MASC 333 Lecture 1 2024 PDF: Unusual Properties of Water

Lecture Objectives To understand the unusual physical and chemical characteristics of seawater and their significance to physical and biological...

Lecture Objectives To understand the unusual physical and chemical characteristics of seawater and their significance to physical and biological MASC333: CHEMICAL OCEANOGRAPHY phenomena. To know the important differences in chemical composition of saltwater and freshwater. LECTURE 1 PHYSICAL AND CHEMICAL CHARACTERISTICS OF SEAWATER 1 2 The Nature and Properties of Water Physical States of Water Water is a compound composed of: – two hydrogen atoms Water exists on the earth in three physical states: – one oxygen atom. – Gas (water vapor) – molecules have high kinetic energies and separated. The water molecule has: – Liquid – molecules have less kinetic energies and link into loose – a bent structure groups. – a small positive charge on each hydrogen atom – Solid (Ice) - molecules are rigidly linked in open crystalline – a small negative charge on the oxygen atom structure. The water molecule is therefore dipolar. 3 4 Changes of State of Water Phase changes of water Pure liquid water is converted to pure ice at 0oC at standard atmospheric pressure and pure ice is converted to water at at the same temp and pressure. Pure water boils at 100oC at standard atmospheric pressure and is converted to steam (vapour) at the same temp and pressure. Steam condenses to water at 100oC at standard atmospheric pressure. 5 6 1  Hydrogen bonding Structure and hydrogen bonding in water The dipolar nature of water allows for hydrogen bonding. © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Water Molecules Hydrogen bonds are weaker than normal covalent bonds but they are more flexible and can easily break and reform. Generally, a hydrogen bond is formed whenever hydrogen in one molecule is bonded to a small electronegative atom such as N, O and F in another molecule. The molecules may be the same e.g., H2O and H2O or different e.g., H2O and NH3. Hydrogen bonding confers unusual properties on water. 7 8 Unusual Properties of Water and their Ecological Boiling points water and comparable compounds Significance Water, when compared with compounds of similar composition, displays most uncommon properties due to hydrogen bonding. 1. Water has a high boiling point and can exist as a liquid under normal environmental conditions relative to comparable compounds e.g., H2S, NH3, HF, CH4 and C2H6 which have low boiling points and exist as gases under normal environmental conditions. Significance: water can exist as a liquid over a broad range of temperatures. 9 10 Unusual Properties of Water and their Ecological Density variation with temperature Significance cont’d 2. Water expands when it freezes (solidifies) so that solid water (ice) has a lower density (0.92 g/cm3) than liquid water (1.00 g/cm3) and hence, ice floats on water. Significance: – Since ice floats on top of water, it prevents further heat loss from the water below. 3. The maximum density of pure water occurs at 4oC. Significance: – It allows water overturn and prevents deep waters from Temperature of maximum density decreases with increasing salinity. freezing to the bottom; only shallow waters may freeze Temperature differences in water layers result in thermal stratification and completely. Thus, in deeper waters, there will always be water prevents the mixing. beneath the ice in which fish and other organisms can live. This Besides temperature, water density is affected by dissolved salts; dissolved salts is important for aquatic life in polar regions. increase the density of water. 11 12 2  Unusual Properties of Water and their Ecological Significance Cont’d Unusual Properties of Water and their Ecological Significance cont’d 5. Water has a very high surface tension The surface of water behaves as a stretched elastic membrane. 4. Water has very high heat capacity, latent heat of fusion and latent heat of vapourization, compared to other substances. Significance: – The water surface serves as a microhabitat for: Significance: neuston (organisms that float on the top of water). – A large amount of heat is required to change appreciably the epineuston are those that live on the water surface e.g. temperature of water. This prevents sudden large changes of water strider temperature in large bodies of water, thereby protecting aquatic organisms from the shock of abrupt temperature variations. hyponeuston are those that live below the water surface but attached to the water surface. e.g. mosquito larvae – Certain organisms such as insects may be trapped by surface tension and die as a result. 13 14 6. Water has high buoyancy and so is able to support floating Surface Tension organisms Formation of water droplets Water strider 15 16 Unusual Properties of Water and their Ecological Significance Unusual Properties of Water and their Ecological Significance Cont’d Cont’d 8. Water is a universal solvent because of its charge polarity, and is 7. Water has high viscosity (a measure of a liquid’s resistance to able to dissolve almost every substances to some extent. flow) as a result of internal friction. Significance: Significance: – Dissolution of gases (e.g. O2 and CO2), minerals (e.g. Fe and Zn) and nutirents – It helps small organisms (flagellates) to stop moving when (e.g. N and P) in water is crucial to the welfare of aquatic organisms. resting; slows the rate of sinking of plankton; retards the The solubility of a gas in water is affected by the temperature of the movement of organisms (essence of streamlined bodies of water and pressure. some organisms). The solubility of gases, unlike solutes, increases with decreasing temperature but increases with pressure. – Viscosity is primarily affected by temperature and is much Thus thermal pollution can have a devastating