IB Topic 2.
2:
Water
Essential Idea: Water is the medium of life
Nature of science: Use theories to explain natural phenomena—the theory that hydrogen
bonds form between water molecules explains the properties of water.
The Chemistry of Water
The polarity of water
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen
atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons.
Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding
the oxygen atom, two pairs involved in covalent bonds with hydrogen,
and two unshared pairs on the opposite side of the oxygen atom. Oxygen
is an "electronegative" or electron "loving" atom compared with
hydrogen.
Water is a "polar" molecule, meaning that there is an uneven
distribution of electron density. Water has a partial negative charge ( )
near the oxygen atom due the unshared pairs of electrons, and partial
positive charges ( ) near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the
partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in
the illustration.
The ability of ions and other molecules to
dissolve in water is due to polarity. For
example, in the illustration below sodium
chloride is shown in its crystalline form
and dissolved in water.
Many other unique properties of water are due to the hydrogen bonds. For example, ice floats
because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where
there is one less hydrogen bond per molecule. The unique physical properties, including a high
heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent
properties of water are also due to hydrogen bonding. The hydrophobic effect, or the exclusion of
compounds containing carbon and hydrogen (nonpolar compounds) is another unique property of
water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the
formation of cell membranes. The best description is to say that water "squeezes" nonpolar
molecules together.
U1: Water molecules are polar and hydrogen bonds form between them.
Exercise 1: Draw two water molecules. Make sure you label the covalent bonds inside
each molecule and the hydrogen bonds between each molecule.
A1: Comparison of the thermal properties of water with those of methane.
Exercise 2: Compare the thermal properties of water with those of methane
Water Methane
er
Could Life be methane Based?
Saturn's moon Titan: "A unique world in the solar system" for life?
Saturn’s largest moon Titan has water frozen as hard as
granite and great lakes-of methane fed by a complete
liquid vapour cycle, much like the water cycle on Earth.
This liquid-gas cycle is made up of methane rather than
water. Methane has a small molecule like water but it
melts at -182°C and boils at -169°C, very different from
water which boils at temperatures 270°C warmer. Why
are the thermal properties of these two small molecules
1 Artists impression of Titan's surface, so different?
Source: www.dailygalaxy.com
The presence of methane on Titan is interesting for
another reason. Methane is common on Earth in places which have living organisms. Could there be
life on this cold moon orbiting Saturn?
Questions to answer
1. What state is methane, solid liquid or gas, in In Titan’s atmosphere it rains liquid methane
the earth's atmosphere? at minus 180°C. At this temperature water
would be a hard and brittle solid.
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2. Why is it that methane can evaporate at -180°C
on Titan whereas water is frozen solid at this
temperature?
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…………………………………………………………………. Property Methane Water
Melting -182 0
3. What is the "special force" which is found point / °C
between water molecules that is not found in
methane? Boiling point -161 100
/ °C
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Specific heat 36 4200
4. What effect does this force have on the melting
capacity /
point and boiling point of water?
J°C-1mol-1
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Cohesion low high
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© David Faure, InThinking http://www.thinkib.net/biology 1
5. Suggest how this “special force” might change the cohesion of water molecules compared to
methane molecules? (Cohesion is the ability of like molecules to stick together)
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6. Sweating cools the skin because when the water in sweat evaporates it carries heat away from
the skin. If humans made sweat with methane instead of water how would it change
the effectiveness of sweating to cool the skin?
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7. If extra-terrestrial life on Titan does exist, and if it uses methane in sweat to cool down, explain
why Titan's organisms would be happier to be sweating with methane and not water.
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8. Another difference between Titans lakes and the oceans of the earth will be the substances
dissolved in each. Water is a polar molecule and an excellent solvent of ionic substances like
salts and other polar molecules like amino acids and sugars. Methane is non-polar and thus will
dissolve lipids and small molecules better.
Suggest why cytoplasm or blood made with water would be better for living things than
cytoplasm or blood made from methane.
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2
U2: Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent
properties of water.
A2: Use of water as a coolant in sweat.
A3: Modes of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in
blood in relation to their solubility in water.
Exercise 3: Use the article called “The Wonderful Water Molecule” and your textbook
to answer the following questions
• Describe, with the help of diagrams, how the distribution of electrons results in the
structure and polarity of the water molecule.
• Molecules with small molecular mass are usually gases at room temperature. Explain
why water is unusual.
• How does the bonding between water molecules change with temperature.
• List 5 other key properties of water that you have learned from reading the article.
U3: Substances can be hydrophilic or hydrophobic.
Exercise 4: Complete the following table using your textbooks and the internet
Substance Hydrophobic Solubility in water How transported in the body
or Hydrophilic
Oxygen
Glucose
Amino Acids
Cholesterol
Fats
Sodium
Chloride
Essential idea: Water is the medium of life.
38
Topic 2: Molecular biology
2.2 Water
Nature of science:
Use theories to explain natural phenomena—the theory that hydrogen bonds form between water molecules explains the properties of water. (2.2)
Understandings: International-mindedness:
Water molecules are polar and hydrogen bonds form between them. There are challenges for the increasing human population in sharing water
Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal resources equitably for drinking and irrigation, electricity generation and a
and solvent properties of water. range of industrial and domestic processes.
Substances can be hydrophilic or hydrophobic. Theory of knowledge:
Applications and skills: Claims about the “memory of water” have been categorized as
pseudoscientific. What are the criteria that can be used to distinguish
Application: Comparison of the thermal properties of water with those of
scientific claims from pseudoscientific claims?
methane.
Utilization:
Application: Use of water as a coolant in sweat.
Application: Modes of transport of glucose, amino acids, cholesterol, fats, Syllabus and cross-curricular links:
oxygen and sodium chloride in blood in relation to their solubility in water. Biology
Topic 4.3 Carbon cycling
Guidance:
Topic 4.4 Climate change
Students should know at least one example of a benefit to living organisms Physics
of each property of water. Topic 3.1 Thermal concepts
Transparency of water and maximum density at 4°C do not need to be Aims:
included.
Aim 6: Probes can be used to determine the effect of different factors likely
Comparison of the thermal properties of water and methane assists in the
to influence cooling with water.
understanding of the significance of hydrogen bonding in water.
Biology guide
