WATERSection 1-3

Four emergent properties of water• ALL ARE A RESULT OF HYDROGEN

BONDING

H-bonds = Hydrogen atom of a molecule is covalently bonded to an electronegative atom of that molecule and bonded to an electronegative atom of ANOTHER MOLECULE; individually, are weak.

Polar covalent bond holds H atom to O atom WITHIN a water molecule. O shares e- unequally ~ highly electronegative.

2 Water Molecules with the Bonds

1.Cohesion

• Collectively, H-bonds hold water molecules together.

• Cohesion – due to constant breaking and reforming of H-bonds. Makes water a very structured liquid.

Related to cohesion:

a) Capillary Action = Helps plants fight gravity. As water leaves the top of the plant as vapor, the water molecules remaining at the top, pulls on the water molecules below as new H-bonds reform.

b)Adhesion = two different substances clinging together.

Example: Water adhering to the cell of the xylem also helps counter act gravitational pull of water downward in a plant.

c)Surface Tension = a property of the surface of liquid that allows it to resist “breaking.”

This characteristic allows animals to “walk on water” even though they have a greater density than water.

2.High specific heat and heat of vaporization

• Specific Heat = the amount of heat that must be lost or absorbed for 1g of a substance to change 1C

Specific heat of water = 1cal/g/CEven though heat is breaking H-bonds,

they reform quickly; makes water temp. difficult to change.

Relative to planet’s temp.? Body temp.?

•Heat of vaporization = the amount of heat a liquid must absorb for 1g of it to turn into vapor.

- As water evaporates, the remaining liquid cools because heat is released with the vapor = evaporative cooling

Helps plants stay cool on warm daysHelps body cool when sweating.

- Humidity = high concentration of water in the air.

Discomfort comes from water vapor in the air inhibiting evaporation of sweat.

3.Insulation of Bodies of Water by Floating Ice

Water expands when frozen; unlike other substances.

Since molecules barely move in solid state, few H-bonds break; forms strong lattice.

Makes solid form less dense than liquid state = floating

The floating water insulates the water below allowing life to exist under ice.

4.Solvent of Life

Background you should know:

Solution = homogenous mixture of two or more substances.

Aqueous solution = a solution where water is the solvent.

Solute = substance being dissolved; Solvent = doing the dissolving.

Water is a good solvent due to its polarity.

Weak H-bonds constantly breaking exposes the + and - ends of water a molecule.

The O (-) of water attracts cations of an ionic compound; H (+) of water attract the anions. The solute is ripped apart.

Hydration Shell

Water molecules surround the ion of the dissolved solute (with their respective oppositely charged ends) and shield them from the other ion of the solute it was bonded with not allowing the ionic bond to reform.

Ex: NaCl with hydration shell

o Both ionic and other polar molecules have an affinity for water = Hydrophilic

o Non-polar molecules do not have an affinity for water = Hydrophobic