Boiling Water Experiment: Understanding the Phenomenon
What happens when you add 100 mL water to a 500-mL round-bottom flask and heat the water until it is boiling?
Why does the boiling stop when you remove the heat and stopper the flask?
What causes the boiling to start again when you run cool water over the neck of the flask?
Answer:
The boiling water experiment with a round-bottom flask demonstrates the principles of Gay Lussac's law, which states that pressure is directly proportional to temperature at constant volume. When you heat the water in the flask until it boils, the pressure inside increases due to the higher temperature.
When you remove the heat and stopper the flask, the boiling stops because the heat source is taken away, and the pressure inside the flask decreases. As a result, the water ceases to boil and remains in a liquid state.
The boiling starts again when you run cool water over the neck of the flask because the cool water absorbs heat from the gas and the liquid inside the flask. This results in a rapid decrease in temperature and pressure, causing the water to boil once more as it tries to reach equilibrium.
When you conduct the boiling water experiment as described, you can observe the fascinating interplay between temperature, pressure, and phase changes. As you heat the water in the round-bottom flask, the energy from the heat source increases the temperature of the water molecules. When the temperature reaches the boiling point, the water transitions from a liquid state to a gaseous state, creating bubbles of water vapor.
By removing the heat and stoppering the flask, you create a closed system where the pressure inside the flask decreases as the temperature decreases. This decrease in pressure causes the boiling to stop, as the water vapor condenses back into liquid form due to the reduced energy and pressure levels.
When you introduce cool water over the neck of the flask, you disrupt the equilibrium by rapidly cooling the system. The cool water absorbs heat from the gas and liquid in the flask, reducing their temperature and pressure. This sudden decrease in pressure triggers the boiling to start again, as the water vaporizes to equalize the pressure inside the flask with the external environment.
Overall, the boiling water experiment serves as a hands-on illustration of the relationship between temperature, pressure, and phase changes, showcasing the principles of thermodynamics in action.