Water
Constituent elements of water
Water is one of the most important molecules in living things. Water makes about 70% of
living cells. Water consists of elements hydrogen (H) and oxygen (O). Hydrogen and oxygen
atoms bond together in the ratio of two hydrogen atoms to every one oxygen atom.
(diagram of a water molecule)
The chemical structure of water molecules
Each water molecule consists of one oxygen atom and two hydrogen atoms bonded
together. An important property of water molecules is that they can stick together because
of the hydrogen bonds that form between water molecules.
Water molecules are neutral but because of the structure of the water molecule, there is an
uneven distribution of charges. The oxygen end has a slight negative charge and the
hydrogen end has a slight positive charge. A molecule where there is an uneven distribution
of charge is called a polar molecule. This means that water molecules can attract each
other. There is an attraction between the hydrogen atom of one molecule and the oxygen
atom of another molecule. Weak hydrogen forms. Lots of hydrogen bonds can form so that
one water molecule can form four hydrogen bonds at one time.
(diagram of hydrogen bonds form between water molecules)
The water molecules form a network in which the molecules can move around and make
and break hydrogen bonds as they move. This makes it difficult for water molecules to
escape from the liquid to become gas. As water is cooled, the molecules move less and
more hydrogen bonds form and the water molecules do not break away easily. The
hydrogen bonds between the water molecules pull the molecules together.
Cohesion
The multiple bonds between molecules of the same substance is called cohesion and gives
water its special properties.
Water is lost from the surface of leaves by evaporation and must be replaced by water
coming up from the roots and xylem. Forces of cohesion hold the water molecules together.
These forces are strong enough to hold the water molecules together in a long chain. As the
molecules are lost at the top of the chain, the whole column is pulled up. This forms the
transpiration stream.
Adhesion
Attraction between molecules of different kinds is called adhesion.
Water is attracted to glass. The level of water in a thin tube is curved because the water
molecules are attracted to the sides of the glass tube.
Capillary action
The same forces that hold the water molecules together also attract water molecules to the
sides of the xylem vessels. The xylem vessels are narrow and this allows the forces of
attraction to pull the water up the sides of the xylem vessel. This is called capillary action.
Water as a solid
Water is unusual in that when it is frozen it expands and becomes less dense than when it is
liquid
Properties of water
The physical properties and chemical properties of water differ from those of most other
liquids and make water uniquely effective in supporting living things.
a) Transparent – Water is transparent. This allows sunlight to penetrate to plants
growing under water. It also means that animals such as fish that live in water can
see food and also potential predators.
Uses - Plants can trap sunlight for photosynthesis even if they grow under water.
b) Solvent – Living things are made up of mixtures that involve water. Water is the
solvent when solutes such as sugars and salts dissolve in water and become evenly
distributed throughout the solution. Water can dissolve ionic compounds such as salt
(NaCl) as well as other polar molecules such as sugars.
Uses - Chemicals that dissolve in water can move around freely and react with
other substances inside cells.
- Many chemical reactions take place in the watery cytoplasm of cells.
- Water is important as a transport medium. In animals, blood transports
many dissolved substances. In plants, the properties of water are
important in transporting substances in xylem and phloem tissue.
c) Specific heat capacity – Water has a high specific heat capacity. This means that
water can absorb a lot of heat without its temperature rising to levels that could
damage proteins. Water also has to lose a lot of heat energy before it starts cooling
down.
Uses - A high specific heat capacity is important as enzymes in cells can function
effectively only in a narrow range of temperatures.
- Aquatic habitats have fairly stable temperatures so that organisms are
not exposed to extreme temperature change.
-
A high specific heat capacity helps mammals to maintain a stable internal
body temperature. Sweating relies on the change of a liquid to a gas. This
requires the transfer of a lot of energy which is effective in cooling the
body.
d) Reagent – Water acts as a reagent in many chemical reactions. It is needed for some
reactions and produced during some reactions. Eg. Water is needed during the
process of photosynthesis but it is produced during the process of respiration.
Water and habitats
Since water is less dense when it is solid, ice floats on water. A frozen layer of ice forms on
ponds or lakes that insulate them from the temperature of the atmosphere. This prevents
water underneath the layer from freezing solid throughout and killing organisms. In winter
months, organisms can still survive beneath the ice.
Large bodies of water tend to have constant temperatures, which mean that oceans and
large bodies of water form a stable habitat for living things such as hippos and fish.
Because of cohesion between water molecules, a skin forms on the water surface. This
makes it possible for insects such as water boatmen to walk on the water surface.