1. Polar Nature of Water

2. Background - Polar Nature of Water
Oxygen has 6 valence (outer) electrons
needs 8 to have a full outer shell.
Hydrogen has 1 valence electron
needs 2 to have a full outer shell.
Neither atom wants to give up electrons.
Instead, they share electrons to form covalent bonds.
Hydrogen
Hydrogen
Oxygen
Polarity

3. Background - Polar Nature of Water
Although they are sharing electrons with the hydrogen atoms, the oxygen atom has a greater attraction for the electrons.
So, the electrons spend more time near oxygen’s end of the molecule.
Hydrogen
Hydrogen
Oxygen
Polarity

4. Background - Polar Nature of Water
Oxygen atom also has two unshared pairs of electrons
These tend to make the oxygen end of the molecule more negative.
Unshared pairs repel the shared pairs and hydrogen atoms off to one side, creating the familiar Mickey Mouse appearance of the water molecule.
Hydrogen
Hydrogen
Oxygen
Polarity

5. Background - Polar Nature of Water
As a result, the oxygen end of water tends to have a slight (partial) negative charge,
while the hydrogen end of water tends to have a slight (partial) positive charge
Since water has positive and negative poles, it is a polar molecule.
Hydrogen
Hydrogen
Oxygen
Polarity

6. Background - Polar Nature of Water
As a result, the oxygen end of water tends to have a slight (partial) negative charge,
while the hydrogen end of water tends to have a slight (partial) positive charge
Since water has positive and negative poles, it is a polar molecule.
Polarity

7. This gives water very special properties…
Water is a polar molecule
Hydrogen
Hydrogen
This gives water very special properties…
Oxygen

8. Cohesion – Water Sticks to itself

9. Adhesion – Water Sticks to
other things

10. Water droplets…
…form due to cohesion

11. Surface Tension
The cohesion of water molecules creates a skin-like surface that we call surface tension. (Tension because the water molecules are pulling on each other.
Hydrogen Bonding

12. Surface Tension
Hydrogen Bonding

13. Surface Tension
Surface tension is actually able to hold up a certain amount of weight.
If the weight gets too high, it will break the surface tension
(Don’t try to float a hammer!!)
Hydrogen Bonding

14. Surface Tension
Surface tension depends on the water molecules pulling on each other.
If you add a non-polar molecule (such as detergent), these new molecules interfere with cohesion and break the surface tension.
Hydrogen Bonding

15. Surface Tension Can you explain how an insect does this?
More detailed description
Hydrogen Bonding

16. Soap Breaks Surface Tension
The hydrophilic (red) end of the soap molecule is attracted to the water molecules.
More detailed description
Hydrogen Bonding

17. Soap Breaks Surface Tension
The long nonpolar chain (gray) is hydrophobic (it repels water).
More detailed description
Hydrogen Bonding

18. Soap Breaks Surface Tension
The soap repels the water, which pushes all of the soap molecules to the top of the liquid.
More detailed description
Hydrogen Bonding

19. Soap Breaks Surface Tension
As the soap collects at the top of the water, the soap molecules break the hydrogen bonds that connected the water molecules on the surface.
The soap “breaks” the surface tension.
More detailed description
Hydrogen Bonding

20. Surface Tension
What would happen if a water spider tried to walk across a pond that had been treated with soapy water?
Hydrogen Bonding

21. Surface Tension in 3D
Breaking a Soap Bubble
Hydrogen Bonding

22. Adhesion – Water Sticks to
other things

23. Water is the Universal Solvent.
Chloride
Water
Sodium

24. Water is the Universal Solvent.

25. Most Liquids are Aqueous Solutions*
*(Stuff dissolved in water)

26. Other Polar Compounds Polar Liquids (besides water):
Acetone – weak (Fingernail Polish Remover)
Ethanol (slightly polar due to OH group)
Polar Gases:
Ammonia (NH3) – liquid on really cold planets and when mixed with water
Sulfur Dioxide (SO2)
Hydrogen Sulfide (H2S).

27. Nonpolar Liquids Gasoline
Ethyl Alcohol (on ethyl end - actually slight polar on OH end)
Oils (Motor Oil and Plant Oils)
Mercury and Bromine (Elements)
Glycerin
Many organic solvents:
Turpentine Benzene
Carbon Tetrachloride Chloroform

28. Nonpolar Liquids Chemical Makeup of Typical De-Greaser
2-Butoxyethoxy-2-ethanol
9-Octadecenoic acid
Ammonium hydroxide
Citric acid
d- limonene Fragrance
Sodium carbonate
Triethanolamine

29. Surface Tension
What do you think would happen if an insect tried to walk on a nonpolar liquid such as the liquid hydrocarbon lakes of Titan?
Hydrogen Bonding

30. Part VI. 3-D Polar Water Models
You will be working with a set of 3-D Polar Water Molecule Models.
The red oxygen models have a South magnetic pole and the white hydrogen models have a North magnetic pole.
This makes them behave in a similar way to the positive hydrogen and negative oxygen electrical poles of the water molecule.
Polarity

31. Cohesion, Adhesion, Surface Tension
Surface Tension – the skin-like surface that forms on water due to cohesion
Note: Tension is a pulling force.
Nonpolar compounds (like detergent) break surface tension.
Cohesion – Water’s ability to stick to itself
Adhesion – Water’s ability to stick to other things (such as a window or metal)

32. VI. Methane and Ethane Look at the gray 3D models: Methane (CH4)
They are both nonpolar molecules, with C-H bonds evenly distributed around their centers:

33. Turning Methane into Methanol
Look at the gray 3D models:
Remove one H from Methane (CH4) and replace it with a hydroxyl group (OH)-
You now have a methanol molecule (CH3OH)

34. Turning Ethane into Ethanol
Look at the gray 3D models:
Remove one H from Ethane (C2H6) and replace it with a hydroxyl group (OH)-
You now have an ethanol molecule (C2H5OH)

35. Methanol and Ethanol: Polar or Non-Polar??
Ethanol is a very polar molecule due to its hydroxyl (OH)- group, with the high electronegativity of oxygen allowing hydrogen bonding to take place with other molecules.
Non-Polar End
Polar End

36. Methanol and Ethanol: Polar or Non-Polar??
The polar OH end of Ethanol therefore dissolves polar and some ionic molecules.

37. Ethanol – Polar or Non-Polar??
The ethyl (C2H5) group in ethanol is nonpolar.
Non-Polar End
Polar End

38. Ethanol – Polar or Non-Polar??
The ethyl (C2H5) group in ethanol is nonpolar.
Ethanol therefore dissolves nonpolar molecules, too!
Thus, ethanol can dissolve both polar and nonpolar substances!
In industrial and consumer products, ethanol is the second most important solvent after water.

39. Attach top layer to bottom layer
Turning Water Into Ice
Build the top layer
Attach top layer to bottom layer
Build the bottom layer

41. Water is the Universal Solvent.
Chloride
Water
Sodium

42. Habitable Zone
The region of the solar system where temperatures allow water to exist in its liquid state.

43. Surface Tension
Predict what will happen when you lay a paperclip on top of each surface. Why? Record your hypothesis and reasons.
Hydrogen Bonding