Chapter 3- 3.2- Water Flashcards
What is meant by the term polar, and how are polar bonds formed?
Polar means the molecules have regions of negativity and regions of positivity.
Polar bonds are formed when the electrons spend more time closer to one of the atoms in the covalent bond than to the other. The atom with the greater share of negative electrons will be slightly negative (delta negative) compared with the other atom in the bond, which will therefore be slightly positive (delta positive).
What are two examples of elements that do not share electrons equally in a covalent bond?
Oxygen- Always has a much greater share of the electrons in an O-H bond.
Hydrogen.
Many organic molecules contain oxygen and hydrogen bonded together in what are called hydroxyl (OH) groups and so they are slightly polar.
Water is an example of this and contains 2 hydroxyl groups.
How do polar molecules interact with each other?
The positive and negative regions of the molecule attract each other and form bonds known as hydrogen bonds. Hydrogen bonds are relatively weak interactions which break and reform between constantly moving water molecules.
Although hydrogen bonds are weak interactions they occur in high numbers. Hydrogen bonds give water its unique characteristics which are essential for life on this planet.
What are the characteristics of water?
1) Unusually high boiling point. Water is a small molecule much lighter than the gases carbon dioxide and oxygen, but it is liquid at room temp rather than a gas . This Is due to the hydrogen bonding between the water molecules. It takes a lot of energy to increase the temp of water and cause water to become gaseous (evaporate).
2) When water freezes it turns to ice. Most substances are more dense in their solid state than in their liquid state, but water in the form of ice is less dense. This is because of the hydrogen bonds formed.
As water cools below 4 (degrees Celsius) the hydrogen bonds fix the positions of the polar molecules slightly further apart than the average distance in the liquid state. This produces a giant, rigid but open structure, with every oxygen atom at the centre of the tetrahedral arrangement of hydrogen atoms, resulting in a solid that is less dense than liquid water. Therefore ice floats.
3) Water therefore has cohesive properties. It moves as one mass because the molecules are attracted to each other (cohesion). It is in this way that plants are able to draw water up their roots and how you are able to drink water through a straw.
4) water also has adhesive properties. This is where water molecules are attracted to other materials. For example; When you wash your hand your hands become wet, the water doesn’t run straight off.
5) Water molecules are more strongly cohesive to each other than to air, this results in water having a “skin” of surface tension.
How does water act as a solvent?
Because it is polar molecule many of the solutes in an organism can be dissolved. The cytosol of prokaryotes (bacterial) and eukaryotes is mainly water.
Many solutes are also polar molecules for example amino acids, proteins and nucleic acids.
Water acts as a medium for chemical reactions and also helps transport dissolved compounds into and out of cells.
Why does water make a very efficient transport medium within things?
Cohesion between water molecules means that when water is transported through the body, molecules will stick together.
Adhesive occurs between water molecules and other polar molecules and surfaces.
The effects of adhesion and cohesion result in water exhibiting capillary action. This is the process by which water can rise up a narrow tube against the force of gravity.
How does water act as a coolant?
It helps to buffer temp changes during chemical reactions in prokaryotic and eukaryotic cells because of the large amounts of energy required to overcome hydrogen bonding.
Maintaining constant temperatures in cellular environments is important as enzymes are often only active in a narrow temperature range.
Why is water a stable environment (habitat) for organism to live in such as fish?
It does not change temperature or become a gas easily. Therefore providing a constant environment. Because ice floats it forms on the surface of the water such as on lakes and ponds it forms an insulating layer above the water below. Aquatic organisms would not be able to survive freezing temperatures if their entire habitat froze solid. Some organisms also inhibit the surface of water. Surface tension is strong enough to support small insects such as pond skaters.
