1. Chapter 8 How Water Behaves
Lecture Presentation
Chapter 8 How Water Behaves
Bradley Sieve
Northern Kentucky University
Highland Heights, KY

2. 8.1 Water Molecules Form an Open Crystalline Structure in Ice
When water forms, the crystalline structure contains many open spaces
This causes water to expand upon freezing

3. 8.1 Water Molecules Form an Open Crystalline Structure in Ice
Ice is then less dense than water
A very rare behavior in liquids
Thin layer of water still forms on the surface of ice due to the three-dimensional structure

4. 8.2 Freezing and Melting Go On at the Same Time
At the molecular level, melting and freezing occur at the same time
At 0°C, the rates of freezing and melting of ice are equal

5. 8.2 Freezing and Melting Go On at the Same Time
To promote freezing, one must increase the rate of ice formation
Remove heat
Slow the motion of the molecules
This will allow the formation of more hydrogen bonds

6. 8.2 Freezing and Melting Go On at the Same Time
Solutes tend to inhibit crystal formation
Limit the interaction of water molecule to water molecule

7. 8.2 Freezing and Melting Go On at the Same Time
Water is densest at 4°C
Lower than 4°C, the water contains significant amounts of ice crystals
Above 4°C, water expands due to molecular motions

8. 8.2 Freezing and Melting Go On at the Same Time
Ice floating is an effect of the 4°C behavior
Because the solid is less dense than 4°C water, the solid will float on the liquid
This allows creatures to survive in bodies of water

9. Concept Check
What was the precise temperature at the bottom of Lake Michigan on New Year’s Eve 1901?

10. Concept Check
If a body of water has 4°C water in it, then the temperature at the bottom of that body of water is 4°C, for the same reason rocks at the bottom are 4°C. Both 4°C water and rocks are denser than water at any other temperature. If the body of water is deep and in a region of short summers, as is the case for Lake Michigan, the water at the bottom is 4°C year round.

11. 8.3 Liquid Water’s Behavior Results from the Stickiness of Its Molecules
Cohesive Forces
How molecules in the liquid phase interact
Hydrogen bonding is the cohesive force in water
Adhesive Forces
Interaction between molecules of different substances

12. Surface of water acts as an elastic film
8.3 Liquid Water’s Behavior Results from the Stickiness of Its Molecules
Surface of water acts as an elastic film
An object such as a paper clip can rest on the surface of water
This behavior is called surface tension
Caused by the hydrogen bonding between water molecules

13. Soap reduces the surface tension of water
8.3 Liquid Water’s Behavior Results from the Stickiness of Its Molecules
Soap reduces the surface tension of water
Soap molecules interfere with the hydrogen bonding of water

14. 8.3 Liquid Water’s Behavior Results from the Stickiness of Its Molecules
Capillary Action
Interplay of adhesive and cohesive forces
Interactions move liquid up the tube

15. 8.3 Liquid Water’s Behavior Results from the Stickiness of Its Molecules
Amount of rise of liquid depends on width of tubing
Curved of liquids surface is called the meniscus

16. Concept Check
An astronaut sticks a narrow glass tube into a blob of floating water while in orbit, and the tube fills with water. Why?

17. Concept Check
Capillary action causes the water to be drawn into the tube. In the free-fall environment of an orbiting spacecraft, however, there is no downward force to stop this capillary action. As a result, the water continues to creep along the inner surface of the tube until the tube is filled.

18. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Evaporation
Converting a liquid phase to a gas phase
Condensation is the reverse

19. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Evaporation is due to high-kinetic-energy molecules escaping the liquid surface

20. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Sublimation
Evaporation of a solid
Happens much slower than evaporation

21. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Condensation and evaporation occur at the same time at liquid’s surface
Slow-moving water vapor sticks to the liquid
Fast-moving molecules break free into the vapor phase

22. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Water vapor can cause weather
Vapor releases energy as it condenses
This energy is source of many weather systems, including hurricanes
1 inch of rain over 1 square mile yields energy equivalent to 32,000 tons of dynamite

23. Concept Check
If the water level in a dish of water remains unchanged from one day to the next, can you conclude that no evaporation or condensation is taking place?

24. Concept Check
Not at all, for much activity is taking place at the molecular level. Both evaporation and condensation occur continuously and simultaneously. The fact that the water level remains constant indicates equal rates of evaporation and condensation—the number of molecules leaving the liquid surface by evaporation is equal to the number entering the liquid by condensation.

25. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Boiling
Evaporation below the liquid surface
Pressure inside the bubble equals or exceeds the combined pressure of the water and atmosphere
Boiling occurs at lower temperatures at higher altitudes
In Denver (1-mile altitude), water boils at 95°C
At sea level, water boils at 100°C

26. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases

27. 8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases
Boiling is a cooling process
Energy is removed from the water in the process
Used to cool refrigerators and air conditioners

28. Concept Check
Is boiling a form of evaporation, or is evaporation a form of boiling?

29. Concept Check
Boiling is evaporation that takes place beneath the surface of a liquid.

30. 8.5 It Takes a Lot of Energy to Change the Temperature of Liquid Water
Specific Heat
Quantity of heat required to change 1 gram of substance 1°C
Differs for different substances
Kind of thermal inertia that must be overcome to change temperature
Corresponds largely to type of intermolecular forces present

31. 8.5 It Takes a Lot of Energy to Change the Temperature of Liquid Water

32. Concept Check
Hydrogen bonds are not broken as heat is applied to ice (providing the ice doesn’t melt) or water vapor. Would you therefore expect ice and water vapor to have specific heats that are greater or less than that of liquid water?

33. Concept Check
As Table 8.1 shows, the specific heats of ice and water vapor are about half that of liquid water. Only liquid water has a remarkable specific heat. This is because the liquid phase is the only phase in which hydrogen bonds are continually breaking and re-forming.

34. 8.5 It Takes a Lot of Energy to Change the Temperature of Liquid Water
Water’s specific heat influences global climate
England is warmed by water from the Caribbean
Winds in North America are westerly
Islands and peninsulas do not have extreme temperature changes because they are surrounded by water

35. 8.6 A Phase Change Requires the Input or Output of Energy
Any phase change involves the formation of breaking od molecular attractions
An energy change is therefore always associated with the change

36. 8.6 A Phase Change Requires the Input or Output of Energy
Heat involved in heating 1 gram of ice

37. 8.6 A Phase Change Requires the Input or Output of Energy
Heat of Melting
Amount of heat energy needed to change a solid to a liquid
+335 joules per gram for water
Heat of Freezing
Amount of heat energy released in a change of a liquid to a solid
−335 joules per gram for water

38. 8.6 A Phase Change Requires the Input or Output of Energy
Heat of Vaporization
Amount of heat energy required to change a liquid to a gas
+2259 joules per gram for water
Amount of heat energy released in a change of a gas to a liquid
−2259 joules per gram for water

39. Concept Check
Can you add heat to ice without melting it?

40. Concept Check
A common misconception is that ice cannot have a temperature lower than 0°C. In fact, ice can have any temperature below 0°C, down to absolute zero, –273°C. Adding heat to ice below 0°C raises its temperature, say, from –200°C to –100°C. As long as its temperature stays below 0°C, the ice does not melt.

41. 8.6 A Phase Change Requires the Input or Output of Energy
Water’s high heat of vaporization can be useful

42. 8.6 A Phase Change Requires the Input or Output of Energy
What remarkable properties of water
can be found in this photograph?