Water Properties
What are the physical and chemical properties of water that make it so unique and
necessary for living things? When you look at water, taste and smell it—well, what could
be more boring? Pure water is virtually colorless and has no taste or smell. But the hidden
qualities of water make it a most interesting subject.
Polarity
You probably know water's chemical description is H20. That is one atom of oxygen bound
to two atoms of hydrogen. The hydrogen atoms are bonded to one side of the oxygen atom,
resulting in a water molecule having a positive charge on the side where the hydrogen
atoms are and a negative charge on the other side, where the oxygen atom is. A compound
with one negatively charged side and one positively charged side is called a polar
molecule. Each end of the water molecule is attracted to the opposite charged end of
another water molecule. Water’s polarity is responsible for the “stickiness” or cohesion
between molecules.
Surface Tension
The cohesive forces between liquid molecules are responsible for the
phenomenon known as surface tension. Water molecules want to
cling to each other. At the surface, however, there are fewer water
molecules to cling to since there is air above (thus, no water
molecules). These results in a stronger bond between those molecules
that actually do come in contact with one another, and a layer of
strongly bonded water (see diagram). This surface layer (held together
by surface tension) creates a considerable barrier
between the atmosphere and the water. Surface tension
allows light objects to float on the surface of water.
The stickiness of water is responsible for two other
important properties: adhesion and cohesion. Cohesion
means that water molecules stick to each other.
Adhesion means that water molecules stick to other
materials. The properties of polarity, cohesion, and adhesion all work together to create
capillary action. Capillary action is the
ability of water to move upward, against
gravity, through small openings. As water
molecule #1 starts climbing, it pulls along
water molecule #2, which, of course, is
dragging water molecule #3, and so on. For example, water
can move upward through soil or porous rocks. Capillary
action is extremely important for living organisms. It is what
allows water (and any substances dissolved in it) to move
through the roots of plants and through the tiny blood
vessels in our bodies.
Universal Solvent
A solution is a homogeneous mixture of two or more substances and looks the same throughout the
solution because it is thoroughly mixed. A solution consists of two parts: the solute and the solvent.
The solute is the substance of lesser amount that is being dissolved in the solvent. The solvent is the
substance of greater amount that is doing the dissolving. Whether or not two substances can form a
solution is determined by the polarity of each substance. A polar solvent will dissolve a polar or ionic
solute but not a nonpolar one. A nonpolar solvent will dissolve a nonpolar solute but not a polar or
ionic one. An easy way to remember this rule is “like dissolves like”.
Water’s polar arrangement of oxygen and hydrogen make it a good polar solvent. In fact it is called
the “universal solvent” because it dissolves more substances than any other liquid. A substance that
can be dissolved in water is referred to as soluble (aq) while one that cannot is called insoluble-(s)
because it forms a solid precipitate. Looking at the molecular structure of water allows us to see why
it is such an effective solvent. Since one side (hydrogen) has a positive charge and the other side
(oxygen) has a negative side, the water molecule is attracted to many other different types of
molecules. For example, water is so heavily attracted to a compound like salt (NaCl), that it disrupts
the attractive forces that hold the sodium and chlorine together and, thus, dissolves it. The positivelycharged side of the water molecules is attracted to the
negatively-charged chloride ions and the negatively-charged
side of the water molecules is attracted to the positivelycharged sodium ions. Essentially, a tug-of-war ensues with the
water molecules winning the match. Water molecules pull the
sodium and chloride ions apart, breaking the ionic bond that
held them together. After the salt compounds are pulled apart,
the sodium and chloride atoms are surrounded by water
molecules, as this diagram to the right shows. As not all
substances are not soluble, a solubility chart is needed to
determine the solubility of a given substance in water.
Density
Density can be defined as mass per unit volume. This is really just a
description of how compact a substance is. Most substances are denser
as a solid than as a liquid because the molecules are closer together and
more ordered. Water is one of the few substances that are less dense as
a solid than as a liquid. In other words, ice floats. While other materials
contract when they become solids, water expands. This again is caused
by its polarity. When water freezes its molecules become locked into a
crystalline lattice where each water molecule bonds to the maximum
four partners. This creates a less dense structure and is the reason ice
floats.
Specific Heat
Water has a very high specific heat capacity. The specific heat of any substance is the amount of heat
needed to raise its temperature 1°C. One of water's most significant properties is that it takes a lot of
heat to make it get hot. If you leave a bucket of water outside in the sun in summer it will certainly get
hot, but not hot enough to boil an egg. However, dropping an egg on the metal of a car hood on the
same day will produce a fried egg because metals have a much lower specific heat capacity than water.
Large bodies of water such as lakes and oceans require much more energy (heat from the sun) to raise
their temperature. This plays an important key role in regulating Earth’s temperatures, weather
patterns, and seasons. The water stays cool longer, and acts as an effective coolant. This is why cool
"sea breezes" are welcome on a hot summer's day. On the contrary, once the water is warmed up, it
takes a great deal of heat loss to cool it down. This creates warm winds blowing off lakes and oceans
and keeps temperatures warmer than they would otherwise be.