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

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

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

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

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

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

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

8.2 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

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

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.

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

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

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

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

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

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?

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.

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

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

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

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

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

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?

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.

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

8.4 Water Molecules Move Freely between the Liquid and Gaseous Phases

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

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

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

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

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

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?

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.

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

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

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

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

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

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

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.

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

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