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Properties of Water

Properties of Water. Objectives. 3.1.4 – Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation. 3.1.5 – Outline the thermal, cohesive, and solvent properties of water.

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Properties of Water

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  1. Properties of Water

  2. Objectives • 3.1.4 – Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation. • 3.1.5 – Outline the thermal, cohesive, and solvent properties of water. • 3.1.6 – Explain the relationship between the pro- perties of water and its uses in living organisms as a coolant, medium of meta- bolic reactions, and transport medium.

  3. Effects of water’s polarity Polarity (the effect of having distinct ends, or poles) gives water opposite charges on opposite ends of the molecule. • Weak hydrogen bonds form between H of one molecule and O of another, to a max- imum of 4 bonds. • Hydrogen bonds are th the easiest to break - just use heat.

  4. Emergent properties of water Water has cohesive behavior (holds together). Cohesion means it sticks to itself. • due to hydrogen bonding among molecules. • aids transport up stems against gravity. Water also has adhesion (it clings to other substances, such as paper towels) due to H-bonds. Water has great surface tension due to H-bonds: it is difficult to break the surface, so it forms beads. Some creatures can walk across it.

  5. Emergent properties of water A Jesus lizard (or basilisk) using water’s property of surface tension to walk on water and escape predators.

  6. Emergent properties of water Cohesive and adhesive properties of water: • Adhesion to the side of the glass capillary tube pulls water up. • Cohesion of water molecules to them- selves pulls more water molecules along, but gravity is also pulling them down, so a meniscus forms.

  7. Emergent properties of water Water resists changes in temperature. • Has a highspecific heat: amount of heat that must be absorbed to change its temperature by 1oC. • To raise 1 g H2O 1oC requires 1 calorie (1 cal/g/oC) • Specific heat of ethanol = 0.6 cal/g/oC. • Specific heat of iron = 0.1 cal/g/oC (Fe heats fast) • Water heats slowly as energy first goes to break the hydrogen bonds, then it heats the molecule. • Result: Bodies of water (& organisms) can absorb a lot of solar energy without heating dangerously high (H-bonds break before molecules heat).

  8. Emergent properties of water A high heat of vaporization cools surfaces as H2O vaporizes. A lot of energy is needed to break the hydrogen bonds. • A lot of heat in the ocean is lost as water evaporates. I It goes into the gas molecule. This energy can fuel hurricanes. • Perspiration absorbs much heat, cools animals and plants. (This is called evaporative cooling.)

  9. Emergent properties of water Water expands as it freezes; greatest density at 4oC • As water cools, more H-bonds form, making a crystal lattice. These H-bonds keep the H2O molecules separ- ated farther apart than the molecules are in liquid water. With more empty space, ice is less dense than liquid water, and it floats.

  10. Emergent properties of water Water expands as it freezes. • If ice sank, frozen bodies of water would take longer to thaw, making life harder: a shorter growing season. • Surface ice in- sulates water beneath, keep- ing it a bit warmer.

  11. Emergent properties of water Water is a versatile solvent (the solvent of life). • Positive, & negative ends of water pull anions and cations from ionically bonded materials.

  12. Emergent properties of water Water is a versatile solvent (the solvent of life). • Water’s charged ends also coat polar sugars, proteins, & other molecules, pulling them into solution.

  13. Emergent properties of water Water is a versatile solvent (the solvent of life). • Hydrophilic: (Gr.- water loving) Substances with an affinity for water - they either dissolve in or absorb water. Ex. sugar, salt, cotton, cellulose. • Hydrophobic: (Gr.- water fearing) Substances that don’t dissolve in or absorb water. Ex. things with many non- polar bonds (esp. C-H), such as vegetable oil.

  14. Emergent properties of water Dissociation of water molecules • In pure water, only 1 molecule of H2O dissociates in every 554,000,000; so the concentration of each ion (H+ and OH-) is 10-7 Molar, at 25oC. • pH = the negative log of concentration = 7.

  15. Emergent properties of water The pH scale: • In any solution, the product of [H+][OH-] = 10-14M2. • In a neutral solution, where [H+] = [OH-], 10-7M x 10-7 M = 10-14 • Scale goes from 0 to 14 pH = -log10 [H+]

  16. Water in living systems Because of water’s properties (due to hydrogen bonding), in living creatures water is useful in many ways. • As a coolant: homeostasis - evaporative cooling • As a solvent it provide a medium in which chemicals can come together and react: metabolism. • Due to adhesion and cohesion and solvent capa- bilities, it transports chemicals throughout the body, which is ~65% water (in blood, which is ~80% water, in urine, etc.).

  17. Water in living systems

  18. Water in living systems Diet water: only one H+ instead of two.

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