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To Verify Charles's Law Experimentally

13th Nov 2019 @ 7 min read

Physical Chemistry

There are a number of laboratory experiments to verify Charles's law. Some are modern while others are traditional. Irrespective of which method one uses the objective and result of an experiment remains the same. Charles's law relates volume to temperature at a constant pressure. Thus, in the experiment below, we will be studying volume versus temperature relationship under a constant pressure.

Experiment

The experiment is performed at a constant atmospheric pressure. The experiment proceeds by placing an empty flask in a boiling water bath. As the temperature increases, the air inside the flask expands. Afterwards, the gas is cooled in a water bath by maintaining the amount of the air in the flask constant. By measuring/determining the initial and final temperature and volume, we verify Charles's law.

Objective

To verify Charles's law by studying volume versus temperature relationship. And also to determine the absolute zero temperature from the volume-temperature graph.

Apparatus

An Erlenmeyer flask (aka a conical flask) of 125 mL, a stand with a clamp, a tripod stand, a beaker of a size such that the flask can be comfortably submerged in it, lab rubber stopper with one hole, a glass tube, a thermometer, a wire gauze, a bunsen burner, a tank of water, and a graduated cylinder.

Charles's law experiment
Experimental diagram to verify Charles's law

Nomenclature

  1. t1 is the temperature of the boiling water.
  2. V1 is the volume of the air in the flask at the boiling point of the water bath.
  3. t2 is the temperature of the air when the flask is submerged in the water bath.
  4. Vw is the volume of the water moved in the flask.
  5. V2 is the volume of the air at temperature t2.

Procedure

  1. Thoroughly clean the conical flask with a paper towel. If possible, rinse it with a small quantity of acetone or ethanol and left it to dry.
  2. Fit the one-holed rubber stopper tightly on the flask and insert the dried glass tube in the rubber stopper.
  3. Place the wire gauze on the tripod stand. The wire gauze gives support to glassware during heating.
  4. Arrange the beaker on the wire gauze. Properly attached the clamp to the neck of the flask and place the flask inside the beaker as shown in the diagram above. The flask should be submerged as low as possible, but it must never touch the bottom of the beaker. There should be a considerable gap between the two.
  5. Pour tap water into the beaker so that the flask submerges in the water. Never completely filled the beaker, because we are going to boil it. Finally, add the burner below the tripod. The setup should look like the diagram above.
  6. Gently heat the water using the burner to get a calm boil.
  7. Place the thermometer in the beaker to measure the temperature of the water. Once the temperature crosses 95 °C, the water is about to start boiling.
  8. Let the heating continue for 6-7 min more. We want the air inside the flask to be at the same temperature as of the boiling water. After continued heating, note the temperature of the boiling water (t1).
  9. Wear safety gloves to avoid burning yourself from the hot water or a hot surface.
  10. Turn off the burner and cover the hole of the glass tube on the rubber stopper by your fingertip.
  11. Detach the flask from the clamp and immediately transfer it into the water tank in the inverted position as shown in the figure below. During the transfer, the finger pressure must on the glass tube to entrap the air in the flask. Otherwise, the entire experiment will be repeated.
  12. Now, release the finger with the flask in the same position and observe the water of the tank moving inside the flask. The movement of water in the flask is the push of the atmospheric pressure.
  13. Water tank with the inverted flask
    Water tank with the inverted flask
  14. Maintain the flask submerged for 5 min to 6 min so that the temperature of the air inside the flask reaches that of the water.
  15. Slowly Raise the flask upwards with the inverted position until the water level inside the flask matches the water level of the tank. When both water level matches, the air pressure inside the flask is the same as the atmospheric pressure.
  16. Place the figure tip back on the flask and remove it from the tank. Place the flask on the bench in its normal position.
  17. Measure the temperature of the water tank (t2).
  18. Remove the rubber stopper and measure the volume of water in the flask using a graduated cylinder (Vw).
  19. Now, fill the flask completely with fresh tap water and place the rubber stopper to let the excess water drain. Remove the stopper and measure the volume of the water in the flask (V1).
  20. Repeat the above procedure twice to get three sets of readings, so we can average them.

Precaution

  1. The rubber stopper and the glass tube must be properly fitted to avoid any seepage of water in the flask when it is inverted in the tank.
  2. The flask must be properly clamped, and it should not touch the bottom of the beaker.
  3. The beaker should never be completely filled to avoid water splashes during the boiling.
  4. Safety glows are requisite to prevent any burns.
  5. To avoid the seepage of the entrapped air from the flask, the fingertip is maintained during the transfer.
  6. The flask is always in the inverted position inside the tank. The air may escape by tilting the flask at an angle. This would cause an experimental error.

Observation

Throughout the experiment, we measure the four parameters: t1, t2, V1, and Vw.

Sample observation table
ParameterValue (set 1)Value (set 2)Value (set 3)Value (average)
Initial temperature, t1100 °C100 °C99.5 °C99.8 °C
Final temperature, t222 °C22 °C22 °C22 °C
Initial volume, V1138 mL141 mL141 mL140 mL
Volume of water, Vw27 mL29 mL28 mL28 mL

Calculation

V2 is still unknown, but we can determine it from Vw. The volume of the air (V2) at t2 is the volume of the flask (140 mL or V2) minus the volume of the water in the flask (Vw).

Finally, we have both volumes and their temperatures. Now, converting temperatures in the kelvin from the degree celsius.

As per Charles's law,

Rearranging the equation above,

Calculating the ratios of volume to temperature,

As we can see both values are almost equal but not equal. The difference between the values is 0.382 − 0.375 = 0.007. Calculating the experimental error,

The error of 1.87 % exists in our experiment.

Absolute zero temperature

The absolute zero temperature can be determined as follows:

We can also determine the absolute zero temperature from plot volume versus temperature (in °C) graph.

Result

  1. The ratio of volume to temperature is 0.375 with an error of 1.87 %.
  2. From the calculation, the value of the absolute zero temperature is −289 C.
  3. The graph of temperature in the kelvin versus volume is as follows:
  4. Charles's law experimental graph
    The graph is linear passing through the origin with a positive slope.
  5. The graph of temperature in the degree celsius versus volume is as follows:
  6. The experimental graph of Charles's law
    The graph is linear with a positive slope making x-intercept at −289°C.

Conclusion

The experiment is successfully studied. The ratio of volume to temperature remains approximately constant. The graphs of volume versus temperature is linear in nature with a positive slope as expected. The value of the absolute zero temperature is estimated from the calculation as well from the graph, and its value is −289 C. The value deviates from the expected value by 16 °C. The reason for this deviation is the fewer experimental data points on the graph.

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