Biology                           October 1, 2008  Answers to worksheet 3-1 

1.  A polar compound is one with an uneven distribution of positive and negative charges.

2. A hydrogen bond is an attraction between a hydrogen atom in one molecule and a region of negative charge on another molecule.

3. Cohesion is an attractive force between particles of the same kind.

4. Adhesion is an attractive force between unlike substances.

 Multiple choice.

     1.  b     2.   a     3.  c     4.  d     5.  d                      

Short Answer.

1.  Water’s polar nature allows it to dissolve other polar substances as well as ionic compounds.

2.   Any molecule that has a region of negative charge can be involved in hydrogen bonding.

3.   Adhesion allows water to stick to a dry surface.

4.   Because of its hydrogen bonds, water must gain or lose a relatively large amount of energy for its temperature to change.  It therefore tends to resist temperature changes.

5.  Cohesive forces cause the water molecules in each drop to move as close as possible to one another, making the drop round.

6.   It suggests that the substances are most often polar molecules or ionic compounds.

 Structures and Functions

 Drawings should show two water molecules below and one above the central water molecule.  The molecules below should have their H atoms facing away from the central molecule, and the molecule above should have one of its H atoms pointing toward the central molecule.  Dashed lines should be drawn between each H atom in the central molecule and the O atom in each of the lower water molecules, and between the O atom in the central molecule and the nearer H atom in the upper water molecule.