PROPERTIES OF WATERThe 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 MoleculeWater’s bent shape and ability to

Hydrogen Bond give it special propertiesHigh Surface TensionLow Vapor PressureHigh Specific HeatHigh Heat of VaporizationHigh boiling point

Water molecules are attracted by Dipole InteractionsWhat are dipole interactions?

High Surface TensionSurface Tension: the inward pull or

force that tends to minimize the surface area of a liquid

Water forms round dropletsThe greater the molecular force, the

higher the surface tension, the more spherical the droplet

Water acts like it has a skinHydrogen 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 TensionGlass has polar molecules and has the

ability to hydrogen bondThis attracts water molecules and causes

them to be drawn up a cylinder wallPlastics 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 PressureVapor 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 CapacityWater has a high heat capacity (also

termed specific heat)It requires a large amount of energy/heat

to be absorbed to change it’s temperatureAllows 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 CondensationWater 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

IceAs 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 expandThis 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