Lec 4: Properties of Seawater

• Water


  Unique chemical properties that make it essential for life on Earth
• Water on Earth: Why do we care?
• Temperature extremes found at places far from the ocean


  Areas close to the ocean rarely experience severe temperature variations
• Almost every other liquid contracts as it approaches its freezing point, but water actually expands as it freezes
• If water's properties followed the pattern of similar chemical compounds, ice would sink and cause all temperate-zone lakes, ponds, rivers, and even oceans to eventually freeze solid from the bottom up
• Instead, a floating skin of ice forms at water's surface and acts as an insulating cover to protect the organisms that live in the liquid water below
• Unique properties of water

  • Thermal
  • Dissolving
• Seawater is mostly water molecules but has dissolved substances
• Ocean salinity, temperature, and density vary with depth
• Presence of water on Earth makes life possible
• Organisms are mostly water - our blood is 83% water!
• The composition and properties of the ocean vary greatly in space and time
• Water is one of the main distinguishing features of our planet (along with life!)
• Atom


  Basic building block of all matter, consist of a nucleus, protons, and electrons
• Molecule


  Two or more atoms held together by mutually shared electrons
• Water Molecule


  Consists of one oxygen atom covalently bonded to two hydrogen atoms, separated by an angle
• Covalent bonds are due to sharing of electrons between oxygen and each hydrogen atom
• Covalent bonds are relatively strong so lots of energy is needed to break them
• Polarity


  Water molecules have a slight negative charge on one side and a positive charge on the other
• Several water molecules orient themselves so the positively charged hydrogen area of one molecule interacts with the negatively charged oxygen end of an adjacent water molecule, forming a hydrogen bond
• Hydrogen Bonding


  • Weaker than covalent bonds but still strong enough to contribute to cohesion, high water surface tension, high solubility of chemical compounds in water, unusual density of water, and unusual thermal properties of water
• Water: The Universal Solvent

  Water molecules are attracted to other polar chemical compounds and can reduce the attraction between ions of opposite charges in other substances
• Ionic Bond


  A chemical bond formed as a result of the electrical attraction between charged particles
• When solid NaCl (table salt) is placed in water, the electrostatic force of attraction (ionic bonding) between the sodium and chloride ions is reduced by 80 times
• This makes it much easier for the sodium ions and chloride ions to separate, with the positively charged sodium ions becoming attracted to the negative ends of the water molecules and the negatively charged chloride ions becoming attracted to the positive ends of the water molecules, dissolving the salt in water
• Hydration


  The process by which water molecules completely surround ions
• Water influences Earth's heat budget and moderates temperature on Earth's surface, allowing equatorial oceans to not boil and polar oceans to not freeze solid
• Heat energy exchanges in the evaporation-condensation cycle make life possible on Earth
• Water has 3 states of matter at the Earth's surface: solid, liquid, and gas
• Van der Waals forces

  Weak interactions when molecules are close together
• Energy must be added for molecules to move fast enough to overcome the attractive forces and change state
• Heat


  Energy moving from a high temperature system to a lower temperature system
• Temperature

  Average kinetic energy of molecules in a substance
• Calorie/Joule


  The amount of heat (energy) needed to raise the temperature of 1 gram of water by 1°C
• Heat Capacity


  The amount of heat needed to change the temperature of 1 g of a substance by 1°C
• Water has a high heat capacity and can take in or lose much heat without changing temperature
• Specific Heat


  Heat capacity per unit mass
• Water has high heat capacity because it takes more energy to increase the kinetic energy of hydrogen-bonded water molecules than for substances with weaker intermolecular interactions
• Sensible Heat

  Heat added or removed that raises or lowers the temperature of an object but does not change its phase
• Latent Heat


  Heat added or removed that does not change the temperature of a substance but leads to a change of phase