2.

5: Properties of Water


Skills to Develop

 Describe the properties of water that are critical to maintaining life

 Explain why water is an excellent solvent

 Provide examples of water’s cohesive and adhesive properties

 Discuss the role of acids, bases, and buffers in homeostasis

Why do scientists spend time looking for water on other planets? Why is water so important? It is because water is essential to life as we
know it. Water is one of the more abundant molecules and the one most critical to life on Earth. Approximately 60–70 percent of the
human body is made up of water. Without it, life as we know it simply would not exist.

The polarity of the water molecule and its resulting hydrogen bonding make water a unique substance with special properties that are
intimately tied to the processes of life. Life originally evolved in a watery environment, and most of an organism’s cellular chemistry and
metabolism occur inside the watery contents of the cell’s cytoplasm. Special properties of water are its high heat capacity and heat of
vaporization, its ability to dissolve polar molecules, its cohesive and adhesive properties, and its dissociation into ions that leads to the
generation of pH. Understanding these characteristics of water helps to elucidate its importance in maintaining life.

Water’s Polarity

One of water’s important properties is that it is composed of polar molecules: the hydrogen and oxygen within water molecules (H 2O) form
polar covalent bonds. While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen
and a slightly negative charge on oxygen, contributing to water’s properties of attraction. Water’s charges are generated because oxygen is
more electronegative than hydrogen, making it more likely that a shared electron would be found near the oxygen nucleus than the
hydrogen nucleus, thus generating the partial negative charge near the oxygen.

As a result of water’s polarity, each water molecule attracts other water molecules because of the opposite charges between water
molecules, forming hydrogen bonds. Water also attracts or is attracted to other polar molecules and ions. A polar substance that interacts
readily with or dissolves in water is referred to as hydrophilic (hydro- = “water”; -philic = “loving”). In contrast, non-polar molecules such as
oils and fats do not interact well with water, as shown in Figure 2.5.12.5.1 and separate from it rather than dissolve in it, as we see in salad
dressings containing oil and vinegar (an acidic water solution). These nonpolar compounds are called hydrophobic (hydro- = “water”; -
phobic = “fearing”).

Figure 2.5.12.5.1: Oil and water do not mix. As this macro image of oil and water shows, oil does
not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound. (credit: Gautam Dogra).

Water’s States: Gas, Liquid, and Solid

The formation of hydrogen bonds is an important quality of the liquid water that is crucial to life as we know it. As water molecules make
hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids and, since living things
have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are
constantly formed and broken as the water molecules slide past each other. The breaking of these bonds is caused by the motion (kinetic
energy) of the water molecules due to the heat contained in the system. When the heat is raised as water is boiled, the higher kinetic
energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas
(steam or water vapor). On the other hand, when the temperature of water is reduced and water freezes, the water molecules form a
crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds) that makes ice less dense
than liquid water, a phenomenon not seen in the solidification of other liquids.

Water’s lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther
apart compared to liquid water. With most other liquids, solidification when the temperature drops includes the lowering of kinetic energy
between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid.

1
The lower density of ice, illustrated and pictured in Figure 2.5.22.5.2, an anomaly, causes it to float at the surface of liquid water, such as in
an iceberg or in the ice cubes in a glass of ice water. In lakes and ponds, ice will form on the surface of the water creating an insulating
barrier that protects the animals and plant life in the pond from freezing. Without this layer of insulating ice, plants and animals living in
the pond would freeze in the solid block of ice and could not survive. The detrimental effect of freezing on living organisms is caused by the
expansion of ice relative to liquid water. The ice crystals that form upon freezing rupture the delicate membranes essential for the function
of living cells, irreversibly damaging them. Cells can only survive freezing if the water in them is temporarily replaced by another liquid like
glycerol.

