1. Water as a Polar Molecule
TAKS: Objective 4
TEKS: 8D

2. Water has a simple molecular structure.
The Polarity of Water
Water has a simple molecular structure.
Composed of one oxygen atom and two hydrogen atoms.
Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons.

3. Electronegativity
What is electronegativity and how does it affect an atom of an element???
With your shoulder partner, discuss the answer to the question and be ready to share with the class…….

4. Electronegativity
Chemical property that describes the ability of an atom (or, more rarely, a functional group) to attract electrons (or electron density) towards itself in a compound
Looking at water, which element has a higher electronegativity?
OXYGEN

5. Since oxygen has a higher electronegativity than hydrogen:
Electrons in molecule group closer to oxygen than with hydrogen
The oxygen side has a slight or partial negative charge
The hydrogen side has a slight or partial positive charge.

6. What does this do the water molecule????
The partial positive and negative ends of the molecule make it a “polar” molecule
There is an uneven distribution of electron density in the molecule

7. Hydrogen bond:
Electrostatic attraction between partial positive charge near hydrogen atoms and partial negative charge near oxygen atom
Weak bonds
Non-covalent bonds
Effective solvent

8. Brainstorm!!!
This polar condition is a great characteristic that all water molecules have…….why do you think?????
With your shoulder partner discuss how is this characteristic of water beneficial to us? Be prepared to share with the class……

9. Polarity of Water Makes it an effective solvent
Polar attracts polar and dissolves (“like dissolves like”)
Dissolves other polar compounds
Ionic compounds dissolve, they break up into positive and negative ions
Example: Water molecules surround sodium ions and chloride ions, thus dissolving the salt

10. Many other unique properties of water are due to the hydrogen bonds.
For example, ice floats because hydrogen bonds hold water molecules farther apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. Therefore, the density decreases.

11. Application
Because ice is less dense than water, lakes begin to freeze from the top down.
The layer of ice that forms on a surface of a lake helps shield the water underneath from cold air temperatures.
This tends to keep lakes from freezing solid and killing the organisms that live there.

12. High Boiling Point On average, each water molecule interacts with
about four others.
In water vapor, the molecules are too far apart for hydrogen bonding to occur.
All hydrogen bonds in liquid water must be broken in order to boil water
Breaking those bonds takes energy, thus the high boiling point for water.

13. Cohesion
The attraction of one water molecule to another resulting from hydrogen bonding.
By placing a drop of water on a surface you can directly observe cohesion in the resistance that water droplet shows to wetting, i.e., water clumps up in a pile despite being a liquid, rather than spreading out over the surface.

14. Adhesion
Similar to cohesion except adhesion involves the attraction of a water molecule to a non-water molecule.
How are adhesion and
cohesion taking place
in this image??

15. Surface Tension
The water molecules on the surface have partners for hydrogen bonding only within the liquid
Above the water surface there are no more molecules available for hydrogen bonding.
This means that molecules at the surface experience a net force pulling them inward.

16. Surface Tension
The result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension

17. Cohesion, Adhesion, and Surface Tension
Capillary Action

18. Cohesion, Adhesion, and Surface Tension
Water adhesion to the walls of the tube is greater than water cohesion
Surface tension increases and causes the meniscus
Water begins to go up through the tube until it can no longer ascend due to the pull of gravity (capillary action)

20. Properties of Water
1. Draw the structure of water. Include the partial charges of each atom.
2. Explain how a water molecule becomes a polar molecule?
3. How does a hydrogen bond form?
5. Explain the difference between adhesion and cohesion.
6. What is surface tension?
7. Explain why water is considered to have a high boiling point.