Download
1 / 34
350 likes | 451 Views
Chapter 4 Physical Properties of Water. Look For The Following Key Ideas In Chapter 4.
E N D
Look For The Following Key Ideas In Chapter 4 • Water is a polar chemical compound composed of two hydrogen atoms and one oxygen atom. Its remarkable thermal properties result from the large number and relatively great strength of hydrogen bonds between water molecules. • Heat and temperature are not the same thing. Heat is energy produced by the random vibration of atoms or molecules. Heat is a measure of how many molecules are vibrating and how rapidly they are vibrating. Temperature records only how rapidly the molecules of a substance are vibrating. Temperature is an object's response to an input (or removal) of heat. • 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.” • Water density is greatly influenced by changes in temperature 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 density of water masses power deep ocean circulation. • Light and sound are affected by the physical properties of water, with refraction and absorption effects playing important roles
Key Ideas Continued… • The polar nature of the water molecule produces some unexpected chemical properties. One of the most important is water's remarkable ability to dissolve more substances than any other natural solvent. • The most abundant ions dissolved in seawater are chloride, sodium, sulfate. • The quantity of dissolved inorganic solids in water is its salinity. The proportion of ions in seawater is not the same as the proportion in concentrated river water, which indicates that ongoing geological and chemical processes affect the ocean's salinity. • Though most solids and gases are soluble in water, the ocean is in chemical equilibrium, and neither the proportion nor the amount of most dissolved substances changes significantly through time. • Gases dissolve in water in proportions that vary with their physical properties. Nitrogen is the most abundant dissolved gas in seawater; oxygen is the second most abundant. Carbon dioxide is the most soluble gas, and one of many substances that affect the ocean's pH balance. • Seawater acts as a buffer to prevent broad swings of pH when acids or bases are introduced.
The Water Molecule A water molecule is composed of two hydrogen atoms and one oxygen atom.Water is a polar molecule, having a positive and a negative side. A molecule is a group of atoms held together by chemical bonds. Chemical bonds, the energy relationships between atoms that hold them together, are formed when electrons - tiny negatively charged particles found toward the outside of an atom - are shared between atoms or moved from one atom to another.
Electron (–1 unit of charge) 105° 2 – Nucleus (+1 unit of charge) Two hydrogen atoms... share their electrons with one oxygen atom... to form a water molecule held together by covalent bonds... which acts as if it has negative and positive ends. Stepped Art Fig. 6-1, p. 122
The Water Molecule What holds water molecules together? Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule. What are two important properties of water molecules? 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
Water Has Unusual Thermal Characteristics What is the difference between heat and temperature? Heat is energy produced by the random vibrations of atoms or molecules. Temperature is an object’s response to input or removal of heat. Heat Capacity is a measure of the heat required to raise the temperature of 1g of a substance by 1C. Water has a very high heat capacity, which means it resists changing temperature when heat is added or removed.
Water’s Temperature Affects Its Density The relationship of density to temperature for pure water.
Water’s Temperature Affects Its Density The three common states of matter – solid, liquid, and gas. A gas is a substance that can expand to fill any empty container. A liquid is a substance that flows freely in response to unbalanced forces but has a free upper surface in container it does not fill. A solid is a substance that resists changes of shape or volume. A solid can typically withstand stresses without yielding permanently. A solid usually breaks suddenly.
Water Becomes Less Dense When It Freezes 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 of the way water molecules are arranged during freezing, ice is less dense than liquid water – so it floats.
Water Removes Heat from Surfaces As It Evaporates For water to evaporate, heat must be added to water in the liquid state. After water reaches 100C, 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.
Water Removes Heat from Surfaces As It Evaporates The energy input or output associated with water in the three states of matter.
Surface Water Moderates Global Temperature San Francisco, California, and Norfolk, Virginia, are on nearly the same line of latitude, yet San Francisco is warmer in the winter and cooler in the summer than Norfolk. Part of the reason is that wind tends to flow from west to east at this latitude. Thus, air in San Francisco has moved over the ocean while air in Norfolk has approached over land. Water doesn’t warm as much as land in the summer nor cool as much in winter – a demonstration of thermal inertia.
Water Is a Powerful Solvent Salt in solution. When a salt such as NaCl is put in water, the positively charged hydrogen end of the polar water molecule is attracted to the negatively charged Cl- ion, and the negatively charged oxygen end is attracted to the positively charged Na+ ion. The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent.
Salinity Is a Measure of Seawater’s Total Dissolved Organic Solids A representation of the most abundant components of a kilogram of seawater at 34.4‰ salinity. Note that the specific ions are represented in grams per kilogram, equivalent to parts per thousand (‰).
The Components of Ocean Salinity Came from, and Have Been Modified by, Earth’s Crust Processes that regulate the major constituents in seawater. Ions are added to seawater by rivers running off crustal rocks, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Ions are removed from the ocean by chemical entrapment as water percolates through the mid-ocean ridge systems and seamounts, sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.
