Oceanography-Chapter 2.pdf

Oceanography Chapter 2. Properties of water The States of Water Water has three states. Below freezing water is a solid (ice or snowflakes) Between freezing and boiling water is a liquid Above its boiling point water is a gas. There are words scientists use to describe water changing from one state...

Oceanography Chapter 2. Properties of water The States of Water Water has three states. Below freezing water is a solid (ice or snowflakes) Between freezing and boiling water is a liquid Above its boiling point water is a gas. There are words scientists use to describe water changing from one state to another. solid to liquid - melting liquid to gas - evaporating gas to liquid – condensation solid to gas - sublimation. gas directly to solid - Frost formation Most liquids contract (get smaller) when they get colder. Water is different. Water contracts until it reaches 4 C then it expands until it is solid. Solid water is less dense that liquid water because of this. If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the bottom up! Water is found on Earth in all three forms. This is because Earth is a very special planet with just the right range of temperatures and air pressures. Earth is said to be at the triple point for water. Adhesion and Cohesion Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion. The oxygen end of water has a negative charge and the hydrogen end has a positive charge. The hydrogens of one water molecule are attracted to the oxygen from other water molecules. This attractive force is what gives water its cohesive and adhesive properties. Surface Tension Refers to the cohesion of water molecules at the surface of a body of water. Molecules at the surface of liquid water have fewer neighbors and, as a result, have a greater attraction to the few water molecules that are nearby. This enhanced attraction is called surface tension. It makes the surface of the liquid slightly more difficult to break through than the interior. When a small object that would normally sink in water is placed carefully on the surface, it can remain suspended on the surface due to surface tension. The Basilisk lizard makes use of the high surface tension of water to accomplish the incredible feat of walking on water's surface. The Basilisk can't actually walk on water; rather, it runs on water, moving its feet before they break through the surface. Capillary Action Surface tension is related to the cohesive properties of water. Capillary action however, is related to the adhesive properties of water. You can see capillary action 'in action' by placing a straw into a glass of water. The water 'climbs' up the straw. What is happening is that the water molecules are attracted to the straw molecules. When one water molecule moves closer to the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw. Capillary action is limited by gravity and the size of the straw. The thinner the straw or tube the higher up capillary action will pull the water Universal solvent It is often referred to as a universal solvent because many substances dissolve in it. These unique properties of water result from the ways in which individual H2O molecules interact with each other. The partial charge that develops across the water molecule helps make it an excellent solvent. Water dissolves many substances by surrounding charged particles and "pulling" them into solution. Sodium chloride contains Na+ and Cl- ions. For example, common table salt, sodium chloride, is an ionic substance that contains alternating sodium and chlorine ions. When table salt is added to water, the partial charges on the water molecule are attracted to the Na+ and Cl- ions. The water molecules work their way into the crystal structure and between the individual ions, surrounding them and slowly dissolving the salt. The water molecules will actually line up differently depending on which ions are being pulled into solution. The negative oxygen ends of water molecules will surround the positive sodium ions; the positive hydrogen ends will surround the negative chlorine ions. In a similar fashion, any substance that carries a net electrical charge, including both ionic compounds and polar covalent molecules (those that have a dipole), can dissolve in water. This idea also explains why some substances do not dissolve in water. Oil, for example, is a nonpolar molecule. Because there is no net electrical charge across an oil molecule, it is not attracted to water molecules and therefore does not dissolve in water. Hydrogen bonding H O H The electrostatic attraction between the ð+ hydrogen and the ðoxygen in adjacent molecules is called hydrogen bonding. High specific heat capacity of water Specific heat