Lesson 3: Cohesion, adhesion & surface tension Objective: 1.Describe cohesion, adhesion and surface tension 2.Understand how and why plants utilize them.

How many drops of water fit on a penny? Part A 1.Rinse a penny in tap water and dry completely 2.Place the penny on a paper towel 3.Use a dropper to place DROPS of WATER on the penny (one at a time) until ANY amount of water runs over the edge of the penny. 4.Record the number of drops for the trial in the table. Trial #1Trial #2Trial #3Trial #4Average

Part B 1.Clean the penny. Rinse the penny in tap water and dry completely. Try to remove as much residue as possible – WITHOUT soap. 2.With tweezers, dip the penny into the testing liquid. Let extra liquid drip off the penny into the beaker. 3.Place the penny on a dry paper towel. Place drops of WATER on the penny (one at a time) until ANY amount of water runs over the edge.

Review Ionic Bonding Ionic bonds form when electrons are transferred from one atom to another. Example: salt NaCl Covalent Bonding Covalent bonds result when two atoms share electrons so each atom has octet of electrons in the outer shell. Ex. Chlorine Cl 2

Polar and Nonpolar bonds * ONLY covalent bonds can be polar/ nonpolar Polar Polar covalent bonds: the sharing of electrons is unequal. In a water molecule, each hydrogen atom has a partial positive charge and the oxygen atom has a partial negative charges. Nonpolar Nonpolar covalent bonds: sharing of electrons is equal. They do not interact with polar molecules. Example Oil and ethane

What will happen when two water molecules bump into each other? When the oxygen atoms of 2 different water molecules come together, they repel. When the hydrogen atoms of 2 different water molecules come together, they repel. When an oxygen atom and a hydrogen atom from two different water molecules come together, they attract

Adhesion and Cohesion The attraction between two like molecules is cohesion. The attraction between two unlike molecules is adhesion. Adhesion and cohesion are intermolecular forces between two molecules.

Surface tension Adhesion between water and glass leads to capillary rise in a glass tube. Water and glass have a smaller attractive force compared to water’s attraction of water. The cohesive force of water molecules is responsible for the phenomenon of surface tension. Water molecules have a strong mutual attraction for one another, enabling them to hold together strongly.

Capillary action in plants Capillary action in plants is a good example of adhesion and cohesion. The inner surface of the xylem (the cell wall of a plant) contains positive and negative charges to which water forms hydrogen bonds. This is called adhesion. As water creeps up the sides of the xylem (adhesion) the water molecules in the middle connect to other water molecules because of cohesion. The water moves up as the water molecules at the top of the xylem enter the leaves and evaporate (move out of the stomata in the leaf). When a water molecule leaves the leaf, the molecule behind it moves up causing a general movement of the water up the tree.