KEY
The Unique and Unusual Properties of Water
Charles Riley, Jr. PE
Reading for 10.1 - 10.4: Intermolecular Forces
1.
(a) Why does the polar nature of water make it an excellent solvent?
b/c it will interact with other polar compounds - especially salts,
alcohols, carboxyl compounds, etc.
(b) Compare water's ability to dissolve to that of other solvents.
more substances dissolve in water than any other solvent
(c) How many elements are able to dissolve in water?
more than half
(d) List 4 things on which water's ability to dissolve a substance depend:
1.
2.
3.
4.
chemical composition
chemical bond strengths of the elements
temperature
pH
(e) Which types of compounds tend to be difficult to dissolve in water? Given an example of this
type of compound.
non-polar compounds - like hydrocarbons
2.
(a) What is surface tension and why is it important?
the attractive force exerted by molecules below the surface on
those at the liquid-air interface; this inward force tends to restrain
the liquid from flowing
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2. Continued:
(b) What is special about water's surface tension?
water has the highest surface tension of any other liquid except
mercury
(c) Generically, which types of compounds tend to have higher surface tensions and which tend to
have lower surface tensions?
polar compounds = high surface tensions
non-polar compounds = low surface tensions
(d) To what is the exceptionally high surface tension of water attributable?
hydrogen bonding of water molecules
(e) Why do drops of water form spheres?
this is the geometric shape with the least surface tension
(f) Why can object that are heavier than water "float" on water?
b/c of water's high surface tension
3.
(a) What does it mean for something to be "hydrophilic"?
water-loving
(b) What does it mean for something to be "hydrophobic"?
water-fearing
(c) List some examples of substances which are "hydrophobic".
oils, fats, waxes, and many synthetics like polypropylene
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3. Continued:
(d) What is the purpose of a "wetting agent"?
to lower the surface tension of water which allows water to "wet"
a substance
(e) What does it mean to "wet" a substance?
the ability of water to adhere to the surface of a substance
4.
(a) What is "capillary action"?
the ability of water to stay in the pores of a substance due to
surface tension
(b) List some examples that make the phenomenon of "capillary action" important.
1. allows the movement of water through soils and blood vessels
2. allows the movement of water carried nutrients through the
roots of plants
5.
(a) List 2 reasons why is water important to biochemical processes.
1. all known biochemical processes occur in aqueous
environments
2. the composition of most living things is 70 to 80 percent water
(b) Explain water's role in photosynthesis.
plants use the energy from sunlight to convert water and carbon
dioxide into carbohydrates
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5. Continued:
(c) List 2 reasons why photosynthesis is so important.
1. supplies the primary nutrients for ALL living organisms
2. the primary source of atmospheric oxygen
6.
(a) What is the "valence" of an element?
the power of an element to combine with other elements to form a
compound
(b) What would be the sign of the valence of an element that makes a cation? an anion? for
inorganic compounds?
cation = valence is (+)
anion = valence is (-)
inorganic compounds = algebraic sum of the valence numbers of
the combining elements must = zero
(c) Do atoms with a (+) valence gain or lose electrons? with a (-) valence?
(+) valence = lose electrons
(-) valence = gain electrons
(d) What is the significance of valence in the formation of compounds?
atoms with opposite valence attract each other to form
compounds
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7. Define the term "ionic bonding" according to the article we are reading.
the electrostatic attraction between oppositely charged ions to
form a compound
8.
(a) What type of bond hold together the hydrogen and oxygen atoms in a water molecule?
covalent
(b) How is the distribution of the electrons in the bonds of water different from the distribution of
electrons in the bonds of sodium chloride?
water = covalent = shared electrons
NaCl = ionic = gain or loss of electrons
9.
(a) In the water molecule, which atom exerts a stronger pull on the shared electrons? How does this
affect the electronegativity both the oxygen and hydrogen atoms?
Oxygen exerts a stronger pull
the hydrogen atoms become electropositive
the oxygen atoms become electronegative
(b) Why does the distribution of the (+) and (-) regions make the water molecule polar?
the regions of charge are not evenly distributed around the center
point of the molecule
(c) How does the polar nature of the water molecule make it behave towards other water molecules
and ions?
it becomes electrostatically attractive to other water molecules as
well as ions in soution
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9. Continued:
(d) What "parts" of separate water molecules are attracted to each other? What is this "bridging"
phenomenon called?
the electronegative oxygen atoms of one water molecule will be
attracted to the electropositive hydrogen atoms on an adjacent
water molecule
the bridging phenomenon is called hydrogen bonding
10.
(a) Hydrogen bonding is responsible for what percentage of the strength of a covalent bond?
10%
(b) List 7 physical phenomenon of water for which hydrogen bonding is responsible.
1. high freezing point
2. high melting point
3. high heat capacity
4. high heat of fusion
5. high heat of evaporation
6. solvency
7. high surface tension
(c) Explain a chemical phenomenon for which hydrogen bonding is responsible.
water does not easily ionize during chemical reactions
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10. Continued:
(d) Why is pure water a relatively poor conductor of electricity?
b/c it has few ions to conduct the charge
(e) Explain the principle that allows water's purity to measured with a conductivity or resistivity
meter.
the purer the water the less resistance will be measured, and viceversa
or
the purer the water the greater the conductivity will be measured,
and vice-versa
11.
(a) Explain hydrogen bonding's role in the relative density of ice and water.
at freezing the water molecules arrange themselves along the
directional lines of the hydrogen bonds - this causes water to
expand and become less dense
(b) What happens to the melting point of water when pressure is increased on ice? How do you
think this affect the hydrogen bonds of ice?
the melting point decreases and ice begins to melt
the hydrogen bonds must become "less rigid" allowing more
movement of the individual water molecules
12.
(a) How does a water molecule align itself in an electric field?
oxygen end = aligns towards the positive pole
hydrogen end = aligns towards the negative pole
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12. Continue:
(b) What is a "dipole moment" according to the article? What do you think this means "in English"
in terms of chemical bonds?
Article = the product of the distance between the charges
multiplied by the magnitude o of the charge
English = the longer the bond and the greater the difference in EN
between the bonded atoms the greater the dipole
moment and vice-versa
(c) How does water's dipole moment compare with that of other inorganic compounds?
it is exceptionally large
(d) How is the dipole moment of water responsible for its ability to dissolve a wide variety of
substances?
it reduces the attractive force of ionized substances in water (like
the attractive force between sodium cations and chlorine anions),
making them easier to "pull apart" and dissolve
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