of chlorine?
an ion is formed, creating a more stable atom due to the filled energy level
Electrons are shared by two or more atoms in
covalent bonds.

17. Review
A polar molecule is one in which
electrons are not equally shared, so there is a slight + and - charge at certain locations.
What do we get if one proton is added to the nucleus of a boron atom?
a carbon ion - (C+)
+ charges = 6
- charges = 5
6 - 5 = +1

18. Review
• The polarity of water allows it to...
dissolve other polar molecules (especially ionic compounds).
• Non-polar compounds are hydrophobic and thus
not easily dissolved in water.
• Many properties of water are due to
hydrogen bonding.
• How might a pine tree take advantage of the cohesion of water ?
If water molecules “stick” to each other because of hydrogen bonds, it is
easier to draw the water up to the leaves.
• How does water resist temperature changes?
by absorbing (and releasing) lots of heat
• Why does ice float in water?
Hydrogen bonds cause water to become less dense as it freezes solid
• What is the term that describes molecules moving from an area of
higher concentration to an area of lower concentration?
diffusion
• What is osmosis?
the diffusion of water across a membrane