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 that 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 most likely to be correct? What, then, is responsible for the second batch of bubbles? [Section 13.4]

Verified step by step guidance

1. Step 1: Understand the initial conditions. The water sample is at room temperature and in contact with air, meaning it contains dissolved gases like oxygen and nitrogen.
2. Step 2: Consider the effect of applying a vacuum. Lowering the pressure above the liquid reduces the solubility of gases in the water, causing the dissolved gases to come out of solution and form bubbles.
3. Step 3: Identify the first batch of bubbles. These are likely the gases (oxygen, nitrogen, etc.) that were dissolved in the water, now escaping due to the reduced pressure.
4. Step 4: Analyze the continued application of the vacuum. As the pressure continues to decrease, the boiling point of water also decreases, eventually reaching a point where the water begins to boil at room temperature.
5. Step 5: Identify the second batch of bubbles. These are likely water vapor, as the reduced pressure has lowered the boiling point, causing the water to transition into the gas phase and form bubbles.

Key Concepts

Boiling Point and Vapor Pressure

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. When the pressure is reduced, as in a vacuum, the boiling point of water decreases, allowing it to boil at lower temperatures. This phenomenon explains why bubbles can form in water under reduced pressure, as the water transitions to the gas phase more readily.

Dissolved Gases in Water

Water can dissolve various gases from the air, such as oxygen and nitrogen, which are present in the water at equilibrium with the atmosphere. When the pressure is decreased, the solubility of these gases decreases, leading to the formation of bubbles as the dissolved gases escape. This process occurs before any significant boiling of the water itself.

Henry's Law

Henry's Law states that the amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid. When the external pressure is reduced, the solubility of the dissolved gases decreases, causing them to come out of solution and form bubbles. This law helps explain the initial bubbles observed when the vacuum is applied.