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Name: Cole
Status: student
Grade: 6-8
Location: LA
Country: USA
Date: Fall 2009

Question:
Why does changing the shape of an object have no effect on the density of that object?



Replies:
Hi Cole,

I have got a couple of answers that might help your thinking on density. First of all, density is a characteristic physical property of a substance. That means this characteristic physical property can be used to identify one substance from another. For example, boiling points and melting points of substances are usually quite different from other substances. Heating a substance and noting when it melted or boiled would give you a clue as to what the substance is. Density is the same kind of characteristic physical property. You could tell gold from brass if you could obtain a mass, volume and do a little division. Density does not change for the substance unless you somehow altar the substance on a molecular level, or have a sample size near the molecular size.

Now, you could take a lump of lead and hit it with a hammer until it became a thin sheet. The density of lead is still 11.3 g/cm3. Flattening the lead has not changed the mass or the volume of the lead. Unless of course some of the lead broke off while you were hitting it. However, the decrease in mass would coupled with a decrease in volume. The ratio between these 2 factors remain the same and the density remains unchanged. Think about a block of aluminum and a piece of aluminum foil. The density of Al is still 2.7 g/cm3 whether its flat or a block. However you can do so much more with that piece of foil. You could form a little boat out of the foil and even get it to float on water. Hey that is crazy, the density of Al is 2.7 g/cm3 and the density of water is 1.0 g/cm3, this should sink! But you and I know it does not. Making a foil boat has not changed the density of the boat; the aluminum still has a density of 2.7 g/cm3 but now we're talking about buoyancy.

I am going to leave you with buoyancy to think about. I hope the density part made sense. Write back if you need a jump start on any thing.

Happy science,

Martha Croll


Cole,

An item's density is its mass divided by its volume. The units of density are: pounds per gallon (lbs/gal), grams / liter, and whatever other units you want to measure density in.

So, since changing the shape of an object does not effect its mass or volume, the object's density remains the same. You might ask, what if I put one gallon of jet fuel in a two gallon tank. The density remains unchanged because although the jet fuel is in a two gallon tank, it still occupies one gallon of space and still weighs 6 pounds. If you added two gallons of jet fuel to the two gallon tank, you will have 12 pounds of jet fuel in a 2 gallon tank which reduces to a density of 6 pounds of jet fuel per gallon.

Another way to think of it is if you change the shape of a pillow by pushing in on it, the pillow will grow a bulge in some other place, maintaining the same weight and volume.

This URL gives a table of some common items along with a history of density:

http://www.visionlearning.com/library/module_viewer.php?mid=37

Sincere regards,
Mike Stewart



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