capacity, is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval. More heat energy is required to increase the temperature of a substance with high specific heat capacity than one with low specific heat capacity. Water has a high specific heat capacity because it is a relatively light molecule (18 grams per mole). The specific heats of molecules are all about the same on a per-molecule basis, especially at higher temperatures. When specific heats are measured on a per-gram basis, lighter molecules have higher specific heats. For example, the specific heat of hydrogen (H2), which has a molecular weight of 2 grams per mole, is much higher than that of water. Liquid water has a higher specific heat than most other liquids (such as alcohols) because its molecules are lighter. Density of seawater On average, seawater in the world's oceans has a salinity of about 3.5%, or 35 parts per thousand (also expressed 35‰ or 35 ppt) Every 1 kg of seawater has approximately 35 grams of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl-). The density of surface seawater ranges from about 1020 to 1029 kg/m-3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg/m-3 or higher. Seawater pH is limited to the range 7.5 to 8.4. The average density of seawater at the surface of the ocean is 1.025 g/ml; seawater is denser than fresh water (which reaches a maximum density of 1.000 g/ml at a temperature of 4°C) because of the added weight of the salts and electrostriction. Electrostriction is a property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. The freezing point of sea water decreases with increasing salinity and is about -2°C (28.4°F) at 35 parts per thousand. Scientific theories behind the origins of sea salt started with Sir Edmond Halley in 1715, who proposed that salt and other minerals were carried into the sea by rivers, having been leached out of the ground by rainfall runoff. Upon reaching the ocean, these salts would be retained and concentrated as the process of evaporation removed the water. Halley noted that of the small number of lakes in the world without ocean outlets (such as the Dead Sea and the Caspian Sea), most have high salt content. Halley termed this process “continental weathering". Thermal expansion The tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its constituent particles move around more vigorously and by doing so generally maintain a greater average separation. Materials that contract with an increase in temperature are very uncommon; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature. Global warming is likely to cause a rise in sea level for a number of reasons, one of which is the thermal expansion of water. Table shows the volume that 1 gram of water occupies as temperature varies. Data corrected for buoyancy and for the thermal expansion of the container. Temperature (°C) Volume (mL) 17.0 1.0022 18.0 1.0024 19.0 1.0026 20.0 1.0028 21.0 1.0030 22.0 1.0033 23.0 1.0035 24.0 1.0037 25.0 1.0040 26.0 1.0043 The thermal coefficient of expansion of water is 0.00021 per 1° Celsius. SEAWATER CHEMICAL COMPOSITION Seawater is more enriched in dissolved ions of all types than fresh water. However, the ratios of various solutes differ dramatically. Although seawater is about 2.8 times more enriched with bicarbonate than river water based on molarity, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water; bicarbonate ions constitute 48% of river water solutes, but only 0.41% of all seawater ions. Differences like these are due to the varying residence times of seawater solutes; sodium and chlorine have very long residence times, while calcium (vital for carbonate formation) tends to precipitate out much more quickly. Total Molar Composition of Seawater (Salinity = 35) Component H2O ClNa+ Mg2+ SO42Ca2+ K+ CT BrBT Sr2+ F- Concentration (mol/kg) 53.6 0.546 0.469 0.0528 0.0282 0.0103 0.0102 0.00206 0.000844 0.000416 0.000091 0.000068 Seawater composition (by mass) (salinity = 35) Element Oxygen Hydrogen Chlorine Sodium Magnesium Chloride (Cl): Sulphate (SO4): Calcium (Ca): Percent 85.84 10.82 1.94 1.08 0.1292 Element Sulfur Calcium Potassium Bromine Carbon Percent 0.091 0.04 0.04 0.0067 0.0028 55.04 wt% 7.68 wt% 1.16 wt.% Sodium (Na): Magnesium (Mg): Potassium (K): 30.61wt% 3.69 wt% 1.10 wt.% There are >70 elements dissolved in seawater but only 6 make up >99% of all the dissolved salts; all occur as ions - electrically charged atoms or groups of atoms Detailed composition of seawater Note: ppm = parts per million = mg/L = 0.001g/kg Dissolved gases in seawater The gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amount of gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperature increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount of gas dissolved.