- Water is the basis of life on our planet. It exists in different physical states – solid, liquid and gas – and makes up 70% of the surface of Earth, plus 65 – 90% of the weight of all living organisms. Water also plays an important role in all vital processes of living organisms.
- The water molecule, H2O, is composed of one oxygen atom and two hydrogen atoms. These atoms are bound covalently (by a covalent bond). In a water molecule, hydrogen carries a positive molecular charge, while oxygen carries a negative molecular charge. Thus, a water molecule is a ‘polar’ molecule, because it has both positive and negative poles.
- Close water molecules are attracted to each other by a relatively low electrical attraction, (negative hydrogen atoms attract positive oxygen atoms in other molecules). This bond is called a ‘hydrogen bond’. Water has unique properties because of its polarity and the hydrogen bonds between its molecules.
1- Water is a polar solvent.
- Water is regarded as the ‘general solvent’ or ‘universal solvent’ due to the polarity of its molecules.
For example, when sodium chloride (NaCl) dissolves in water, it produces positive sodium ions and negative chlorine ions. The positive oxygen atoms in water attract the negative chlorine ions, and the negative hydrogen atoms attract the positive sodium ions. All polar substances (substances containing ions) can dissolve in polar solvents, such as water.
- All the essential substances for living organisms (vitamins, salts, amino acids, gases, and glucose) transport inside their bodies in the form of solutes dissolved in water. These substances take part in metabolic reactions inside the cells.
2- Water has the ability to ionize molecules, which are necessary for life.
- This means that water has the ability to disassociate the molecules necessary for life into positive and negative ions (water can do so due to the polarity of its molecules).
For example, the pancreas secretes sodium bicarbonate (NaHCO3). This compound ionizes in water into positive hydrogen ions and negative bicarbonate ions, which makes the medium alkaline and thus suitable for the enzymes’ work.
3- Water has high specific heat.
- Specific heat is the amount of heat required to increase the temperature of one gram of matter by 1 degree Celsius.
- Water has the highest specific heat on Earth due to the hydrogen bonds between its molecules.
- As a result of having high specific heat, water needs a great amount of energy to increase its temperature and loses a great amount of energy when its temperature decreases. This helps living organisms to have a constant temperature which is essential for the vital processes occurring within their bodies. Cells contain lots of water to keep their temperature constant.
- Animals and plants lose water by sweating and transpiration processes to decrease their temperature.
- The high specific heat of water provides living organisms with temperatures suitable for life on Earth.
- Water forms almost 70% of the surface area of Earth. If water didn’t exist in such a great amount, the temperature of the Earth would decrease dramatically because the substances forming the Earth’s crust have low specific heat.
- The water that makes up oceans absorb a great number of sun rays in the morning and spread them into the atmosphere at night in order to keep the temperature of the Earth suitable for living organisms.
4- Water has low viscosity and high surface tension.
- Surface tension is the cohesion of the molecules on the surface of a fluid to occupy the least possible volume. Viscosity is the resistance of a 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.
These properties are important because:
1- They work on the cohesion of cell substances.
2- It slows down water loss in plants’ leaves through pores.
3- Some insects can walk on water due to the cohesion of the molecules on its surface.
5- Water density decreases under 4◦C.
- Water expands when its temperature becomes less than 4◦C (instead of shrinking). THis decreases its density and makes it float. In frozen lakes, we find ice on the surface, while we find liquid water underneath.
- This property is because of the hydrogen bonds between water molecules.
- This property is important because it enables living organisms to live in oceans and seas. Without this property, all oceans and seas will turn into ice, rather than just the surface. Surface freezing works as an insulator to prevent the rest of water from freezing.
6- The freezing point of water decreases if it has substances dissolved in it.
- This property is very important for living organisms, as it prevents the water in the cells of from freezing when exposed to temperatures less than 0◦C.
7- Water can turn into vapour in temperatures lower than boiling point (100◦C).
- Water vapour formed on the surfaces of oceans is carried by convection currents to cold layers in the atmosphere. This changes into clouds which provide living organisms with rain and water.
8- Water rise in capillary tubes.
- Water has the ability to rise in capillary tubes without being pumped and in opposition to external forces such as gravity. This property helps water transport from trees’ roots to all of its parts.