Question

You take a sample of water that is at room temperature and in contact with air and put it under a vacuum. Right away, you see bubbles leave the water, but after a little while, the bubbles stop. As you keep applying the vacuum , more bubbles appear. A friend tells you that the first bubbles were water vapor, and the low pressure had reduced the boiling point of water, causing the water to boil. Another friend tells you that the first bubbles were gas molecules from the air (oxygen, nitrogen , and so forth) that were dissolved in the water. Which friend is mostly likely to be correct? What, then, is responsible for the second batch of bubbles?

Answer 1

The first bubbles in the water under vacuum are from dissolved gases, while the second batch of bubbles are due to the water boiling at a reduced boiling point caused by the vacuum. As the vacuum is applied, the water begins to boil at room temperature due to the lowered pressure which allows the vapor pressure of water to match the surrounding pressure.

Step-by-step explanation:

When you take a sample of water that is at room temperature and put it under a vacuum, the first bubbles you see are likely due to dissolved gases (such as oxygen and nitrogen) that were present in the water escaping as the pressure above the water decreases. This happens because gases are less soluble in liquids at lower pressures. The initial appearance of bubbles can be attributed to outgassing of these dissolved air molecules.

As for the second batch of bubbles seen when the vacuum is continually applied, these are likely because the boiling point of the water has been lowered below room temperature due to the reduced pressure. Under vacuum, the vapor pressure of water can quickly exceed the surrounding pressure and when it equals the atmospheric pressure, boiling occurs, hence more bubbles form - these are water vapor bubbles.

At normal atmospheric pressure, the boiling point of water is 100°C, but when placed under a vacuum, it can boil at a much lower temperature. This is due to the water vapor pressure becoming equal to the reduced external pressure within the vacuum environment.

Answer 2

The second friend is mostly correct.

Step-by-step explanation:

When pressure decreases, gas solubility decreases. The bubbles are gas molecules that were dissolved in the water at atmospheric pressure. The bubbles started coming out as a result of the decrease pressure. The second bubbles would be water vapor because as the pressure continue to decrease, the amount of vapor pressure ( vapor leaving the surface of the liquid ) equals the external pressure and water begin to boil. So the second bubbles must be water vapor.