Problem 8 - Chemistry: The Central Science

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? [Section 13.4]

Step-by-Step Solution

Answer

The most likely explanation for the first batch of bubbles is Friend 2's suggestion that the bubbles are gas molecules (oxygen, nitrogen, etc.) from the air, which were dissolved in the water. The reduced pressure under vacuum decreases the solubility of these gases, releasing them as bubbles. The second batch of bubbles appears because the continued application of vacuum leads to a further reduction in pressure, lowering the boiling point of water and eventually causing the water to boil, forming bubbles of water vapor.

Step 1: Identify the given explanations

Friend 1 suggests that the bubbles are water vapor, and the low pressure reduced the boiling point of water, causing the water to boil. Friend 2 suggests that the bubbles are gas molecules (oxygen, nitrogen, etc.) from the air, which were dissolved in the water. #Step 2: Understanding the boiling point of water under vacuum#

Step 2: Understand the concept of boiling point

Boiling occurs when the vapor pressure of a liquid is equal to the atmospheric pressure. When water is under a vacuum, the pressure above the water decreases, and the boiling point can get reduced as compared to room temperature. #Step 3: Understanding the dissolved gases in water#

Step 3: Understand the solubility of gases in water

When water is in contact with air, it can dissolve gases like oxygen, nitrogen, and carbon dioxide. The solubility of these gases in water decreases with a decrease in pressure. #Step 4: Evaluating Friend 1's explanation (boiling point)#

Step 4: Evaluate the boiling point explanation

Since the pressure reduces under vacuum, the boiling point of water does get lower, which could potentially cause the water to boil. However, this would only be true if other conditions have not changed. As such, Friend 1's explanation is plausible, but not necessarily guaranteed to be correct. #Step 5: Evaluating Friend 2's explanation (dissolved gases)#

Step 5: Evaluate the dissolved gases explanation

With the reduction of pressure under vacuum, the solubility of the gases in water decreases. The dissolved gases (oxygen, nitrogen, etc.) come out of the water as bubbles, which is a more likely scenario for the first batch of bubbles as it would happen as soon as the vacuum is applied. #Step 6: Concluding the most likely explanation for the first batch of bubbles#

Step 6: Conclude the most likely explanation

Considering the promptness of the first batch of bubbles upon applying the vacuum, Friend 2's explanation of dissolved gases from air is more likely to be the correct explanation for the first batch of bubbles. #Step 7: Identifying the cause for the second batch of bubbles#

Step 7: Identify the cause of the second batch of bubbles

As the vacuum continues to be applied, the pressure above the water continues to decrease. After some time, the pressure drops enough that the boiling point gets lower, and the water begins to boil, causing water vapor to form bubbles. So, the second batch of bubbles is most likely caused by the reduced boiling point of the water and its subsequent boiling.