Seawater Chemistry: Key Ideas Water is a polar molecule with the remarkable ability to dissolve more substances than any other natural solvent. Salinity is the measure of dissolved inorganic solids in water. The most abundant ions dissolved in seawater are chloride, sodium, sulfate, and magnesium. The ocean is in steady state (approx. equilibrium). Water density is greatly affected by temperature and salinity Light and sound travel differently in water than they do in air. Oxygen and carbon dioxide are the most important dissolved gases. 1 The Water Molecule Water is a polar molecule with a positive and a negative side. 2 1 Water Molecule Asymmetry of a water molecule and distribution of electrons result in a dipole structure with the oxygen end of the molecule negatively charged and the hydrogen end of the molecule positively charged. 3 The Water Molecule Dipole structure of water molecule produces an electrostatic bond (hydrogen bond) between water molecules. Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule. 4 2 Figure 4.1 5 The Dissolving Power of Water As solid sodium chloride dissolves, the positive and negative ions are attracted to the positive and negative ends of the polar water molecules. 6 3 Formation of Hydrated Ions Water dissolves salts by surrounding the atoms in the salt crystal and neutralizing the ionic bond holding the atoms together. 7 Important Property of Water: Heat Capacity Amount of heat to raise T of 1 g by 1oC Water has high heat capacity - 1 calorie Rocks and minerals have low HC ~ 0.2 cal. Significance: For a given addition of heat Water T increases less than rock T That’s why sand at the beach feels hotter than water 8 4 Water And Heat Note the high heat capacity of water. 9 10 5 Energy absorbed and released during phase changes of water 11 The Three States of Matter The energy input or output associated with water in the three states of matter. 12 6 Evaporating Water For water to evaporate, heat must be added to water in the liquid state. After water reaches 100°C, an input of 540 cal/g is required to break the hydrogen bonds and allow evaporation. The amount of energy required to break the bonds is the latent heat of vaporization. Water has the highest latent heat of vaporization of any known substance. 13 The Unique Water Molecule Unique properties of water include: Higher melting and boiling point than other hydrogen compounds. High heat capacity Greater solvent power than any other substance. 14 7 15 Water Molecule Ice floats in water because all of the molecules in ice are held in hexagons and the center of the hexagon is open space, making ice 8% less dense than water. 16 8 The arrangement of water molecules in an ice crystal 17 Water Molecule Water reaches its maximum density at 3.98oC. Below this temperature water molecules become structured and density decreases. Above this temperature water molecules are increasingly energetic, move farther apart, and density decreases. Hydrogen bonding is responsible for many of the unique properties of water because energy is required to break the hydrogen bonds and separate the water molecules. 18 9 Water Temperature And Density The relationship of density to temperature for pure water. 19 Salinity Addition of salt modifies the properties of water Pure water freezes at 0oC. Adding salt increasingly lowers the freezing point because salt ions interfere with the formation of the hexagonal structure of ice. Boiling point increases because ions retard evaporation. Density of water increases as salinity increases. 20 10 Seawater as a Solvent Salinity is the total amount of salts dissolved in the water. It is measured in parts of salt per thousand parts of salt water and is expressed as ppt (parts per thousand) or abbreviated as o/oo. Average salinity of the ocean is about 35 o/oo. 21 22 11 Table 4.4 23 Density of Seawater: Function of Temperature, Salinity and Pressure. Density increases as temperature decreases and as salinity and pressure increase. Pressure increases regularly with depth, but temperature and salinity are more variable. 24 12 Refraction, Light, and Sound Sound and light both travel in waves. Refraction is the bending of waves, which occurs when waves travel from one medium to another. The refractive index is a ratio that expresses how much light is refracted from one medium to another. 25 26 13 Light in the Ocean Sunlight does not travel well in the ocean. Scattering and absorption weaken light. Scattering occurs when light is bounced between air and water molecules, dust and other objects. Absorption occurs when light’s electromagnetic energy is converted to heat in the molecules of seawater. 27 Light Penetration Amount of light entering the ocean depends upon the height of the sun above the horizon and the smoothness of the sea surface. 65% of light entering the ocean is absorbed within the first meter and converted into heat. Only 1% of light entering the ocean reaches 100 m. Water displays selective absorption of light with long wavelengths absorbed first and short wavelengths absorbed last. In the open ocean, blue light penetrates the deepest. 28 14 Figure 4.8 29 30 15 Sound in Seawater The speed of sound in water increases as salinity, temperature and pressure increase, but in the ocean speed of sound is mainly a function of temperature and pressure. Near the surface, as pressure increases the speed of sound increases despite the gradual decrease in temperature. At mid-depths, the speed of sound decreases because of the decrease in temperature and only slight increase in pressure. 31 Sound in the Ocean In the deepest part the speed of sound gradually increases because pressure increases, but temperature declines only slightly. SOFAR Channel is located where sound speed is at a minimum. Refraction of sound waves within the channel prevents dispersion of the sound energy Sound waves travel for 1000s of kilometers within the channel. 32 16 Sound in the Ocean The relationship between water depth and sound velocity. 33 34 17 35 36 18 Sea Ice As ice forms, the salt remains in solution, increasing salinity and further lowering the freezing point of the water. Depending upon how quickly the ice freezes, some salt may be trapped within the ice mass, but is gradually released. 37 Salinity Salinity is the total quantity of dissolved inorganic solids in water. As salinity increases: Heat capacity of water decreases Freezing point decreases Evaporation slows (i.e., Boiling Pt. increases) Osmotic pressure increases 38 19 Definition of Salinity Salinity is the total mass, expressed in grams, of all substances dissolved in one kilogram of sea water when all carbonate has been converted to oxide, all bromine and iodine has been replaced by chlorine and all organic compounds have been oxidized at a temperature of 480oC. 39 Dissolved Salts in Sea Water + - = +2 +2 99% of salt: Na , Cl , SO4 , Mg , Ca and K+ Sodium and chloride comprise about 86% of the salt Major chemical constituents display little variation over time - conservative property of sea water 40 20 Salinity and Chlorinity Chlorinity is the amount of halogens (chlorinity, bromine, iodine and fluorine) in seawater and is expressed as grams/kg or o/oo. Salinity - 1.8065 times chlorinity Salinity determined from the electrical conductivity of seawater produced by the dissolved salts - salinometer 41 The Components of Salinity The most abundant components of a kilogram of seawater 42 21 43 44 22 Principle of Constant Proportions Principle of constant proportion states that the absolute amount of salt in sea water varies, but the relative proportions of the ions is constant. • Because of this principle, it is necessary to test for only one salt ion, usually chloride, to determine the total amount of salt present. 45 Determining Salinity Salinity can be determined by measuring the chlorinity of the sample. Since the chlorinity is easy to measure, and the principle of constant proportions applies to all seawater, the following formula is used to determine salinity: Salinity (in parts per thousand) = 1.80655 × chlorinity (in ppt) 46 23 Sources of Sea Salt Weathering of continents Volcanic eruptions Hydrothermal vents 47 Removal of Salts Salt sinks include the following: Precipitation of evaporites - evaporation removes only water molecules Wind-blown spray - minute droplets of saltwater carried inland Adsorption onto clays and authigenic minerals Formation of shells by organisms 48 24 Circulation of Seawater Through Oceanic Ridges 49 25.