Science is
Cool!
Under Pressure: Boiling Water
Summary: We're investigating the boiling point of water. Is it the same everywhere, even at the South Pole? Perform the experiments here and find out. After you're done you can submit your results to us and they will appear here on our web site along with data contributed by other students all over the world!

I. JUST THE FACTS?
Heat changes the properties of water. If we add enough heat to water in its solid form (ice) it will change its state of matter to a liquid. We call this melting. If more heat is added the liquid will change to gas (water vapor). When enough vapor forms so that the pressure of the vapor is equal to the pressure of the atmosphere above the water, the vapor can then push the air above the container away and allow vapor bubbles to be released. We call this process boiling.

II. DON'T GET HURT/WATCH OUT!
This experiment requires proper laboratory equipment. Do not try this if you do not have the proper equipment.

III. PROBLEM
Is the boiling point of water the same everywhere?

IV. WHAT DO YOU THINK?
Make a prediction as to how you think the water will behave in different locations, different altitudes, and different pressures.

V. STUFF
  • Heat source (to boil the water)
  • Metal thermometer (Celsius, accurate to 1/10th of a degree) (we recommend a digital thermometer)
  • Distilled water
  • Pyrex beaker
  • Ring stand
  • Clamp that will hold the thermometer securely

Note: In case your thermometer is in Fahrenheit (F) you can convert to Celsius (C) using the relation C=(F-32)(5/9)

VI. JUST DO IT

  1. Fill the beaker about half full with distilled water.
  2. Place it on the heat source and heat until the distilled water boils.
  3. Using the ring stand and the clamp, suspend the thermometer in the water (be sure the thermometer is secure, and is not touching the beaker).
  4. After one (1) minute record the temperature.
  5. Repeat steps 1,2,3 & 4 three (3) times and determine the average.
  6. Look up and record the elevation of your city, town, or locale and record it.
  7. In the local newspaper look up and record the barometric air pressure on the day you carried out the experiment and record it.
VII. SCORE BOARD
Create a table like the one below and record your data. Include location, elevation, air pressure, and boiling point.

Location Elevation (meters)  Barometric Pressure (mm Hg) 
   
   
Liquid Boiling Point (oC) Average Boiling Point (oC)
Distilled Water
Trial 1 Trial 2 Trial 3
   
 

VIII. DO YOU SEE WHAT I SEE?
Submit the results of your experiment to Randy and Janice! As we collect data from students around the world we'll post the results here daily. To contribute your data simply fill in the blanks below and hit the "Submit" button.
Your Name:
Email Address:
Location (City/State):
Elevation:
Barometric Pressure:
Average Boiling Point:
Check out data already submitted. Inspect the data and answer the following questions:

  1. How are the boiling points similar?
  2. How are the boiling points different?
  3. What pattern did you notice about the boiling points?
  4. What general statement can you make concerning the boiling point water at different elevations? barometric pressures?
IX. EXTENSIONS Investigate the relationship between temperature and time in water that is being heated. Water, in an open beaker, does not get hotter than its boiling point. Measure the water temperature at 30 second intervals as it is heating up a for a few intervals after it begins to boil and then graph temperature versus time.

Look for "high altitude" baking directions on a cake mix package. How do the "high altitude" directions differ from the regular directions. Why?

More Information
The two principal factors that affect boiling are the pressure and the temperature. Under one atmosphere of pressure, pure water boils at 100 C. If the pressure changes, then the temperature will also change. More pressure means that the water vapor has to push harder on the air above it and less pressure means that there is less air to push out of the way. Because when we increase our altitude we generally find an accompanying decrease in atmospheric pressure, changing altitude can change a boiling point.

At the South Pole the barometric air pressure is about 20% less than expected for an elevation of 2,835 meters. This is the result of the general weather pattern that persists at the Pole and the effect of cooling the air over the Pole. This extra "thinning" of the atmosphere, combined with the calm weather pattern and the fact that air is extremely dry, make the South Pole an excellent place for astronomy.