PHYS 1090: Fundamentals of the Physical Universe

Unit 12: Intermolecular Forces
Due November 9, 2011
20 points

Answer one, and only one, of the following questions.

These questions are for you to combine different ideas from the unit and apply them to situations not directly addressed in class or in the textbook.  Think about the situation, and about the physics that applies to it.  See me for help if you are stuck.

1. Surface Tension.

Even though paper clips are denser than water, you managed to get a paper clip to float on a water surface.

• Explain how this occurred.  What forces or interactions kept the paper clip from sinking?
• Have you ever observed other objects float on water this way?  What were they?
2. Clean-up.

Explain why a mixture of detergent in water can clean things.

3. Solvents and solutes.

In class you contacted a variety of solids with several different liquids.  Sometimes the solids dissolved in the liquids, and sometimes they did not.  Give an example of a solid we did not test in class, a liquid that dissolves it, and a liquid that does not dissolve it.

4. Types of mixtures.

The mixtures of sugar, corn starch, and milk powder with water were used as examples of a solution, suspension, and colloid of a solid in a liquid.  Complete the chart below, identifying the types of mixtures and the natures of the components.  (The first three have been completed as examples—you don’t need to do anything further with them.)  Also explain your reasoning for each classification you make.

Mixture	Type	Nature of Components
sugar in water	solution	solid in liquid
corn starch in water	suspension	solid in liquid
milk powder in water	colloid	solid in liquid
dust storm
cloud or fog
humid air
spray from a waterfall
silty water
smoke
coffee

5. Rock candy.

Have you ever tried to make rock candy?  The recipe is similar to what you did in the “Sugar Crystals” activity, except that you start with white sugar instead of brown sugar.  Sugar is dissolved in hot water, the solution is cooled, and crystals slowly grow from the syrup.  (Often a surface is placed into the syrup to provide a place for the crystals to grow.)

If you try to make colored rock candy by adding food coloring to the water, the crystals will be only faintly colored, if at all.  Why is this?  Explain this as you would to a disappointed second-grader who was hoping to make brightly-colored rock candy.

Take your choice of these tasks.  Do not submit answers to more than one of them. Just choose one, and submit it.  Your answer should thoroughly address all points of the question.  An adequate answer will be several paragraphs long.

If you submit any part of this assignment as an electronic document, upload it (or them) to Sakai using the “Assignments” tool.  That way I will be able to find them easily and I will not tend to lose them.  Or, you may enter your text directly into the “Assignments”tool.  Hard copy submissions are also allowed.

I entreat you: If you submit your answer in hard copy, do not turn in paper that has ragged edges!  If you write your work in a spiral-bound notebook, trim the edge before submitting it.  Papers with ragged edges will be returned unmarked.