effect on aquatic higher at lower temperatures organisms. Water is easily contaminated/polluted 17 18 3  Important differences in Seawater and Freshwater Important differences in Seawater and Freshwater Physical and Chemical Characteristics Physical and Chemical Characteristics cont’d The presence of salts in seawater results in: The presence of salts in seawater results in: – a higher density than that of freshwater – a high ionic strength or higher conductivity than freshwater – a higher boiling point than that of freshwater – a higher sound absorption than freshwater – a lower melting/freezing point than that of freshwater (i.e. sea water can remain liquid at temperatures below 0°C) – Seawater is slightly alkaline with a pH between 7.5 and 8.5 (average value for the world’s oceans over all depths is approx. – a lower temperature of maximum density 7.8). The pH of seawater remains relatively constant because of the buffering action of carbon dioxide in water. 19 20 Salinity of Seawater Density of Seawater The salinity of seawater is a measure of its salt content. It is expressed as o/oo (parts per thousand) or % or psu (practical salinity The density (ρ) of sea water depends on temperature T, salinity S units). and pressure p. Average ocean salinity is 35.0 ppt (3.5%). Average seawater density is 1.025 gcm-3 The refractometer (a handheld optical instrument) can be used to measure salinity (inexact). Water is densest near 4oC at low salinity The salinometer measures salinity more accurately, with Density increases with an increase in salinity and decrease in measurement based on the electrical conductivity of the water temperature, except temperatures below the density maximum. sample compared to that of a standard at 15oC. The CTD (conductivity, temperature, depth) can measure conductivity in situ at the same time temperature and pressure are measured to give a profile of salinity and temperature versus depth (pressure) in the ocean. Variations in seawater salinity may be due to addition of river water or rainwater (causes salinity to decrease) and excess evaporation or formation of ice (causes salinity to increase). 21 22 Temperature Variations in the Oceans Density Variations in the Oceans The temperature of the ocean is highly variable with warmest water Variations in salinity and temperature cause variations in the density occurring in surface tropical ocean and coldest water occurring at the of sea water. poles. As seawater freezes, it excludes salts and makes the remaining The ocean is heated from the surface downward by solar radiation, seawater even more saline. but at depth most of the ocean is very cold. Sea ice is less dense than surrounding seawater and floats. Heating in mid-latitudes (only during summer) and the tropics (always) causes mixing of surface ocean to depths of several hundred As evaporation occurs, the remaining seawater also becomes more meters. saline. The region below the mixed layer where a rapid change in As a water mass changes density, it sinks or rises until it joins other temperature occurs with depth is called the thermocline. water with similar density. The average ocean temperature is 3.8°C; even at the equator the Physical oceanographers use density differences among different average temperature is as low as 4.9°C. water masses to track movements of water in the ocean. 23 24 4  Density Variations in the Oceans Cont’d Temperature, salinity and density stratification in the ocean Density variation in the ocean can lead to some kind of stratification in the ocean. Solar heat only directly penetrates a short distance into the ocean. The surface water is generally warmer and less dense than deep water, which gives most of the ocean a stable density structure. The temperature and density are often relatively constant in the surface zone or mixed layer due to wind and wave action but begin to change more abruptly in deeper water. The layer where density changes rapidly with depth is called the pycnocline. Below the pycnocline lies the deep zone of cold, dense water where temperature and density are relatively constant. The pycnocline may coincide with the thermocline or the halocline (the region where salinity changes rapidly with depth). 26 25 26 Water Masses and Water Types Some Important Water Masses in the Oceans A water mass is a volume of ocean water with a characteristic North Atlantic Deep Water (NADW) - a cold, high-salinity water mass range of temperature (T) and salinity (S). left after formation of ice in Arctic which sinks and flows southward in the Atlantic. Formation of water masses is a fundamental part of ocean circulation. Antarctic Bottom Water (AABW) - a cold, very high- salinity water mass formed in the Antarctic which sinks and flows northward into Water masses, after their formation, move to balance changes the Atlantic, Pacific, and Indian Oceans. It is the densest and most resulting in a global circulation pattern called thermohaline voluminous deep water in the ocean. circulation. Mediterranean Intermediate Water (MIW) - a warm, high-salinity A water type has a single value of T and S. water mass formed in the Mediterranean Sea which flows into the Atlantic. 27 28 27 28 Light in the Ocean Most of the ocean lies in complete darkness. Sunlight reaches only depths of about 100 m in clear open water. This lighted layer is referred to as the photic zone. The depth of light penetration is decreased by particles in the water, including any algal cells that are growing there. THANK YOU FOR YOUR ATTENTION Coastal waters with a high sediment content, or waters in which an algal bloom is occurring, have much shallower light penetration than clear open-ocean water. Water absorbs different wavelengths of light differently. By a depth of 10 m, mostly blue-green light remains, explaining the bluish color of underwater photos taken in natural light. 29 29 30 5