Density (mass/volume)

We shall begin the semester with a study of one of the INTRINSIC properties of matter - density. Everything has a density. You know that the metal lead is denser than aluminum, and that aluminum is denser than water - since aluminum will sink in water. Because water is so common, water is usually the standard for density. Usually density is expressed in grams per cubic centimeter (g/cc; gm/cc; gm/ml). Another way of speaking about density is by indicating how dense something is relative to water. This is called "specific gravity," and is unit-less. If something has a specific gravity of 2, it means its density is twice that of water, and will sink in water. Thus will something that has a specific gravity of 0.5 float on water, or sink?

Today we shall determine the densities of several things, and then start asking ourselves "So what? What good is knowing that?" We shall progress from the dry subject of just knowing how to determine densities of various things - solids, liquids, and odd things, to using our findings to make an inquiry into the state of the world as well as to crack open the door onto biotechnology. (And hidden away until a later day will be Einstein's work.)

Along the way, the topics of precision, accuracy and lab safety will be discussed.

What to Do and How to Do It

FIRST - Determining the density of a solid

We shall learn how to determine the densities of solids - both those with regular shapes and others like a king's crown (hint: do you know the story about that Spartan guy Archimedes and his shouting "Eureka!" while running down the street naked?)
First, determine the weight of your assigned solid (a) coal, (b) aluminum, (c) lead sinker, or (d) glass marble. Second, using whatever means you can think of, including math formulae for cubes, cylinders, or spheres, determine the volumes of your assigned item.
Write your findings in your lab book and then on the blackboard for the other groups to copy.

SECOND - Determining the density of "mixed solids"

Who knows how to determine the density of a piece of wood? What would be the problem? For our homework, we shall learn how to determine the density of something like DNA or a bacterium. Pick up a copy of this Mary Lou experiment on your way out later.

THIRD - Developing a Bouyancy Law

Things more dense than water sink in water; conversely, things less dense than water float.

Does iron float or sink in water?

FOURTH - Determining the density of a liquid

What is the density of your assigned liquid? Let's see how good you are. How careful are you. To save time today, we shall divide the class into four sections, each assigned to one of these - (a) ice-cold fresh water, (b) cool seawater, (c) warm seawater, and room-temperature rubbing alcohol. Then we will learn a little trick and see if we can get them to float upon each other as we expect they should once we know their relative densities. Find your thermometer. Be mucho careful. You WILL learn how to clean up mercury if you break it! Ask the instructor about some ways to be careful with your thermometer so you won't have that clean-up learning experience!
Make room in your notebook for all four liquids. You will do yours and write it on the "scorecard" taped to the blackboard, and copy down the results of the other groups' data.
(a) Fill a beaker half full with some ice from the ice machine (seek and ye shall find!), then add water until the beak is about 3/4 full. (b) Find some seawater in a beaker in the refrigerator. Leave it there until you are ready to use it! (c) Heat up some room temperature seawater until it is 40°C. (d) Find the bottle of rubbing alcohol on the side shelf (remember: you are working with a flammable liquid, so keep it away from flames. Don't use a match for a light to see if the bottle is full!).
In the "weigh-room" you will find a balance with your group number listed overhead. Always use that balance! Carefully weigh a 100 ml graduated cylinder and write your value in your notebook. Next, add between 90 and 98 ml of your assigned type of water, and write down the volume to the nearest tenth of a ml. Think: How much does your fluid weigh PER ML?
(How could you do this more precisely? Why, for example, would wearing gloves help? What might be the results if you used the next available balance? What other things could you do to add precision? Why don't you merely fill the cylinder to 100.0 ml to make the math easy?)

FIFTH - avoiding the dragons

Clean up! If your space is not "neat and tidy" by the time you leave the lab, you will be docked 1 deportment point.

SIXTH - So what?

When you really do science - do research, you want to work on something that matters. You want to be able to respond intelligently to someone who hears what you have done and asks "So what? Who cares?" So let's take a look at the RAMIFICATIONS of one of the things you did.
So-, if global warming is happening, what are the consequences of the melting Arctic and Greenland ices?

Homework: Report #1

This is done as a group effort. All members in the group will share the grade given. Your title for Report #1 should be something like "Implications of Water Densities Influenced by Temperature and Salinity." You might be able to think of another wording. When is this due? By the end of September. Just remember, it is to be done in stages: rough draft, intermediate drafts, final draft. See guidelines for writing reports a la journal format.

There will be at least three reports done in the semester. Do not fret! Science writing nearly writes itself IF you understand what you did, and why you did it.


This is neither required nor for course credit, but may be the most important thing you do in college as it may give you a really big boost in your attaining a career in the sciences. For example, if you do the project mentioned here, there is a good possibility you will get accepted for presenting it at some national or international conference in chemistry or physics. That will look amazingly good on your resumé. It is about a force in nature that has been little studied UNTIL YOU STUDY IT. You will be among the very first in the history of the world to do it!