The Ratio of Dissolved Solids in the Ocean is Constant Forchhammer’s principle, also known as the principle of constant proportions states thatalthough the salinity of various samples of seawater may vary, the ratio of major salts is constant. How do scientists determine the salinity of seawater? 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, scientists can use the following formula to determine salinity: Salinity in ‰ = 1.80655 Chlorinity in ‰
The Ocean Is in Chemical Equilibrium Is the ocean becoming progressively saltier with age? No, the ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain constant. This concept is known as the “steady state ocean.“ Ions are being added to and removed from the ocean at the same rate. Residence time is the average length of time an element spends in the ocean. Residence time can be calculated by the equation: Residence Time = ___Amount of element in the ocean___ The rate at which the element is added to (or removed from) the ocean
Gases Dissolve in Seawater How concentrations of oxygen and carbon dioxide vary with depth. Oxygen is abundant near the surface because of the photosynthetic activity of marine plants. Oxygen concentration decreases below the sunlit layer because of the respiration of marine animals and bacteria, and because of the oxygen consumed by the decay of tiny dead organisms slowly sinking through the area.
Acid-Base Balance What are acids and bases? An acid is a substance that releases a hydrogen ion in solution. A base is a substance that combines with a hydrogen ion in solution. A solution containing a base is called an alkaline solution. Acidity or alkalinity is measured on the pH scale.
Acid-Base Balance The pH scale A solution at pH 7 is neutral; higher numbers represent alkaline bases, and lower numbers represent acids.
The Ocean Is Stratified by Density The complex relationship among the temperature, salinity and density of seawater. Note that two samples of water can have the same density at different combinations of temperature and salinity.
The Ocean Is Stratified by Density The ocean is divided into three density zones: • Surface zone - the upper layer of the ocean, containing the least dense water. The surface zone is only about 2% of total ocean volume. • Pycnocline - a zone in which density increases with depth, containing about 18% of all ocean water • Deep zone – contains about 80% of all ocean water. There is little change in density throughout this layer.
The Ocean Is Stratified into Three Density Zones by Temperature and Salinity Density stratification in the ocean. (a) In most of the ocean, a surface zone (or mixed layer) or relatively warm, low-density water overlies a layer called the pycnocline. Density increases rapidly with depth in the pycnocline. Below the pycnocline lies the deep zone of cold, dense water – about 80% of total ocean volume. (b) The rapid density increase in the pycnocline is mainly due to a decrease in temperature with depth in this area – the thermocline. (c) In some regions, especially in shallow water near rivers, a pycnocline may develop in which the density increase with depth is due to vertical variations in salinity. In this case, the pycnocline is a halocline.
The Ocean Is Stratified into Three Density Zones by Temperature and Salinity Typical temperature profiles at polar, tropical, and middle (temperate) latitudes. Note that polar waters lack a thermocline.
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. 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 occurswhen light’s electromagnetic energy is converted to heat in the molecules of seawater.
Water Transmits Blue Light More Efficiently Than Red Only a thin film of seawater is illuminated by the sun. Except for light generated by living organisms, most of the ocean lies in complete blackness. (a) The table shows the percentage of light absorbed in the uppermost meter of the ocean and the depths at which only 1% of the light of each wavelength remains. (b) The bars show the depths of penetration of 1% of the light of each wavelength (as in the last column of the table)
Sound Travels Much Farther Than Light in the Ocean The relationship between water depth and sound velocity.
Refraction Can Bend the Paths of Light and Sound through Water An analogy for refraction. The ranks of marchers represent light or sound waves; the pavement and sand represent different media. (a) If the marchers head off the pavement at an angle other than 90°, their path will bend (refract) as they hit the sand because some will be walking more slowly than others. (b) If they march straight off the pavement, the ranks will slow down but not bend as they hit the sand.
Refraction Causes Sofar Layers and Shadow Zones The sofar layer, in which sound waves travel at minimum speed. Sound transmission is particularly efficient - that is, sounds can be heard for great distances - because refraction tends to keep sound waves within the layer
Sonar Systems Use Sound to Detect Underwater Objects The principle of active sonar. Pulses of high-frequency sound are radiated from the sonar array of the sending vessel. Some of the energy of this ping reflects from the submerged submarine and returns to the sending vessel. The echo is analyzed to plot the position of the submarine.
Sonar Systems Use Sound to Detect Underwater Objects Side-scan sonar in action. Sound pulses leave the submerged towed array.
Chapter 4 Summary Water, a chemical compound composed of two hydrogen atoms and one oxygen atom, is abundant on and within Earth. The polar nature of the water molecule, and the hydrogen bonds that form between water molecules, result in some unexpected physical and chemical properties. The thermal properties of water are responsible for the mild physical conditions at Earth’s surface. Liquid water is remarkably resistant to temperature change with the addition or removal of heat; and ice, with its large latent heat of fusion and low density, melts and refreezes over large areas of the ocean to absorb or release heat with no change in temperature. These thermostatic effects, combined with the mass movement of water and water vapor, prevent large swings in Earth’s surface temperature. The physical characteristics of the world ocean are largely determined by the physical properties of seawater. These properties include water’s heat capacity, density, salinity, and its ability to transmit light and sound. Changes in temperature and salinity greatly influence water density. Ocean water is usually layered by density, with the densest water on or near the bottom. Sound and light in the sea are affected by the physical properties of water, with refraction and absorption effects playing important roles. Water also has the remarkable ability to dissolve more substances than any other natural solvent. Though most solids and gases are water-soluble, the ocean is in chemical equilibrium and neither the proportion nor amount of most dissolved substances changes significantly through time. Most of the properties of seawater differ from those of pure water because of the substances dissolved in the seawater.
