Water Properties Lab

First, a bit of chemistry…

Water is a Polar Molecule (covalent)

Nonpolar Oxygen (even sharing of e-)

Uneven sharing of e-

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Hydrogen Bonds:Reason for Water’s Unique Properties

• Bonds between a hydrogen (H) atom and usually a nitrogen (N), oxygen  (O) or fluorine (F). 

• Individually, weak bonds, but can be strong in large numbers.

Okay, properties recap…

Cohesion on a Penny

• Cohesion - attraction of water to itself – Because of H-bonds – Makes a round droplet (surface tension)

Surface Tension - Paperclip

• Surface tension - an example of cohesion.• Reason for round droplets and “film” on the surface.• Water molecules at the surface do not have other

water molecules on all sides of them.– Making them bond more strongly to those around them at

the surface!

Can you break surface tension?

• You can…with soap!

Detergent on a Penny

Pepper, Water, and Soap

Soap is a Surfactant(Surface Active Agent)

• Surfactants act to reduce surface tension– Gets between water molecules at the surface

• Reduces cohesion of water molecules

Adhesion

• Adhesion is the attraction of water to a different substance.

• A drop of water is flatter on glass than on wax paper. – Why?

Strong Adhesion - Meniscus

• Water’s adhesion to glass is stronger than its cohesion to itself. – Creating the concave

meniscus!

• Mercury’s cohesion to itself is stronger than its adhesion to glass!– Creating the convex

meniscus!

Water Drop Shapes

• More round the drop, the stronger the H-bonds in the drop of water.

• Round drop because water has stronger cohesion to itself than adhesion to wax paper.

• Flatter droplet because water has stronger adhesion to glass than cohesion to itself.– Pulled down by glass

On wax paper On glass

Polar is attracted to Polar

• Water (Polar) has no adhesion to the wax (nonpolar) paper. It would roll right off.

• Water has more adhesion to a glass (polar) plate.

Capillary Action – Water Climbs Thin Tubes

• The small spaces in the paper towel adhere to the water, allowing it to climb up.

• The water climbs by cohering to itself, pulling water up as it climbs.

Likes Dissolves LikePolar dissolves polar; not nonpolar

• Water, being polar, can dissolve polar substances. – Ionic compounds, like salt, are very polar

• Vegetable oil is nonpolar – it cannot dissolve polar substances like salt

Oil is Hydrophobic(afraid of water)

• Oil (nonpolar) and water (polar) do not mix – H-bonding only occurs in polar substances

Oil is on topNon-polar

Water is on bottom

Polar

Dissolving

• A solvent surrounds the molecules of a solute to break it up and make a solution.

Oil Spills

a. dead sea life

• b. human life ills

• c. cost of clean-up

Oil spills – the key is dissolving or physically removing oil from water

Slick Sack

Absorbent padHand washing

Novel way to get rid of oil?

Getting rid of oil on water

• Detergent breaks up the oil into very small droplets.

Booms

Burn Off

Hi-pressure water

Amphipathic Molecules(bipolar)

• Molecules have (hydrophobic) nonpolar end and to attract oil structure.

• Also have a region that is polar (hydrophilic) to attract water to wash everything away.

Like detergent.