Cohesion, Surface Tension, and Adhesion

Content Objectives

• SWBAT describe how hydrogen bonding allows water molecules to maintain strong cohesion, adhesion, and surface tension.

Van der Waals Forces

• The sum of all attractive forces between molecules.

Hydrogen Bonding

• Attraction between hydrogen atoms who are bonded to strongly electronegative atoms like O, F, or Cl and another polar molecule.

• Water is attracted to itself due to hydrogen bonding between H---O

Surface Tension

• Refers to how strong molecules hold onto each other against gravity or other forces.

• Water has strong surface tension that allows it to form almost completely spherical droplets.

Surface Tension

Water Striders Take Advantage of Surface Tension

Why Do Ice Cubes Float?• Hydrogen bonding keeps the water molecules well

spaced as they form their crystalline structure.

• Thus, the density of the solid is actually less than the liquid. This is rare among compounds.

Hydrogen Bonding of DNA and Proteins• Hydrogen bonding is what keeps a DNA

molecule in its double helix.

• It is also what allows proteins to bend into the critical shapes.

• If you heat a protein, it will break the hydrogen bonds and it will fall apart.

• If that happens you die…

Cohesion• Attraction between like molecules

• Mercury is more attracted to itself than the glass, so it does not “wet” it.

• Mercury will roll over surfaces without attaching to them.

Adhesion

• Attraction between dissimilar molecules

• Water has a tendency to stick to many other substances, like these spider webs or pine needles.

Adhesion

• These attractions can be due hydrogen bonding, polar attraction, or other dipole-dipole interactions.

• Geckos use adhesion to climb upside down and on almost any surface.

Meniscus

• Water is attracted to glass and has a tendency to climb up its surface due to adhesion.

• Glass is polar, because it is primarily SiO2

Wetting Vs. Non-wetting

• Gallium does not have much cohesion. It is much more attracted to other surfaces than itself.

• Here it has wet the surface of the glass and it is hard to get off again.

Wetting vs. Non-wetting

• A is a liquid with little adhesion to the surface (thus non-wetting)

• B is a liquid with moderate adhesion

• C is a liquid with significant adhesion and thus is wetting the surface.

Capillary Action• Is the ability of a liquid to climb against gravity

on the surface of something else.

• The higher the level of adhesion between the two and cohesion within the liquid, the more rapidly this can occur.

Capillary Action of Water in Concrete

Capillaries

• Capillaries are tiny blood vessels that deliver blood to individual cells.

• At their tiny size, the pressure of the heart is not enough to force the blood through them.

• The blood moves through the blood vessels through capillary action.

Using Surface Tension to Walk on Water

• Aquatic insects have surface hairs that do not allow water to wet it. Then, they use the surface tension of water to crawl across it.

Definitions• Cohesion allows the water droplets to stay together.

• Surface Tension makes them bead up.

• Adhesion keeps them attached to the leaves instead of running off.

What to Do?

• What do you do if you get butter or vegetable oil on your hands?

Soap

• Water won’t get grease off your hands, so you use soap.

What IS soap anyways?

• Lard (animal fat)

• Vegetable oil

• Caustic NaOH or KOH pellets or lye water

Soap• How does soap do it?

• Soap molecules contains a polar head and a non-polar tail.

• The oils dissolve into the non-polar tail and then the polar head is attracted to and pulled away by the running water.

What About Tough Grease?• Soap often is not strong enough to get off motor

or bike grease

• Use vegetable oil first!

• Non-polar grease will dissolve into the oil.

• Then, soap will remove the vegetable oil.

Surfactant• A substance that disrupts and lowers the surface

tension of a liquid.

• Detergents contain a polar and non-polar end. When added to water, they attach to water, but do not allow waters to attach to each other, thus lowering the surface tension.

How Bubbles Form

• Bubbles form when surfactants surround water molecules.

Content Objectives

• SWBAT describe how hydrogen bonding allows water molecules to maintain strong cohesion, adhesion, and surface tension.