MadSci Network: Chemistry

Re: Measuring matter - tools

Date: Tue Jan 5 21:21:20 1999
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Chemistry
ID: 915490050.Ch

Since you have been measuring solids and liquids, you might have been 
exploring some of their properties.  Solids keep their shape, but they 
also might be heavy or light, hard or soft, rubbery or breakable.  Many 
scientists spend all their time studying the many properties of solids, of 
which there are hunreds.  Liquids are equally interesting.  Water is a 
common liquid and is safe to work with.  Some common liquids(but more 
dangerous; so be careful investigating them), are mercury, gasoline, motor 
oil.  Mercury, used in some thermometers, is a liquid metal.  It is also 
poisonous and can be absorbed into your body through your skin, so do not 
touch it.  Gasoline burns very easily, so it must be handled carefully, 
but it weighs less than water.  You can use a simple scale and a 
polyethylene (not a foamed cup) cup to measure equal volumes of water and 
gasoline.  The same amount of gasoline weighs less than water. The same 
amount of oil weighs less than water but more than oil.  The same amount 
of mercury weighs more than water.

Gases, such as air, are like liquids, in that they fill a volume (like a 
balloon), but are very hard to weigh.  One reason they are hard to weigh 
is they are compressible.  That means they can be squeezed, putting more 
gas into the same space.  The other reaon they are hard to weigh is they 
are very light.  A lot of air weighs very little.  Did you know that the 
air above one square inch of the earth, from the ground to the top of the 
atmosphere, which is about 15 miles, weighs a little less than 15 pounds?

One property of a gas that you can measure easily, and is very important 
in our daily lives, is pressure.  When you use a bicycle pump, you 
compress air, squeezing it into a tire.  The air holds the tire off the 
road and makes the ride softer.  Although the air in the tires of a car 
weighs almost nothing, it can hold a car, which weighs several thousand 
pounds, off the ground.

There are other gases than air that we play with, such as helium.  A 
ballon filled with helium will float, even though the rubber ballon 
without helium inside would fall to the ground.  That is because the 
density of air and helium are different.  The property known as density is 
important for gases and liquids and solids.  Density can be described as 
the weight of something in a given volume.  More simply, if something 
floats on something else, whatever floats is less dense.  If it sinks, it 
is more dense.  Wood must be lighter than water for it to float on water.  
Rocks sink because they are denser.  A foam cup will float on water, but a 
foam cup filled with water will not float or sink.  The water it contains 
weighs the same as the surrounding water, so the water in the cup 
willreach the same level as the surrounding water.

There is one simple experiment that you can do to measure the buoyancy of 
a gas like helium to that of air.  You will need a sensitive scale for 
best results.  Also, a math teacher may be needed.  You will need some 
helium, balloons, a scale, a spring that is very light and easy to 
stretch, a ruler, and some pens.

1) Obtain some round ballons of the same type.  Different colors are 
2)  Put a mark or number on one ballon.
3)   Weigh the marked balloon along with some string.  
4)    Fill the marked ballon with helium until it floats and you have to 
hold it down.
5)     Attach the balloon to one end of the spring.
6)      Hold the other end of the spring, or attached it to a table with 
tape, allowing the ballon to stretch the spring a measurable amount.
7)        Measure the distance the spring streches.
8)         Measure the diameter of the ballon.
9)           Let the math teacher tell you the volume of the balloon.
10)Using calibrated weights, measure the distance the spring stretches.  
The distance the spring stretches when attached to the weight and the 
distance the spring stretches when attached to the balloon tells you the 
amount of upward pull the balloon had on the spring.
11)   Calculate the upward force divided by the volume of the balloon.  Be 
sure to subtract from this number the weight of the balloon, the weight of 
the string and the weight of the spring, too. The result is the buoyancy 
of the helium in air.

Can you calculate the volume of helium that would be needed to lift one of 
the students off the ground?

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