Lesson 3: Cohesion, adhesion & surface tension

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

2. How many drops of water fit on a penny? Part A • Rinse a penny in tap water and dry completely • Place the penny on a paper towel • 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. • Record the number of drops for the trial in the table.

3. Part B • Cleanthepenny. Rinsethepenny in tap water and dry completely. Try to remove as much residue as possible – WITHOUT soap. • With tweezers, dip the penny into the testing liquid. Let extra liquid drip off the penny into the beaker. • 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.

4. Review Ionic BondingIonic bonds form when electrons are transferred from one atom to another. Example: salt NaCl Covalent BondingCovalent bonds result when two atoms share electrons so each atom has octet of electrons in the outer shell. Ex. Chlorine Cl2

5. Polar and Nonpolar bonds * ONLY covalent bonds can be polar/ nonpolar Polar Polarcovalent 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

6. 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

7. 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.

8. 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.

9. 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.