Water

• Properties of water

– Polar molecule

• Hydrogen bonds with other water molecules

• This is the foundation for all the properties of water

Water• Water is the solvent, the medium and the participant

in most of the chemical reactions occurring in the environment.

• On earth, water is found as a liquid, as a solid (ice) or as a gas (water vapor).

• Water molecules are attracted to each other, creating hydrogen bonds.

• These strong bonds determine almost every physical property of water and many of its chemical properties.

Cohesion

• Water molecules stick to each other. • The attraction between the water

molecules is greater than the attraction between water and air. – Creates a surface tension (film)– Permits water to hold up substances heavier

and denser than itself.• Steel needle carefully placed on the surface of a

glass of water will float.

• Cohesion of water

molecules permits the flow of water in streams and rivers.

• Surface tension is essential for the transfer of energy from wind to water to create waves which are necessary for rapid oxygen diffusion in lakes and seas.

• Some aquatic insects such as this water strider (predators on smaller invertebrates) rely on surface tension to walk on water.

Adhesion• Water molecules bind to many other

substances such as glass, cotton, plant tissues, and soils.

• For example, in a thin glass tube, when the molecules at the edge reach for and adhere to the molecules of glass just above them, they tow other water molecules along with them due to cohesion. The water surface, in turn, pulls the entire body of water to a new level until the downward force of gravity is too great to be overcome. This process is called capillary action.

Adhesion• Adhesion allows the flow

of water through the vascular tissues of plants and the flow of blood through blood vessels.

Adhesion

• Adhesion also produces viscosity– Water flows in layers

• Laminar viscosity: friction between layers

– Flow is fastest in middle

– Slowest near edges of stream and bottom due to friction of layers

Laminar viscosity: friction between layers of water. Flow is fastest in middle, and slowest near edges of stream and bottom due to friction between layers.

Eddy viscosity: mixing of layers creates turbulence which keeps small organisms suspended in water column

Plankton swirling in the turbulent flow of an ocean eddy

Water’s High Specific Heat

• The specific heat is the amount of heat that must be absorbed or lost for one gram of that substance to change its temperature by 1oC

• Therefore, the specific heat of water is defined as:

calorie/gram °C = 4.186 joule/gram

4.184 Joules = 1.00 calorie

Highest of any common substance

How Water Stabilizes Temperature

• Water’s high specific heat is linked to hydrogen bonding.

• Much of the heat energy is used to disrupt the hydrogen bonds before the water molecules can begin moving faster.

• When the temperature of the water drops slightly, many hydrogen bonds form, releasing a considerable amount of energy in the form of heat.

High Heat Capacity

• Water absorbs or releases more heat than many substances for each degree of temperature increase or decrease. – Warms slowly– Cools slowly– Evaporative cooling: sweating, evapo-

transpiration in plants

High Heat Capacity

• Differences in temperature between lakes and rivers and the surrounding air may have a variety of effects. – Local fog or mist is likely to occur if a lake

cools the surrounding air enough to cause saturation; small water droplets are suspended in the air.

High Heat Capacity

• Water vapor forms a kind of global "blanket" which helps to keep the earth warm. – Heat radiated from the sun-warmed

surface of the earth is absorbed and held by the vapor.

– Water vapor contributes to global warming • Is it a greenhouse gas?

High Heat Capacity

• Large bodies of water, such as the oceans or the Great Lakes, have a profound influence on climate. – Heat reservoirs and heat exchangers– Sources of moisture that falls as rain and

snow over adjacent land masses.– When water is colder than the air,

precipitation is curbed, winds are reduced, and fog banks are formed.

Freezing and Boiling• Pure water at sea level boils at 100°C and freezes

at 0°C. At higher elevations (lower atmospheric pressures) water's boiling temperature decreases. – Takes longer to boil an egg at higher altitudes

• Doesn’t get high enough to cook the egg properly

– If a substance is dissolved in water, the freezing point is lowered.

• Spread salt on streets in winter to prevent ice formation.

Biological Antifreeze

• Must prevent formation of ice crystals inside cells.

– Insects cells replace water with glycerol

– Plants use sugars and proteins to increase solutes in the cytoplasm

– Antarctic fish use glycoproteins

Water Vapor

• Absolute humidity: actual mass of water vapor in the air

• Relative humidity: % of max. water vapor for a particular temperature

• Dew point: saturation point for a particular temperature

Air Temp. and Humidity

• Warm air holds more water vapor than cold air. – This explains why indoor air is so dry in

winter. • Cold air outside holds little water.• Air coming into house is dry.• Heating dries it further.

Absolute Humidity

Dew PointDew Point• If air can no longer hold water vapor without If air can no longer hold water vapor without

condensing, itcondensing, it

– is is saturatedsaturated

– has a has a relative humidity of 100%relative humidity of 100%

– has reached its has reached its dew point temperaturedew point temperature

Dew Point

• The temperature at which the air is completely saturated with water (i.e., RH = 100%).

• At this temperature water precipitates (goes from gas to liquid or solid) and forms clouds (and dew on grass and ice tea glasses) and potentially rains/snows.

• The dew point is given to indicate the amount of moisture in the air (specific humidity).

Dew Point

• The current dew point will never be higher then the current temperature.

• If the temperature is at the dew point and the temperature falls, the dew point must follow.

• The higher the dew point temperature, the more moisture there is in the air. – Example: Death Valley(desert) California has low dew

points, thus feels dry.

– Key West Fl. has high dew points, thus feels sticky.

Universal Solvent• An important property of water is its ability to

dissolve other substances.

• Were it not for the solvent property of water, life could not exist because water transfers nutrients vital to life in animals and plants.

• A drop of rain water falling through the air dissolves atmospheric gases. When rain reaches the earth, it affects the quality of the land, lakes and rivers. – Acid rain

Selection for Life on Land

• Water is a stable environment

• The invasion of the land is really the invasion of the air– Adaptations are really adaptations for life in

the relatively drier atmosphere

Selection for Life on Land

• What problems would have to be overcome?– Desiccation: drying out

– Water for reproduction:

• Free water needed for gamete fusion

• Protection of embryo/prevent desiccation

– Support:

• Can’t depend on water for buoyancy

– Dispersal of offspring to colonize new habitats

• Can’t depend on currents or swimming

Onychophora

Chicago’s Weather

• There are two geographical factors that contribute to Chicago's weather.– Lake Michigan: 450 miles long, 80 miles

wide, 900 feet deep. – Location: there really aren’t any major

geographical features to the west or south of Chicago for nearly a thousand miles (1600 kilometers).

• Location effect

– Once weather events start heading to Chicago, there is nothing in the geography to stop them.

– This means that there are usually two to three days warning before any major weather systems hit Chicago.

– This also means that any weather system that happens to head to Chicago tends to get there.

– This makes Chicago weather very variable in the long term.

– But it also tends to be predictable in the short term.

• Lake effect

– As any huge mass of water does, Lake Michigan makes a dandy thermal mass.

– It tends to be much warmer that the air over it in winter, and much cooler than the air in summer.

– In winter, this bring lake effect snows that can drop six inches of snow on downtown while leaving the suburbs dry.

– In summer, this brings the lake breeze, which can drop the temperature dramatically downtown, while it’s broiling hot in the western ‘burbs.

• This also explains why NW Indiana, SE

Michigan, and Buffalo, N.Y. are hit so hard by snowstorms in the winter. – As air passes over the warm lake water, it

picks up moisture. – Then as it passes over the land, it cools

and drops its moisture in the form of snow.