Properties of Water Notes
1) Explain why water is considered to be a polar molecule.
There is a positive and negative end. Oxygen is partial negative due to the stronger pull it has on the
electrons that it shares with hydrogen (remember, water is a covalent compound). Hydrogen is
partial positive due to the lesser pull on the shared electrons. The fact that water is a polar molecule
is also reflected in its shape (104.5 0) which is a result of the unequal sharing of electrons between
oxygen and the two hydrogen atoms.
2) Explain why a drop of water keeps its shape.
Hydrogen bonding between individual water molecules keeps the shape of the
water molecule. Hydrogen bonding occurs when the slightly + charged hydrogen atoms are attracted
to the slightly – charged oxygen atoms between adjacent water molecules. This is the reason that
you could put so many water drops on the penny in the Properties of Water Lab. This is also an
example of cohesion in water molecules. See 5) below.
3) What can change this shape? Why?
Soap and oil which consist of nonpolar molecules, break the bonds between the water molecules
making the water drop lose its shape and flatten out (as you saw in the Properties of Water Lab when
you poked the water drop with the toothpick dipped in detergent). Also, nonpolar substances such as
soap and oil are hydrophobic (“water hating”) and polar substances are usually hydrophilic (“water
loving”)
4) Why does ice float on liquid water? Explain your answer in terms of density (d = m / v).
Hydrogen bonding results in yet another unique property of water: The ability to expand upon
freezing. When water freezes, its mass stays the same, but its volume increases. This can be
demonstrated by applying the density equation: density = mass / volume. If volume rises, then
density must decrease. Ice therefore (fortunately) floats on water. What would happen in a pond or
lake in winter if the opposite were true?
5) What is the difference between cohesion and adhesion?
Cohesion is the attraction between molecules of the same substance. Adhesion is the attraction
between molecules of different substances. Water climbing up the glass tubing in the lab was an
example of both cohesion and adhesion. The + end (hydrogen) of the water molecule is attracted to
the – charged glass (adhesion). Individual water molecule are attracted to each other through
hydrogen bonding which keeps the water column intact (cohesion). This is how water climbs trees!
Water molecule are attracted to the xylem (water conducting vessels in the roots) and subsequently
to each other.
6) How does salt dissolve in water?
Salt dissolves in water because it, like water, is also a polar substance. The Na + ions and Cl – ions
attract each other to form ionic bonds. When salt dissolves in water hydration of Na+ ions and Cl –
ions occur. The water molecules orient themselves with the O- atoms facing inward for the
hydration of Na + ions and they orient themselves with the H+ atoms facing inward for the hydration
of Cl – ions. Remember, opposites attract. Try looking up “Hydration of NaCl” and “Ionic bonding in
NaCl” for some additional information and graphics.
Passage of Materials through a Membrane Notes
A) Importance of the Lipid bilayer
The call membrane is amphipathic (derived from the Greek prefix amphi- meaning “both
kinds”). It contains both polar (usually hydrophilic) and nonpolar (usually hydrophobic)
regions. The heads (polar) form the outside of the membranes and the tails (nonpolar) form
the interior of the membrane. (Refer to your drawing in JE #17 or look online or in the
textbook for an example). This arrangement is important because the nonpolar tails keep
polar substances from “drifting” in and out of the membrane.
The membrane contains a lipid layer which is insoluble in water. Why is this so important?
B) How does water get through the membrane?
Water is essential to cells, but according to the information above it should not be able to pass
through cell membranes (because it is a polar molecule). Size, however, is the key here.
Water, despite its polar nature, can move through the membrane pores due to its small size.
C) Importance of the protein channel.
Essential polar substances, such as Na+ and K+ are essentially blocked by the nonpolar interior
of the cell membrane. The protein channel provides these larger polar substances with an
alternate path through the cell membrane.