Hydrogen bonds Relatively weak electrostatic forces between oppositely charged ends of adjacent water molecules The dipolar structure of the water molecule and the formation of hydrogen bonds between water molecules are responsible for many of the unique properties of water. p. 108-109

As water cools it contracts and reaches a maximum density at 3 As water cools it contracts and reaches a maximum density at 3.98°C, but as cooling continues more hydrogen bonds form between water molecules which tend to push the molecules apart. As water freezes to ice the bonds form an open crystal structure less dense than water.

Temporary molecular clusters in water cause higher density than in ice.

Open structure in ice leads to lower density than in water Open structure in ice leads to lower density than in water. Note that hydrogen bonds connect the water molecules.

Because water has hydrogen bonds between molecules, it has relatively high boiling and freezing points compared to other H2X that lack hydrogen bonds. Consequently water can exist as ice, liquid and vapor on the Earth.

Properties of Water water has a high heat capacity water absorbs and releases a considerable amount of heat with little change in temperature Sensible Heat = vibration of water molecules, the temperature measured with a thermometer heat is measured in Calories; 1 cal = amount of energy needed to raise the temperature of 1 g of water by 1oC = 4.185 J (joules)

without a change of temperature Properties of Water Water has a high latent heat of vaporization and a high latent heat of fusion considerable amounts of heat must be added or removed to convert H2O from one phase to another: ice  water  vapor Latent Heat = the heat required to change phase without a change of temperature p. 110

Water & Climate heat energy is required to break hydrogen bonds when ice melts (80 cal/g) and when water evaporates (540 cal/g): “latent heat of vaporization” heat energy is released to the atmosphere as hydrogen bonds form when water freezes (80 cal/g) and when water vapor condenses (540 cal/g): “latent heat of fusion” the Hydrologic Cycle (evaporation-precipitation) redistributes heat around the planet by cycling water between the ocean, atmosphere, & land p. 112

Figure 3.5

540 cal/g released to the atmosphere 540 cal/g removed from the ocean Evaporation Latent heat is removed from the ocean Condensation & Precipitation Latent heat is released to the atmosphere p. 113

Sublimation Deposition phase change directly from solid to vapor e.g., what happens to your ice cubes in the freezer (they shrink over time) Deposition phase change directly from vapor to solid e.g., frost p. 112

Hurricane Floyd, 9/99 Tropical cyclones (hurricanes & typhoons) represent nature’s safety valve for the release of excess heat that builds up in the tropics and subtropics every year

 Energy output/ Energy input  540 cal/g of latent heat are needed to evaporate water, 540 cal/g of latent heat are released to the atmosphere when water vapor condenses to form clouds, rain, or snow 80 cal/g of latent heat are needed to melt ice, 80 cal/g of latent heat are released to the atmosphere when water freezes p. 113  Energy output/ Energy input 

A Few Examples It’s a hot summer day with temperatures on the pavement equal to 50oC (= 122oF). How much latent heat is required to evaporate a rain puddle created the night before? Ever watch a puddle on a hot summer day? Does it boil? Of course not! But the water in that puddle must absorb a lot of energy to change phase from liquid to gas (vapor). This includes both sensible heat and latent heat.

50 + 540 = 590 cal/g 50 cal. 540 cal.

Examples (cont.) Now it’s a cool fall day with temperatures on the pavement equal to 5oC (= 41oF). How much latent heat is required to evaporate a puddle? You notice that the ice-cubes in your freezer seem to shrink in size if they are not used in some time; your freezer is -5oC (= 23oF). How much latent heat is required to turn ice into water vapor? Changing from solid directly to gas is called sublimation.

95 + 540 = 635 cal/g 5 + 80 + 100 + 540 = 725 cal/g Note that this slide has an error. The heat capacity of ice below zero C is 0.5 calories per gram, not 1 calorie per gram. So to go from -5 deg to zero requires only 2.5 calories per gram, not 5….JBG 2/2010 5 -5