Lesson

Background

If a gas is heated, it will expand, and if cooled, it will contract. In 1787 a French scientist named Jacques Charles showed the mathematical relationship between Kelvin Temperature (T) and the Volume (V) of a gas. Charles’ Law is named after him for his pioneering work. It states that the volume (V) of an ideal gas is directly related to its Kelvin temperature (T), shown by the equation:

V₁T₂ = V₂T₁ or V = kT

This relationship between volume and temperature is a linear relationship. The x-intercept of this relationship (where the best-fit line crosses the x-axis) is called absolute zero, which is the coldest possible temperature. At 0 K ideal gases have a volume of 0 L. Unfortunately, it is not possible to reach absolute zero, and even if it were, gases would not behave like ideal gases at this temperature. But V and T data can be measured at temperatures at which gases do behave ideally. The trends found can then be extrapolated to find the coldest possible temperature, or absolute zero.

Purpose

To find absolute zero in °C using Charles’ Law.

Safety

• Use caution around open flames and wear protective gear when handling hot glassware.
• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Materials

• 2x1000-mL beakers
• Bunsen burner
• Hotplate
• Thermometer
• Table salt (NaCl)
• Ice
• 30-mL syringe
• Timer
• Glass stirring rod
• Tongs
• Heat-resistant gloves

Procedure

1. Fill one beaker with tap water to 500 mL and place it on the Bunsen burner.
2. To the other beaker, add 250 mL of tap water and add two heaping scoops of NaCl and enough ice to bring the total volume to 500 mL. Stir with a stirring rod and place a thermometer in the beaker.
3. While we wait for both beakers to reach their final temperatures (~100 °C on the burner, and ~-5 °C with the ice), turn on the hot plate to high
4. Get a 30-mL syringe and pull the plunger back to 30 mL. This will be your test vessel. You will be heating the air inside it with the boiling water and then putting it in the colder water. It is important that the entire airspace in the syringe is submerged and that the plunger does not move. If it does, that trial is ruined and must be thrown out.
5. When the water on the Bunsen burner starts to boil, submerge the test vessel in the water for exactly one minute (watch the clock). Using a thermometer, record the temperature of the boiling water as the max temperature in the table on the next page.
6. By now, the ice water should be as cold as it’s going to get, Record its temperature in the table on the next page and remove the syringe from the boiling water and quickly submerge it in the ice water. Make sure that the entire vessel, including the opening, is under water. Do not let the plunger move! As the air in the vessel cools and contracts, water will be drawn in.
7. To record the volume of air remaining in the syringe, remove it from the beaker and hold it with the opening up. Read the waterline—this is the amount of air remaining. Record this in the table provided.
8. Move the beaker of boiling water to the hot plate, place the ice water on the Bunsen burner and continue to monitor the ice water temperature.
9. Empty the syringe, reset it to 30 mL and return it to the boiling water for one more minute.
10. As one minute approaches, record the temperature of the ice water (it should be warmer than before), submerge the syringe in the ice water and repeat the steps to get a new volume of air for this temperature. Record temperature and volume in the table.
11. Continue this process and collect data points for 20–25 minutes (six to eight data points). Record your data in the table below.
12. Plot your data points on the graph on the next page.
13. Using a ruler or straight edge, draw a best-fit line for your data that crosses the x-axis and goes through or near all of your data points.
14. Estimate, to the nearest degree, where your best-fit line crosses the x-axis

Results/Observations

Analysis

1. As the temperature of a gas decreases, its volume _______________. This is known as Charles’ Law.
2. Explain your answer from question one in terms of the kinetic molecular theory.
3. What happens to the particles of a gas at absolute zero?
4. Why did you need to submerge the test vessel completely in the water?
5. What does it mean to extrapolate?
6. Your value for “absolute zero” will be the point where your line crosses the x-axis (volume = 0). Based on your graph, what is your value in degrees Celsius
7. You took your measurements in degrees Celsius, but should also be familiar with the Kelvin scale. What would 0 ºC be on the Kelvin scale? 0 ºC _____
8. 100 ºC? _____ 22 ºC? _____ -78 ºC _____
9. What is your value for Absolute zero in Kelvin?
10. The actual value for Absolute Zero in degrees Celsius is -273.15. What is your % error?
11. Which is a more appropriate temperature scale to use when discussing the kinetic energy of particles, Celsius or Kelvin? Explain your reasoning.
12. If a gas with bigger molecules were used, why wouldn’t this affect the value for absolute zero? (Hint: think about the definition of an ideal gas)
13. What are two possible sources of error for this lab, and how would you correct them?