If gases expand as they are heated, it stands to reason that they would contract as they cool. Here is a video of an inflated balloon being placed in liquid nitrogen.
What if you cool the gas so much that none of the particles moved around anymore? That would also mean the molecules could be infinitely close together, making the volume of the gas 0. This can’t actually happen, but scientists like to think theoretically. This temperature at which all molecular motion stops is called absolute zero.
Even though you can’t lower the temperature of a gas to absolute zero, you can do an experiment that gives you a quantitative value for the relationship between gas and volume. Once you have figured out two volume-to-temperature points, you can plot a line, and once you have line, you can extrapolate (extend using known information) your line to see what temperature absolute zero would be.
How can the value of absolute zero be determined?
All the glass materials must be made of some sort of Pyrex-like material, to prevent shattering during temperature changes.
- Big spoon
- Safety goggles
- 125 mL Erlenmeyer flask
- Large beaker, at least 500 mL
- One-hole rubber stopper that fits the 125 mL flask
- Short glass rod (5- 10 cm) that fits snuggly into the stopper hole
- Ring stand
- Hot plate
- Tongs big enough to hold the 125 mL flask
- Celsius thermometer
- Graduated cylinder
- Graph paper
- Make an ice bath by filling the bucket with ice and adding cool water. The bucket should be 3/4 full.
- Using the big spoon, stir the ice bath occasionally.
- Put the one hole stopper into the dry 125 mL flask.
- Set up a hot water bath. The 500 mL beaker should be on the hot plate.
- Clamp the 125 mL flask upright so that most of flask is submerged in the hot water bath. (See diagram 1)
- Boil the water with the empty flask inside it for five minutes. You are doing this so that the gas inside the flask reaches the same temperature as that of the boiling water.
- Record the temperature of the boiling water (which should be the same as that of the gas inside the flask). Try to get a measurement to 0.1 degrees C. The more precise your measurements are, the more precise your determination of absolute zero will be.
- While the beaker is still in the hot water bath, carefully place the short glass rod into the hole in the flask’s stopper.
- Using the tongs, remove the flask from the hot bath.
- Invert the Erlenmeyer flask and place into the ice bath. (See diagram 2)
- Making sure the flask is underwater, remove the glass rod in the stopper.
- Keep the flask submerged in the ice bath for 6-7 minutes, stirring the ice bath occasionally. Water should creep inside the flask. Why?
- Using your fingers, which might get chilly, raise the inverted flask until the water level inside and outside the flask are the same. This is to ensure that the pressure inside the flask is equal to atmospheric pressure. The only variable you want in this experiment is temperature.
- Measure the temperature of the water in the ice bath.
- 16. Reinsert the glass rod into the stopper. Take the flask out of the water and set it upright.
- Using the graduated cylinder, record the volume of water in the flask as precisely as possible.
- Fill the flask to the level where the stopper was.
- Using the graduated cylinder, record the total volume of the flask.
- Subtract the volume of the water that moved into the flask from the total volume of the flask. This is the volume of the gas inside the flask after cooling.
- Your data table should look something like this:
Temperature in Celsius
Volume in mL
(record temperature of the flask in the boiling water)
(total volume of flask)
(record temperature of the ice bath)
(total volume of flask minus volume of water that moved in)
- On your graph, temperature should be on the x-axis. The range of temperatures should be from -300 to about 150 degrees C. Volume in mL should be on your y-axis. The range of volumes should be from 150 mL to 0 mL.
- Plot your two points.
- Using your ruler, draw a straight line between them.
- Line up your ruler with the line you already started. Make a dotted line extending the line you drew all the way to where the volume is equal to 0 on the x-axis.
- Carefully determine the temperature, where your line crosses the X axis. This is your determination of absolute zero!
The expected value for absolute zero is -273.15 C. If your value falls between -250 C and -300, you did well considering the limitations of your equipment and graph paper.
As you cooled the flask in the ice bath, the molecules of gas inside the flask moved around less and less. The gas molecules took up less space inside the flask, so water from the outside slowly moved in. Charles’ law predicts that as the temperature decreases, the volume also decreases.
Another important concept related to gases, pressure and temperature is Gay-Lussac’s Law: the pressure of a fixed mass of a gas at a constant temperature varies directly with the temperature. The equation for this relationship is P1/T1=P/T2. You might remember that the pressure of a gas is the amount of force exerts per unit of surface. An example of this phenomenon is when the lid of a plastic container pops off when you heat it in a microwave. The temperature is going up, but the volume of the gas remains the same because the lid. Pressure therefore increases with temperature until it is high enough to pop open the lid.