7Chemistry and Physics of Water I. Water a. Water has a very unique chemical structure i. Two Hydrogen atoms bonded to one Oxygen ii. Because oxygen pulls so strongly on the hydrogen atoms, it causes the hydrogens to be slightly positive, while the oxygen becomes slightly negative iii. Water is therefore a polar molecule, meaning it has positive end and a negative end. iv. Because opposite charges attract, the negative ends of one molecule are attracted to the positive ends of another forming hydrogen bonds. 1. Hydrogen bonds are not true chemical bonds and they are not very strong but they are responsible for many of water’s characteristics. b. Water exists in three states, all of which occur naturally on the earth. i. Liquid: Most familiar ii. Gas: Water Vapor iii. Solid: Ice c. The different states of water are a function of the amount of energy the molecules have. i. The more energy that water molecules have, the more they move. 1. The movement of molecules is registered as heat. The more the molecules move, the hotter the substance. 2. Heat is measured in terms of temperature. a. The greater the temperature, the more heat, the more the molecules move, the more space they occupy and therefore the greater the volume, while their mass remains the same. b. Because Density = mass/volume, an increase in volume with constant mass means a decrease in density. i. This is why hot air, or water, rises. 3. Hydrogen bonds tend to keep water molecules together and therefore require more energy in order to heat water. a. This is why water is such a good coolant. ii. If enough energy is added to water, the hydrogen bonds break and the water leaves the liquid state and becomes a gas. iii. If enough energy is taken from water, the hydrogen bonds get stronger and the water molecules form a crystal lattice. 1. Because of this crystal lattice, the space occupied by the water molecules (i.e. the volume) increases when water is solid, even though it has the same mass as when it was liquid. 2. Therefore, the density of the solid, ice, is less than the liquid, water. a. This is why ice floats. 3. Breaking those hydrogen bonds to change ice to liquid, takes a lot of energy.8a. The Latent Heat of Melting is the amount of energy required to melt a substance. i. Water has a higher latent heat of melting than any other commonly occurring substance b. Heat capacity is the hat that must be added to raise the temperature a substance by a given amount. i. Water has a very high heat capacity 1. Most marine organisms do not experience extreme changes in temperature. c. Latent heat of Evaporation is the energy it takes to change water from a liquid to a gas. i. Water has the highest latent heat of evaporation of any natural substance. 1. This makes water very good for cooling organisms. Think sweat. d. Water as a solvent i. Water can dissolve more things than any other natural substance, and is thus called the universal solvent 1. Water is a polar molecule, meaning it has a slightly negative end and a slightly positive end. 2. It therefore tends to attract to charged particles, called ions, a. Anions are negatively charged particles b. Cations are positively charged particles 3. Salts are substances that release ions a. Example: NaCl or Sodium Chloride i. Na+ and Cl- are released in water. 4. When a salt is mixed in water it dissolves, or dissociates, meaning that it is pulled apart into separate ions. a. Water is the solvent, the thing that does the dissolving b. The salt is the solute the thing that gets dissolved. II. Seawater a. Seawater is salty because of ions. i. Ions enter the ocean from a few different sources 1. The weathering, or erosion, of rocks releases ions, which are carried into the ocean via runoff a. Ex: Sodium, Potassium, Magnesium 2. Raining down from the atmosphere a. Ex: Sulfide, Chloride 3. Release from the earth’s interior through hydrothermal vents or cracks in the earth’s crust a. Sulfide, Chloride ii. If seawater is evaporated, the ions are left behind forming salts. iii. Though seawater contains almost every naturally occurring element, only six ions comprise over 98% of the solids dissolved in seawater. 1. Chloride, Sodium, Sulfate, Magnesium, Calcium, Potassium iv. The salinity is defined as the total amount of salt dissolved in seawater.91. Usually expressed as the grams of salt left behind if one were to evaporate 1000 grams of seawater, called parts per thousand or parts per mille. a. The average salinity of seawater is about 35 ppm 2. The rule of constant proportions states that the relative amounts of the various ions in seawater always stay the same. a. The overall concentration of ions can change, but the ratios of the different ions to is always the same. b. This implies that the ocean is very well mixed. 3. The salinity of seawater is a function of the balance between precipitation and evaporation. a. The greater the precipitation, the lower the salinity. i. The salinity off our coast is about 33ppm b. The greater the evaporation, the higher the salinity i. The salinity of the Red Sea is about 40ppm b. Temperature, Salinity and Density i. The density of a substance is equal to the mass divided by the volume, or how much stuff is in how big of a space. ii. The density of seawater is influenced by both temperature and salinity 1. Density is inversely correlated with temperature. a. As temperature increases, density decreases. As temperature decreases, density increases. b. The temperature of ocean water varies from -2° to +30° C i. The presence of salt in seawater allows it go below 0° C. ii. Seawater actually freezes at -2° C and its density increases all the way to -2° C. 2. Density is directly correlated with salinity. a. As salinity increases, density increases iii. Denser water lies below less dense water. 1. Because we want to know what is going on inside the ocean, not just at the surface, it is necessary to know how things in the ocean vary with depth. 2. It is therefore necessary to know the density profile of a given shaft of water, also known as the water column. a. A profile is a plot that shows temperature, salinity, or any other characteristic of seawater at various depths in the water column. c. Light in the Ocean i. Transparency is the ability for of light to pass through a substance. ii. Some wavelengths of light penetrate deeper than others 1.