The Chemistry of Seawater Unit 3
Water occurs naturally on earth in 3 phases: solid, liquid, or gas (liquid is most abundant) Water Phases
Basic Chemistry Review What is an atom? Smallest particles of matter What is an element? Substance made entirely from one type of atom What is a molecule? Formed when two or more atoms are joined chemically What is a compound? Formed when two or more different atoms are chemically joined; can only separate chemically; has different properties than original compounds What is a mixture? Substance made from two of more compounds, no chemical reaction occurs; can be physically separated back out; retains original properties
The Water Molecule
Water Characteristics Electrons between O and H are shared Covalent Bond, but it is also a Hydrogen Bond which means that the bond between the H and the O is STRONG (similar to an ionic bond in strength very hard to break) The partial positive (δ+) of 1 H on a molecule is attracted to the partial negative (δ-) of the O on another molecule acts like a magnet between the two molecules When water comes into contact with another molecule with opposite charges (like itself), it will separate the components from each other - this is why water is known as the Universal Solvent
Hydrogen Bonding
Water Characteristics Electrons are not shared equally (oxygen has greater electronegativity), so water is POLAR Electronegativity tendency of an atom to attract a bonding pair of electrons; basically how badly it wants to bond with another atom
Water Characteristics Water s polarity is responsible for Surface Tension - surface molecules cling together; create an ability to hold things up Cohesion - strong attraction between water molecules Adhesion - attraction of water molecules to other substances Capillary Action ability of a liquid (water) to move within a space Water spreading to a towel from a corner Xylem of plant carrying water to leaves (colored water to petals) Ink in a pen
Water Characteristics Heat Capacity: amount of energy required to raise the temperature of 1 gram by 1 degree Celsius Because of water s bond strength & the high number of bonds within water, more energy must be added to speed the movement of water molecules & break their bonds Water has one of the largest heat capacity s Water s is 1 cal/g/ C - Water can absorb or release LARGE amounts of heat while changing relatively little in temperature
Water Characteristics Density - how tightly packed molecules are Mass per unit of volume Pure water becomes denser as heat is removed, but less dense at freezing Why? Water forms bubbles as it freezes (expands 9%) that serve as air pockets that hold it up & allow it to float
Ice Crystal
Water Characteristics Viscosity - tendency of a substance to resist flow Viscosity increases when Temperature decreases Salinity increases Pressure decreases
Seawater Characteristics What it s made of: 96.5% Pure Water 3.5% Dissolved Solids Lowers freezing point; acts like antifreeze - the saltier the water, the lower the freezing point Ice crystals that form are free of solids/salt; most salts freefall to the bottom of the ocean & cause deep ocean currents Latent heat is lowered by 4%, so only 0.96 calories are needed to raise 1 g of water Evaporates more slowly because the salts/dissolved solids attract and hold water molecules more closely, so it takes even more energy to break the bonds
Seawater
Thermal Properties For water to change phases it requires the input or removal of energy. More heat - breaks H bonds More pressure - forms H bonds Night vs. Day: 1 C difference in ocean temperature 15-30 C difference in land Water helps to regulate climate changes Acts as a equalizer between polar and tropical waters due to its heat capacity Helps create ocean currents & atmospheric weather
Thermal Properties
Density Density - how tightly packed molecules of a substance are in a specified space Affected by salinity, pressure, and temperature 1L of seawater weighs 2-3% more than pure water due to the dissolved solids/salts Cold seawater is MORE dense than warm seawater Density INCREASES with: INCREASE in Salinity INCREASE in Pressure DECREASE in Temperature
Density Zones Ocean is divided into three density zones, which is known as Density Stratification: Surface (least dense) 2% of ocean water Temp & salinity are constant ~150m to 1000m Pycnocline (density increases with depth) 18% of ocean water Deep Zone (little change in density throughout this layer) 80% of ocean water
Density Stratification
Pycnocline Why does density increase? Due to a decrease in temperature AKA: Thermocline Not identical in all latitudes More pronounced in the tropics where sunlight penetrates further Polar waters tend to lack a thermocline
The Deep Zone AKA: Halocline Zone of rapid salinity increase with depth