Figure 2.5.22.5.2: Hydrogen bonding makes ice less dense than liquid
water. The (a) lattice structure of ice makes it less dense than the freely flowing molecules of liquid water, enabling it to (b) float on water.
(credit a: modification of work by Jane Whitney, image created using Visual Molecular Dynamics (VMD) software 1; credit b: modification of
work by Carlos Ponte)

Link to Learning

Video: Click here to see a 3-D animation of the structure of an ice lattice. (Image credit: Jane Whitney. Image created using Visual
Molecular Dynamics VMD software.2)

Water’s High Heat Capacity

Water’s high heat capacity is a property caused by hydrogen bonding among water molecules. Water has the highest specific heat
capacity of any liquids. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its
temperature by one degree Celsius. For water, this amount is one calorie. It therefore takes water a long time to heat and long time to cool.
In fact, the specific heat capacity of water is about five times more than that of sand. This explains why the land cools faster than the sea.
Due to its high heat capacity, water is used by warm blooded animals to more evenly disperse heat in their bodies: it acts in a similar
manner to a car’s cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even
temperature.

Water’s Heat of Vaporization

Water also has a high heat of vaporization, the amount of energy required to change one gram of a liquid substance to a gas. A
considerable amount of heat energy (586 cal) is required to accomplish this change in water. This process occurs on the surface of water. As
liquid water heats up, hydrogen bonding makes it difficult to separate the liquid water molecules from each other, which is required for it
to enter its gaseous phase (steam). As a result, water acts as a heat sink or heat reservoir and requires much more heat to boil than does a
liquid such as ethanol (grain alcohol), whose hydrogen bonding with other ethanol molecules is weaker than water’s hydrogen bonding.
Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), the heat is able to break the hydrogen bonds between the
water molecules, and the kinetic energy (motion) between the water molecules allows them to escape from the liquid as a gas. Even when
below its boiling point, water’s individual molecules acquire enough energy from other water molecules such that some surface water
molecules can escape and vaporize: this process is known as evaporation.

The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process.
As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place. In many living
organisms, including in humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of
body temperature can be maintained.

Water’s Solvent Properties

Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it.
Therefore, water is referred to as a solvent, a substance capable of dissolving other polar molecules and ionic compounds. The charges
associated with these molecules will form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as
a sphere of hydration, or a hydration shell, as illustrated in Figure 2.5.32.5.3 and serves to keep the particles separated or dispersed in the
water.

When ionic compounds are added to water, the individual ions react with the polar regions of the water molecules and their ionic bonds
are disrupted in the process of dissociation. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions.
Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na + and
Cl– ions, and spheres of hydration form around the ions, illustrated in Figure 2.5.32.5.3. The positively charged sodium ion is surrounded by
the partially negative charge of the water molecule’s oxygen. The negatively charged chloride ion is surrounded by the partially positive
charge of the hydrogen on the water molecule.

2
 Figure 2.5.32.5.3: When table salt (NaCl) is mixed
in water, spheres of hydration are formed around the ions.

Water’s Cohesive and Adhesive Properties

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-
like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water
molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air)
interface, although there is no more room in the glass.

Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under
tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a
small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (heavier)
than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top.
It’s even possible to “float” a needle on top of a glass of water if it is placed gently without breaking the surface tension, as shown in
Figure 2.5.42.5.4.

Figure 2.5.42.5.4: The weight of the needle is pulling the surface

These cohesive forces are related to water’s property of adhesion, or the attraction between water molecules and other molecules. This
attraction is sometimes stronger than water’s cohesive forces, especially when the water is exposed to charged surfaces such as those
found on the inside of thin glass tubes known as capillary tubes. Adhesion is observed when water “climbs” up the tube placed in a glass of
water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are
attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is
called capillary action, and is illustrated in Figure 2.5.52.5.5.

3
 Figure 2.5.52.5.5: Capillary action in a glass tube is caused by the adhesive
forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves. (credit:
modification of work by Pearson-Scott Foresman, donated to the Wikimedia Foundation)

Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the
roots to the leaves in plants. These forces create a “pull” on the water column. This pull results from the tendency of water molecules
being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use
this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be
unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider, shown in
Figure 2.5.62.5.6, use the surface tension of water to stay afloat on the surface layer of water and even mate there.

