2. Explain why water molecules are polar. Oxygen has a stronger affinity (electronegativity) for electrons than hydrogen, therefore, the electrons spend more time hovering around the oxygen nucleus. This gives the oxygen a slightly negative charg), the hydrogen a slightly positive charge. This phenomenon creates a polar molecule, a molecule that has positive and negative charges on it.
Mini Lab #1: Penny Lab
In this mini-lab, you will count test the number of drops of water, oil, and alcohol that will fit on the head of a penny. Make your predictions here and on the white board in the front of the class room:
# drops of oil
# drops of alcohol
# drops of water
Place your 3 pennies on the tray and or paper towel
Using the liquids in the plastic dropper bottles, carefully count the number of drops of each substance you can place on a penny before the penny overflows. Don’t cross-contaminate the liquids!
Record results on white-board.
Record class-wide results on the table below.
Data Table #1: Penny drops
# of drops of oil
# of drops of alcohol
# of drops of water
Class Prediction (Average)
Class Results (Average)
On a piece of plastic or on the tray, place one drop of water next to one drop of alcohol and one drop of oil, about 1 inch apart (do not mix!), Draw what you see.
Draw a molecular drawing of the water drop (more than one molecule - refer to your drawing in #1). Name and indicate the type of bond that holds the molecules together.
Explain your results in terms of adhesion, hydrogen, and cohesion. What phenomenon is going on here? Explain why the 3 substances have different drop shapes. (Refer to your book and background reading).
Water: water molecules adhere to table, they cohere to each other, Hydrogen bonds are formed between molecules, top has more H-bonds which gives it surface tension. Creates a dome shape.
Oil: viscous material (thick) creates a dome shape
Alcohol: Neither viscous nor has H-bonds and therefore does not create a dome shape.
Mini Lab #2: Sink or Swim
Fill a Petri dish with water and sprinkle the surface with black pepper.
Why does the pepper float?
Surface tension of water (more H-bonds at surface because there are less neighboring water molecules) keeps pepper afloat. Pepper IS NOT less dense than water.
Dip the end the end of the toothpick into dish soap then touch the surface of the water. Watch closely this happens fast!
What happened? Why?
Pepper spread out to sides and some sank. Soap is an amphipathic material which reacts with water and breaks the H-bonds therefore breaking the surface tension
Explain how detergents act in molecular terms, considering the cohesion among water molecules and the effects of amphipathic molecules on cohesion.
The charged side of an amphipathic molecule can interact with polar water molecules –it disrupts the H-bonds which destroys the cohesive nature of water.
Explain some of the consequences of oil spills in the sea. What effects do they have on sea life and bird life, and what methods would be used to clean up oil spills?
A small amount of oil will form a monolayer of molecules on the surface of the water, and the monolayer can be seen as a sheen. This is what often occurs in oil spills, making a slick on the surface of the water that is difficult to clean up. The heavier oil in spills forms a coating on any animals that pass through it, often killing them because the coating restricts their movements and oxygen exchange processes, and its toxins are absorbed through their skins.
Mini Lab #3: How a 200 foot tree gets water to its top
Locate the piece of celery that has been soaking in red water for a couple of hours.
Describe what you see.
Red dye up to the top of the celery
Explain what has happened using biological terms from this lab. (How does water go against gravity?)
Water adheres to sides of vessels, water…