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1.3.1.2. Properties and functions of water

Great dissolving power

Water is the universal solvent , the one that can dissolve the most substances.

The dipole character of water allows it to dissolve polar compounds and ionic compounds.

  • The polar compounds alcohols , aldehydes , amino acids ...) establish hydrogen bonds between water and the polar groups of these compounds. Remember that polar compounds have no net electrical charge, but they do have partial electrical charges due to the difference in electronegativity between the atoms.
  • The ionic compounds , such as mineral salts , due to the electrostatic attraction established between the dipoles of water and ions are dissolved, forming solvated ions, that is, surrounded by a layer of water molecules or solvation shell. The water interposes between the ionic compounds, considerably reducing the force of attraction between the ions, causing their separation and, therefore, their dissolution.

Disolución de cloruro sódico

By Andy Schmitz (http://2012books.lardbucket.org/) [CC BY 3.0], via Wikimedia Commons

We can classify substances according to their solubility in water:

  • Hydrophilic: soluble in water, like common salt (ionic compound) and sugar (polar compound).
  • Hydrophobic: insoluble in water, like fats and other non-polar substances.

The high solvent capacity is responsible for two important functions of water:

  • Transport function: Water is the main means of transport for organisms (blood , raw and processed sap).
  • Metabolic and biochemical function: The biochemical reactions of life take place in water (which take place between molecules dissolved in water). It is involved in many chemical reactions, such as hydrolysis (breakage of bonds with the intervention of water) that occurs during the digestion of food, as a source of hydrogen in photosynthesis, etc.

Video: Dissolution of NaCl in water .

High cohesion strength

Water molecules have a high internal cohesion due to hydrogen bonds. The main functions that derive from this property are:

  • Great incompressibility. It takes a lot of energy to bring two water molecules together, which makes water practically incompressible, ideal as a hydrostatic skeleton to give volume to cells, cause turgor in plants, constitute the hydrostatic skeleton of annelids and coelenterates, etc.
  • Structural function. The volume and shape of cells that lack a rigid membrane are maintained thanks to the pressure exerted by internal water. By losing water, cells lose their natural turgor, wrinkle and can even break (lysis).
  • High surface tension. Its surface offers great resistance to breaking. This allows many organisms to live associated with that surface film.

Tensión superficialBy Eclipse2009 (Own work) [CC BY-SA 3.0], via Wikimedia Commons

High bond strength

This causes the water to adhere to the surface of the container that contains it, and the raw sap to rise up the capillary tubes.

The function derived from this property is the phenomenon of capillarity, which depends both on the adhesion of the water molecules to the walls of the ducts and on the cohesion of the water molecules with each other.

Capilaridad

By MesserWoland [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons

High specific heat

The specific heat is the quantity of heat that is necessary to communicate to one gram of a substance to increase its temperature 1° C.

Due to the high specific heat of water, it takes a lot of heat to raise its temperature. It is capable of absorbing a lot of heat without hardly increasing its temperature. It is also a consequence of the formation of hydrogen bonds between water molecules, since energy is used to break hydrogen bonds, not to increase the temperature by molecular agitation. Thus, by communicating a certain amount of heat, the temperature rises little and, in the same way, when releasing energy by cooling, the temperature falls more slowly than in the case of other liquids.

This property makes it have a thermoregulatory function, being a thermal stabilizer, keeping the body temperature relatively constant, despite environmental fluctuations.

The water is thermoregulatory for its high specific heat.

High heat of vaporization

It takes a lot of energy for the water to evaporate, to go from a liquid to a gas, as it is necessary to break the existing hydrogen bonds in the liquid phase.

The hydrogen bonds between water molecules allow remain bonded together at a temperature at which other molecules, chemically comparable, such as H2S or NH3, are in a gaseous state.

The function derived from this property, together with the previous one, is thermoregulatory, since a decrease in the temperature of an organism is achieved by losing an amount of heat that is used in the evaporation of water. Allows you to remove large amounts of heat with little water loss.

As a consequence of the water being liquid at room temperature, water is used as a fluid means of transport between the different parts of an organism.

Also function mechanical damping, as in vertebrates, which have in their joints bags synovial fluid which prevents friction between the bones.

Higher density in liquid state than in solid state

In ice, each water molecule is joined by four hydrogen bonds with its neighbors, forming a more open structure than in the liquid state, being further apart. This causes the ice to float on the water and form a heat-insulating surface layer that allows life, under it, in rivers, seas and lakes. Water reaches its maximum density at 4ºC. If ice were denser than water, all the water would eventually freeze. The function that would derive from this property would then be ecological.

 

Low degree of ionization

In pure water, at 25 ºC, out of every 551,000,000 water molecules, only one is ionized, dissociated into H+ and OH- , which makes the concentration of hydronium ions (H3O+) and of hydroxyl ions (OH-) are very low, specifically 10-7 moles per liter ([H3O+  = [OH-] = 10-7).

With these low levels of H3O+ and OH- , if an acid (H3O+ is added ) or a base (OH- is added ) is added to the water, even in a very small quantity, these levels vary. abruptly. Therefore, water has a buffer function , and the pH of pure water is equal to 7.

Disociación del agua

By Sergi.vicente (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Fundamental ideas about the properties and functions of water

The linked by hydrogen bonds are responsible for much of the properties is water. These properties of water allow it to perform certain functions.

Check out the graphic at the top of the page.

As an outline, some of the properties of water and its functions are: