08 - So far we've studied what builds the ocean basins.now...

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Unformatted text preview: So far, we've studied what builds the ocean basins... ...now we will fill them! Our WATER planet... We live on a planet that is dominated by water. More than 70% of the Earth's surface is covered with Scientists estimate that the hydrosphere contains about this simple molecule. 1.36 billion cubic kilometers of this substance mostly in the form of a liquid (water) that occupies topographic depressions on the Earth. The second most common form of the water molecule on our planet is ice. If all our planet's ice melted, sealevel would rise by about 70 meters. Water is also essential for life. Water is the major constituent of almost all life forms. Most animals and plants contain more than 60% water by volume. Without water life would probably never have developed on our planet. Water is odd... odd... Many of the unique characteristics of the ocean can be ascribed to the nature of water itself What is so special about water? only substance that occurs in all 3 phases as solid, liquid, and gas on Earth's surface Earth' dissolves more substances in greater quantities than any other common liquid highest surface tension, heat conduction, and heat capacity of all common liquids 1 Two hydrogen atoms... Water is a POLAR MOLECULE Water is a polar chemical compound composed of two hydrogen atoms and one oxygen atom. share their electrons with one oxygen atom... to form a water molecule held together by covalent bonds... (very strong bonds) Electron (1 unit of charge) 2 105 H2O Nucleus (+1 unit of charge) which acts as if it has negative and positive ends. Stepped Art Fig. 6-1, p. 155 What holds water molecules together? H bonds! Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule. Same polar nature gives water ability to dissolve compounds (such as salt, gas) Its remarkable thermal properties result from the large number and relatively great strength of H bonds between molecules. Fig. 6-2, p. 156 H-bonds between water molecules creates important properties... properties... Cohesion the ability of water molecules to stick to each other, creating surface tension. Adhesion the tendency of water molecules to stick to other substances 2 One of the most important physical properties of water relates to its behavior as it absorbs or loses heat moderates day-night => winter-summer daywintervariations permits heat flow with global circulation (equator => polar region) powers storms, waves, currents To understand how these things work, we need to examine H2O thermal characteristics... characteristics... Heat vs Temperature Heat = energy produced by the random vibrations of atoms or molecules. Temperature = an object's object' response to input or removal of heat (degrees F, or C). Heat Capacity = a measure of the heat required to raise the temperature of 1g of a substance by 1C. 1 ... which is why on a hot summer day, the sand will burn your toes, but the water will feel great... Nice... Hot!! Table 6-1, p. 157 Water has a very high heat capacity; it resists changing temperature when heat is added or removed. H2O is the only substance that exists on Earth's surface in all 3 states... H2O unique density characteristics review: density = mass/volume pure water = 1.0 g/cm3 granite rock = 2.7 g/cm3 air 0.0012 g/cm3 MOST substances become more dense as For water, it's a little more complex... it' complex... they colder (ex: basalt at mid-ocean ridges) mid- 3 ...imagine you just put an ice cube tray in the freezer... freezer... Temperature vs Density for pure water initially, water is at room temp (A) as expected, density increases as temp drops (B) as temp drops, density increases slows... framework of H bonds slows... the freezer continues to remove heat and the water changes as the ice gets colder, it will get slightly more dense (E) Note that points C and D both represent 0C (32F) but different densities and thus different states of water. Ice floats because the density of ice is lower than the density of liquid water. expands, becoming less dense until it begins to freeze... (C) freeze... decreases abruptly (D) state from liquid to a solid- as this transition occurs, the density solid- The lattice structure of an ice crystal, showing its hexagonal arrangement at the molecular level. The space taken by 24 water molecules in the solid lattice could be occupied by 27 water molecules in liquid state, so water expands about 9% as the crystal forms. Because molecules of liquid water are packed less efficiently, ice is less dense than liquid water and floats. H2O is the ONLY substance 2 that behaves this way! Latent vs sensible heat For water to evaporate, heat must be added to water in the liquid state. After water reaches 100C 100 (sensible heat change), an input of 540 cal/gram is required to break the hydrogen bonds and allow evaporation. The amount of energy required to break the bonds is termed the latent heat of vaporization. Water has the highest latent heat of vaporization of any known substance. What's going on as water changes state? What' What's Fig. 6-6, p. 159 4 Energy needed for H2O phase change We must add 80 calories of heat energy to change a gram of ice to liquid water. latent heat of fusion ~only some H bonds are broken After the ice is melted, about 1 calorie of heat is needed to raise each gram of water by 1C. 1 sensible heat change But 540 calories must be added to each gram of water to vaporize it--to boil it away. it-- latent heat of vaporization ~all H bonds are broken The process is reversed for condensation and freezing. Fig. 6-8, p. 161 What does that mean for the ocean? Without water's unique thermal properties, temperatures on Earth's surface would change dramatically with only minor changes in atmospheric transparency or solar output. Water acts as a "global thermostat." thermostat." heat released as Antarctic water turns to ice ...One example... ...One example... About 20 million square kilometers (7.7 million square miles) of ocean surface thaws and refreezes in the Southern Hemisphere each year--an year-- area of ocean larger than South America! The autumn cooling of the atmosphere is delayed because heat energy is energy released as masses of water turn to ice. Heat is absorbed during ice melt in the spring. Seasonal extremes are moderated by the absorption and release of heat energy as ice thaws and refreezes. (Remember that the seasons are are reversed in the Southern Hemisphere.) The scale shows the percentage of percentage ocean surface completely covered by ice. scale shows the percentage of ocean surface completely covered by ice by Fig. 6-11, p. 164 But oceans are not "pure" water pure" "pure" 3.5% dissolved solids and gases changes thermal characteristics lowers latent heat solids interfere with formation of ice lattice, lowering freezing point ("antifreeze") (" antifreeze" salt excluded as seawater freezes (ice is pure H2O), making remaining seawater saltier (and denser)! Fig. 6-9, p. 163 5 Temp & Salinity affect Density Water density is greatly influenced by changes in temperature and and salinity. Water masses are usually layered by density, with the densest (coldest and saltiest) water on or near the ocean floor. Differences in the Differences density of water masses power deep ocean circulation. Complex relationship between temperature, salinity and density of seawater. Note that two samples of water can have the same density at different combinations of temperature and salinity. Fig. 6-17, p. 169 6 ...
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