Figure 2.5.62.5.6: Water’s cohesive and adhesive properties allow this

pH, Buffers, Acids, and Bases

The pH of a solution indicates its acidity or alkalinity.

H2O(I)⇋H+(aq)+O−(aq)H2O(I)⇋H+(aq)+O−(aq)

litmus or pH paper, filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much
acid (acidity) or base (alkalinity) exists in a solution. You might have even used some to test whether the water in a swimming pool is
properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution.

Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into
equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. While the hydroxide ions are kept in solution by their hydrogen bonding
with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules,
forming hydronium ions (H30+). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this
state in liquid water.

The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. Moles (mol) are a way to
express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 x 10 23 particles of the
substance. Therefore, 1 mole of water is equal to 6.02 x 1023 water molecules. The pH is calculated as the negative of the base 10 logarithm
of this concentration. The log10 of 1 × 10-7 is -7.0, and the negative of this number (indicated by the “p” of “pH”) yields a pH of 7.0, which is
also known as neutral pH. The pH inside of human cells and blood are examples of two areas of the body where near-neutral pH is
maintained.

4
Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high
concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that
increases the concentration of hydrogen ions (H+) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides
either hydroxide ions (OH–) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution
and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water
molecules.

The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and
chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids.
Conversely, strong bases are those substances that readily donate OH– or take up hydrogen ions. Sodium hydroxide (NaOH) and many
household cleaners are highly alkaline and give up OH– rapidly when placed in water, thereby raising the pH. An example of a weak basic
solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are
able to thrive in it.

The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure 2.5.72.5.7). Anything below 7.0 (ranging
from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually
considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the
environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment?
How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying.
New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of
the human stomach is completely replaced every seven to ten days.

Figure 2.5.72.5.7: The pH scale measures the concentration of hydrogen ions (H+)
in a solution. (credit: modification of work by Edward Stevens)

Link to Learning

Watch this video for a straightforward explanation of pH and its logarithmic scale.

So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for
example) and survive? Buffers are the key. Buffers readily absorb excess H+ or OH–, keeping the pH of the body carefully maintained in the
narrow range required for survival. Maintaining a constant blood pH is critical to a person’s well-being. The buffer maintaining the pH of
human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free
hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, as shown in Figure 2.5.82.5.8,
excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions
from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH– is introduced into the system, carbonic
acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put
survival in jeopardy.

Figure 2.5.82.5.8: This diagram shows the body’s buffering of blood pH levels. The blue arrows show the process of raising pH as more
CO2 is made. The purple arrows indicate the reverse process: the lowering of pH as more bicarbonate is created.

5
Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same
way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer
“heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH—as well as water’s other
characteristics—are essential to sustaining life on Earth.

Link to Learning

To learn more about water. Visit the U.S. Geological Survey Water Science for Schools All About Water! website.

Summary

Water has many properties that are critical to maintaining life. It is a polar molecule, allowing for the formation of hydrogen bonds.
Hydrogen bonds allow ions and other polar molecules to dissolve in water. Therefore, water is an excellent solvent. The hydrogen bonds
between water molecules cause the water to have a high heat capacity, meaning it takes a lot of added heat to raise its temperature. As
the temperature rises, the hydrogen bonds between water continually break and form anew. This allows for the overall temperature to
remain stable, although energy is added to the system. Water also exhibits a high heat of vaporization, which is key to how organisms cool
themselves by the evaporation of sweat. Water’s cohesive forces allow for the property of surface tension, whereas its adhesive properties
are seen as water rises inside capillary tubes. The pH value is a measure of hydrogen ion concentration in a solution and is one of many
chemical characteristics that is highly regulated in living organisms through homeostasis. Acids and bases can change pH values, but buffers
tend to moderate the changes they cause. These properties of water are intimately connected to the biochemical and physical processes
performed by living organisms, and life would be very different if these properties were altered, if it could exist at all.

Footnotes

1. 1 W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-
38.

2. 2 W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-
38.