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lec 9 role of water.docx

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University of Lethbridge
BIOL 1010
Igor Kovalchuk

Role of water Water is the basis of life - Life on earth probably evolved in water - Living cells are 70-95% H O 2 - Water covers about ¾ of the earth - In nature, water exists in all three physical states of matter- solid, liquid and gas. - It can be quite correctly argued that life exists on earth because of the abundant liquid water - Other plants have water, but it is either gas (Venus) or water (Mars) - Recent studies of Mars reveal the presence sometime in the past of running fluid, possibly water. Polarity of water - Water is a polar molecule. Its polar bonds and asymmetrical shape give water molecules opposite charges on opposite sides - 4 valance orbitals of O point to corners of a tetrahedron - Two corners are orbitals with unshared pairs of electrons and weak negative charge - Two corners are occupied by H atoms which are in polar covalent bonds with O. - Oxygen is also electronegative, that shared electrons spend more time around O causing a weak positive charge near H’s. - Hydrogen bonding orders water into a higher level of structural organization. - The polar molecules of water are held together by hydrogen bonds. - Positively charged H of one molecule is attracted to the negatively charged O of another molecule - Each water molecule can form a maximum of 4 hydrogen bonds with neighboring water molecules. - Water has extraordinary properties that emerge as a consequence of its polarity and hydrogen- bonding. Come of these properties are that water: o Has cohesive behavior o Resists changes in temperature o Has a high heat of vaporization and cools surfaces as it evaporates o Expands when it freezes o Is a versatile solvent - Water is a polar molecule (having both a positive and negative side) Hydrogen bonds - Consequently, water has a great connectivity of individual molecules, which is caused by the individually weak hydrogen bonds that can be quite strong when taken by the billons Cohesion of water molecule - Collectively, the hydrogen bonds hold the substance together- a phenomena called cohesion. - Cohesion contributes to the transport of water against gravity in plants. Adhesion of water to the walls of the vessels helps counter the downward pull of gravity. - Water has a great surface tension. At interface between water and air, hydrogen is bounded to one another and to the water below. Water moderates temperatures on Earth - Water absorbs heat from the warmer air and releases the stored hear to the cooler air. - Everything that moves has kinetic energy. The faster a molecule moves, the greater E . k - Heat is a measure of the total quantity of E due to molecular motion in a body of matter. k - Temperature measures the intensity of heat due to average E of the mklecules. - When the 2 objects are brought together, molecules in the cooler object speed up at the expense of the E ok the warmer one. - A calorie is the amount of heat energy required for raising the temperature of 1g of water by 1 degree Celsius. - Conversely, one calorie is the amount of heat given by 1 g of water releases when it cools down by one degree Celsius. Water’s high specific heat - Water has a high specific heat, which means it resists temperature changes when it absorbs or releases heat. - The specific heat of a substance is defined as the amount go heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1 degree Celsius. - The specific heat of water is 1 calorie per gram per degree C, abbreviated as 1 cal/g/C - It is unusually high when compared to other substances, (0.6 for alcohol). - As a result of hydrogen bonding water molecules, it takes a relatively large heat loss or gain for each 1 degree change in temperature - Hydrogen bonds must absorb heat to break, and they release heat when they form - Much of the absorbed heat energy is used to disrupt hydrogen bond s before water molecules can move faster (increase temperature) - A large body of water can act as a heat sink, absorbing heat from sunlight during the day and the summer (while warming only a few degrees) and releasing heat when the night and winter as the water gradually cools. - As a result o Water, which covers ¾ of the planet keeps temperature fluctuations within a range suitable for life o Coastal areas have milder climates then inland o The marine environment has a relatively stable temperature Evaporative cooling - The transformation from the liquid to gas is called vaporization o Molecules with enough E tokovercome the mutual attraction of molecules in a liquid; can escape into the air - Heat of vaporization is the quantity of heat a liquid must absorb for 1g of it to be converted to the gaseous state o For water molecules to evaporate, hydrogen bonds must be broke which requires energy - Water has a high heat of vaporization- for each gram of water 580 cal are needed. - With liquid evaporation the surface of the
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