Water

Water• Unique properties – important for understanding

interaction between ocean & atmosphere– Climate

• Dissolved constituents and how they affect water’s properties

• Phenomena in the ocean that are important – light and sound in water– Why is the ocean blue?

• Then ocean & atmospheric circulation – waves & tides

What is Water• H2O

• Covalent & hydrogen bonding

• Water as a solvent

• Density

• Heat Capacity and specific heat

• Salinity (next time)

A covalent bond exists when two electrons are shared by two non-metallic atoms. Dipolar.

**Electron density higher around oxygen, giving that end a partial negative charge.

•Oxygen has 6 electrons, and wants 2 more electrons for stability. •Hydrogen has 1 electron and wants 1 more electron for stability.

104.5°

Electrons and “charge” not distributed evenly so moleculebecomes polar.

Hydrogen bond is a weak bond formed when a charged part of a molecule having polar covalent bonds forms an electrostatic (charge, as in positive

attracted to negative) interaction with a substance of opposite charge. The weak bonds are easily and rapidly formed and broken under normal biological conditions. They are extremely important in biological systems and Their

presence explains many of the properties of water.

H-bonding impartsadhesion and cohesion of water.

Creates surface tensionand wetting ability

Water cohesion (imparted by the H-bonding between water molecules) causes surface tension & capillary action. Adhesion allows water molecules to “stick” to other things or get wet.

Water is a powerful solvent

Ethanol dissolveswell in water becauseit is polar.

On the other hand, oil is repelled by water because it is non-polar, and thus does not dissolve well in water.

Ionic molecules dissolve well in water.

Water & heat

• Temperature – how rapidly molecules are vibrating– Response to addition and removal of heat

• Heat capacity– Amount of heat required to change the temperature of

a substance (raise or lower)– Calorie is the amount of heat required to raise

temperature of 1 gram water by 1 oC– Heat capacity of water is among the highest of all

known substances– This high heat capacity is a result of water’s structure

(H-bonding)

Many metals have low heat capacities(change temperaturewith small changesin heat content)

Significance of heat capacity

• Water is a good buffer for heat

• Land temperatures change more easily and rapidly than water– Compare daily changes on land and in the

sea– Compare land near the coast with inland

4 J ≈ 1 calories (kg)

Sensible heat versus latent heat

Latent heat

• Latent heat does not cause temperature change

• Much higher for vaporization than for fusion

• Must break H bonds for vaporization• Evaporation has a cooling effect because

heat is removed from water• Evaporation is a huge source of heat flow

between the atmosphere and the ocean

Water and density

• Density is mass per unit volume• Density curve is NOT linear

– Normally, as you heat something, molecules move faster and then density decreases and vice versa

• Density and state (liquid, gas, solid)• Solid ice is less dense than liquid water

– Changes bond angle between oxygen and hydrogen atoms in water (from 104.5 to 109o) allowing lattice structure

Maximum density of water is about 4oC

Normal substances

Water3-D

Implications of density

• Ice floats

• Thermohaline circulation

• Salt increases density of water (fresh water should float on salty water)

• Water column stable when density stratified (less dense water rises and denser water sinks)

Salt and properties of water

• Salt water is 96.5% water and 3.5% dissolved solids and gases

• Salt changes water’s density• Salt changes water’s freezing behavior• Maximum density of seawater is about –

2oC• Why does ice that forms from seawater

still float? (salts don’t freeze!)• We’ll talk more about salinity next time.

Now let’s think about the ocean

• Distribution of temperature and heat with latitude

• Distribution of temperature and heat with depth

Thermostatic effects

• Ice and vapor

• Water and air movement– Because of the higher latent heat of

vaporization, atmosphere transfers more heat per unit mass than liquid water.

Annual changes in air temperature depending on air movement

Ocean breezes buffer heat by causing evaporation

Annual changes in sea ice in Antarctica

Over 18,000 km3 of polar ice thaws and refreezes each year

Ocean and air currents

• Ocean and air currents result from unequal solar heating (more on these later)

• Transport heats from equator to poles• Gulf stream is about 10oC warmer than water

moving south to replace it• Half the solar radiation entering the water results

in evaporation• Condensation is usually removed from where

evaporation is occurring• Air moves about 2/3 of heat and ocean currents

the other 1/3.

TropicalTemperatePolar

Warming Cycle: 1. March 2. May 3. June 4. August

Cooling Cycle: 1. August 2. September3. October4. November5. January

Temp. sea water

Temperature degree C

Land shows greater extremes in temperature near poles

Global climates

Water Vapor

Take home points

• Unique properties of water• Hydrogen bonding• Heat capacity• Latent heat versus sensible heat• High latent heat of fusion and vaporization• Relative density of states of water• Effect of salt on properties of water• Transport of heat by ocean and atmosphere