Hydrologic Cycle Powered by solar radiation 85% of all water in the atmosphere originated in the ocean 80% falls back to Earth as precipitation to the ocean Water stays about 4100 years in the ocean before evaporating; only remains for 9 days in the atmosphere Oceans have remained about the same size (except with large climate changes)
Chemistry Terms to Know Solution - Homogeneous solution of two compounds Solute - Solid or gas that is being dissolved; usually lowest amt Solvent - Liquid/water that does the dissolving; usually larger amt Homogeneous - Solution with uniform properties throughout Heterogeneous - Mixture with different properties throughout
Chemistry Terms to Know Ions - Atom that has a charge; has gained or lost an electron Create an attraction through opposite charges; ionic Polarity reduces the electrostatic attraction of ionic bonds & causes the 2 ions to separate (dissociate) NaCl does not act as salt in seawater, but as dissociated ions Na+ & Cl- Diffusion - Ability of molecules to move from an area of high concentration to an area of low concentration What allows gases, atoms, ions, & molecules to move through and disperse in water
Salinity Salinity is the amount of dissolved substances in seawater; measured by hydrometer If all the seawater on Earth evaporated, 150ft (45m) layer of solids would be left on the ocean floor Dissolved Solids/Salts - 35 parts per thousand or 3.5% Six ions make up 99% of the dissolved solids in seawater: Chloride Sodium Sulfate Magnesium Calcium Potassium
1 kg of Seawater
Salinity Varies with latitude (ex. lower at equator due to rain) Red and Mediterranean Seas - hot, dry areas; high evaporation + less precipitation = more salinity Coastal areas (where rivers and streams enter) - lower salinities
Colligative Properties Properties of solutions, bind together; the more concentrated water is, the more important these properties become 1. Heat Capacity - HC decreases as salinity increases 2. Freezing/Boiling Point - FP decreases as salinity increases; BP increases as salinity increases 3. Evaporation - dissolved salts attract water molecules; seawater = slower evaporation 4. Osmotic Pressure pressure exerted on a biological membrane when the salinity of the environment is different than the cells in the organism; OP increases as salinity increases
Where do solids come from? Dissolved solids come from erosion, weathering, hydrothermal vents, and calcium carbonate shells
Constant Proportions Forchhammer s Principle states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant Percentages of each salt is the same in all seawater Forchhammer s Principle is also known as the Principle of Constant Proportions Confirmed through samples taken over a 10 year period on the HMS Challenger
Developed in 1950s Steady State Ocean Ions are added to the ocean at the same rate they are being removed Explains why the ocean is not growing saltier
Dissolved Gases Most gases in the air dissolve readily in the ocean Plants & animals require these gases to survive No marine animal has the ability to break down water molecules to obtain O2 directly No marine plant can manufacture enough CO2 to support its own metabolism Major gases found Nitrogen Oxygen Carbon Dioxide Proportions are NOT the same in the ocean as they are in the atmosphere Gas % in Atmosphere % in Ocean Nitrogen (N2) 78 48 Oxygen (O2) 21 36 Carbon Dioxide (CO2) Less than 1 15
Dissolved Gases Nitrogen Saturates the upper layers, meaning no additional N2 will be able to dissolve Required by organisms to build proteins & biochemical Most can t use N2 directly, require the use of specialized fixing bacteria to manufacture usable nitrates Oxygen There is 100x more gaseous O2 on Earth than there is dissolved in oceans 6ppm per liter of water Vital for any organism with gills Sources of O2 in ocean are from the photosynthetic activity of plants and plantlike organisms (phytoplankton) & diffusion from the atmosphere
Carbon Dioxide Dissolved Gases Lower on land due to the CO2 demand of plants Quickly used for marine photosynthesis Moves quickly from the atmosphere to the ocean, but more slowly from the ocean back to the atmosphere Known as the largest carbon sink on Earth; most is stored as limestone rock & calcium carbonate skeletons on the bottom of the seafloor
Acid/Base Balance Acid - release H+ ions into solution Base - combines with H+ ions in solution AKA: Alkaline Solution Seawater is slightly alkaline Increased acidity in the ocean decreases the amount of CaCO3 in the water Makes it unavailable for organisms to construct hard parts of their shells Accelerated ocean acidification and coral bleaching
Measure of H + ions in solution Water molecules separate into H + and OH - ions Scale of 0-14 0-6 acid 7 neutral 8-14 base Ocean water - ph of 8-9 ph
Night vs. Day Day: Algae photosynthesize using carbon dioxide Hydrogen ions removed from water ph rises (becomes basic) Night: No photosynthesis Respiration adds carbon dioxide Hydrogen ions increase ph lowers