III. Water
A. Polarity
1. In some covalent bonds, electrons are attracted
more strongly to one atom than another.
2. One end of the molecule will then be partially
positive, & the other end will be partially
negative. These molecules are polar.
3. Ex: water!
B. Hydrogen bonds (H-bonds)
1. When bonded to O2, N2, or F, a hydrogen
has a partial positive charge nearly as great
as a proton.
2. This hydrogen is then attracted to the
negative region of polar molecules, forming
hydrogen bonds.
C. Properties of Water – most result because
water forms hydrogen bonds with itself
1. Polarity
2. Density = Ice Floats – solid water is less
dense than liquid water (h-bonds)
3. Water absorbs & retains heat – large
bodies of water keep Earth’s temp.
regulated; water maintains organisms’
internal body temp.
• High Heat of Vaporization: water absorbs
a lot of energy before it evaporates
3. Water absorbs & retains heat – large
bodies of water keep Earth’s temp. regulated;
water maintains organisms’ internal body
temp.
• High Heat of Vaporization: water absorbs
a lot of energy before it evaporates
• High Specific Heat: water absorbs a lot of
energy before its temperature is raised
4. Cohesion – h-bonds hold water molecules
together
5. Adhesion – water sticks to other polar
substances
•
both cohesion and adhesion allow water to move
upward through roots and stems of plants
D. Solutions – one or more substances mixed
evenly in another substance (water)
1. Solute – substance that is dissolved (salt)
2. Solvent – substance that does the dissolving (water)
3. Concentration – every solution has a certain amount
of solute per amount of solvent
•
Conc. = solute (g) / solvent (mL)
4. Saturated Solution – solution that cannot hold any
more solute
5. Aqueous Solution – solution in which the solvent is
always water.
E. Acids & Bases
1. Dissociation – a molecule breaks into its ions
•
H2O ↔ H+ + OH-
H+ = hydronium ion
OH- = hydroxide ion
2. Acid – solution with more hydronium ions
•
HCl = hydrochloric acid
3. Base – solution with more hydroxide ions
•
NaOH = sodium hydroxide
4. pH scale – measures the concentration of
hydronium ions; is logarithmic, so each step is
10 times more acidic or basic
•
•
•
pH of 0-6 is acidic
pH of 7 is neutral
pH of 8-14 is basic or alkaline
o pH of 1 is 10 times more acidic than pH of 2
o pH of 14 is 100 times more basic than pH of 12
5. Buffers – chemicals that neutralize acids and
bases; they prevent changes in pH