Lesson (3) Water

 Water is the basis of life on our planet, it exists in different physical states (solid, liquid, gas). Water makes up:-

 70% of the surface of earth

 (65% - 90%) of the weight of living organisms

The importance of water

It plays an important role in all vital processes within living organisms.

The molecular structure of water

water molecule H2O is composed of one oxygen atom and two hydrogen ones, these atoms are bound covalently (by covalent bond).

In water molecule, hydrogen carries molecular negative charge, while oxygen carries molecular positive charge. Thus, water molecule is polar molecule because it has positive and negative poles.

Fig. (10) The molecular structure of water

 Close water molecules are attracted to each other by relatively low electric attraction (as negative hydrogen atoms attract positive oxygen atoms in another molecules). This bond is called Hydrogen bond

Fig. (11) Hydrogen bond diagram

 Water has unique properties because of its polarity and hydrogen bonds between its molecules

1- Water is a polar solvent

Water is from the best solvents and regarded as "the general solvent" due to the polarity of its molecules

Example:-

When sodium chloride NaCl dissolves in water, it produced positive sodium ions and negative chlorine ions. Thus, positive oxygen atoms of water attract negative chlorine ions, and negative hydrogen atoms attract positive sodium ions.

 All polar substances (substances containing ions) can dissolve in polar solvents such as water

Importance of this property

 All the essential substances for living organisms (vitamin, salts, amine acids, gases, glucose) transport inside their bodies in the form of solutes dissolved in water, these substances take part in metabolism reactions inside cells.

2- Water has the ability to ionize molecules which are necessary for life

This means the ability of water to disassociate the molecules necessary for life into positive and negative ions (water can do so due to the polarity of its molecules)

Example:-

 Pancreas secretes sodium bicarbonate NaHCO3 , this compound ionizes in water into positive hydrogen ions and negative bicarbonate ones, which makes the medium alkaline and suitable for enzymes work.

Importance of this property

It's essential for the occurrence of some chemical reactions inside living organisms (see the previous example)

3- Water has high specific heat

Specific heat: The amount of heat required to increase the temp. of one gram of matter 1 Celsius degree

Water has the highest specific heat on earth due to the hydrogen bonds of between its molecules

 As a result of having high specific heat, water needs great amount of energy to increase its temperature and loses great amount of energy when its temperature decreases

The importance of this property

1- It helps living organisms have constant temperature which is essential for the vital processes occurring within their bodies, as cells contain lots of water to keep their temp. constant.

 Animals and plants lose water by sweating and transpiration processes to decrease their temperature.

2- The high specific heat of water provides living organisms with temperatures suitable for life on earth.\

 Water form almost 70% of the surface area of earth, if water didn't exist with such great amount, the temp. of earth would decrease dramatically because the substances forming the earth crust have low specific heat.

 Waters of oceans absorb great amount of sunrays in the morning, and spreads them in atmosphere at night to keep the temperature of earth suitable for living organisms

4-Water has low viscosity and high surface tension

Surface tension: The cohesion of the molecules on the surface of a fluid to occupy the least possible volume.

Viscosity: The resistance of fluid to flowing.

 Water has low viscosity and high surface tension due to the hydrogen bonds between its molecules, these conditions are suitable for life.