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Properties of Water. The Universal Solvent. The Water Molecule. Simple tri-atomic molecule, H 2 O Each O-H bond is highly polar due to the high electronegativity of Oxygen Bond angle = 105° H 2 O has a bent shape, meaning the O-H bonds due not cancel = Polar Molecule. The Water Molecule.
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Properties of Water The Universal Solvent
The Water Molecule • Simple tri-atomic molecule, H2O • Each O-H bond is highly polar due to the high electronegativity of Oxygen • Bond angle = 105° • H2O has a bent shape, meaning the O-H bonds due not cancel = Polar Molecule
The Water Molecule • δ is the Greek symbol for Delta • δ+ means there is a partial positive charge • δ – means there is a partial negative charge • Since there is a large difference in the electronegativity of the Oxygen and Hydrogen atoms, the element is Polar
The Water Molecule • Water’s bent shape and ability to Hydrogen Bond give it special properties • High Surface Tension • Low Vapor Pressure • High Specific Heat • High Heat of Vaporization • High boiling point • Water molecules are attracted by Dipole Interactions • What are dipole interactions?
High Surface Tension • Surface Tension: the inward pull or force that tends to minimize the surface area of a liquid • Water forms round droplets • The greater the molecular force, the higher the surface tension, the more spherical the droplet • Water acts like it has a skin • Hydrogen bonding creates a large network of bonds between adjacent molecules
Surface Tension • Water molecules hydrogen bond to one another due to electrostatic attraction • Molecules in the middle are pulled in various direction to bond with adjacent molecules • Molecules at the surface are pulled downward and to the side since water is not attracted to air
Surface Tension • Glass has polar molecules and has the ability to hydrogen bond • This attracts water molecules and causes them to be drawn up a cylinder wall • Plastics are non-wetting; have no attraction to water molecules
Can we decrease Surface Tension? • Use a Surfactant – Surface Active Agent • Also called a “wetting agent”, like detergent or soap • How does it work? • It interferes with Hydrogen Bonding
Low Vapor Pressure • Vapor Pressure is caused by molecules that escape the surface of a liquid and enter gaseous state • Hydrogen Bonding holds water molecules close so there is a low tendency to escape • Why is this important?
Specific Heat Capacity • Water has a high heat capacity (also termed specific heat) • It requires a large amount of energy/heat to be absorbed to change it’s temperature • Allows moderation of daily temperatures • Warm days: water absorbs heat from warmer environment decreasing air temperature • Cool Nights: transfer of heat from water to cooler environment increasing air temperature
Evaporation and Condensation • Water absorbs large amounts of heat as it evaporates or vaporizes • Water gives off large amounts of heat as it condenses • Water = 18.02 g/mol, Liquid rather than a gas, an important exception, BP is 100°C • Extensive Hydrogen bonding requires more heat to disrupt bonds
Ice • As most liquids cool, they tend to contract (get smaller) • Water does not act in this manner, Why? • It will decrease in volume until it reaches about 4°C, then it starts to expand • This occurs as water molecules slow down arranging themselves in honeycomb crystal shapes • This expansion causes the volume of the ice to increase while the mass remains the same • Ice has a 10% lower density than water
