Problem 8 - Chemistry The Central Science In Si Units

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?

Step-by-Step Solution

Answer

The first bubbles are likely from dissolved gases escaping, and the second set is water vapor as boiling starts under low pressure.

Step 1: Understanding Bubble Formation

The process of bubble formation in water under a vacuum can occur due to two possible phenomena: boiling and degassing. In a vacuum, the pressure is reduced which can lead to water boiling or gases dissolved in the water escaping as bubbles.

Step 2: First Set of Bubbles: Degassing

When you first apply a vacuum, gases dissolved in the water (such as oxygen, nitrogen, and other atmospheric gases) are less soluble as the pressure decreases. This causes these gases to escape from the solution and form bubbles. The first friend’s suggestion that the bubbles are water vapor caused by boiling is less likely to be correct initially because the temperature has not increased.

Step 3: Conditions for Water to Boil

For water to boil due to a reduction in atmospheric pressure, the pressure needs to fall below the vapor pressure of water at room temperature (around 20°C), which is approximately 17.5 mm Hg. Before this point, degassing of air dissolved in the water is more prominent.

Step 4: Second Set of Bubbles: Boiling

As the vacuum continues and the pressure drops further, reaching below the vapor pressure of water at room temperature, water molecules start to transition to the vapor phase, forming more bubbles. This is when boiling occurs due to the low pressure environment.

Key Concepts

Degassing

When we talk about degassing, we're referring to the process where gases dissolved in a liquid are removed or escape from the solution. In our case, it's all about those gas molecules that sneak into water from the air. At room temperature, water has tiny amounts of gases like oxygen and nitrogen dissolved in it. It's a silent process that happens all around us.

Now, when you apply a vacuum to this water, the pressure around the water drops. This reduction in pressure lessens the solubility of these gases. Imagine trying to push air into a tight balloon; with less pressure, the balloon doesn't hold well, and the air escapes.

Here’s why it happens:

• Gases in water become less soluble at lower pressures.
• The dissolved gases form bubbles as they escape into the atmosphere.
• The initial bubbles seen during vacuum application are these escaping gases.

This degassing phenomenon proves that the first bubbles observed are indeed gas molecules leaving the solution.

Boiling Point Reduction

Boiling point reduction is an interesting phenomenon that directly ties into the behavior of liquids under different pressures. Normally, water boils at 100°C (212°F) at sea level, where atmospheric pressure is standard. But things change dramatically in a vacuum.

As pressure decreases, so does the boiling point of water. This is because the vapor pressure, which we'll touch on later, needs to equal the pressure of the environment for boiling to occur.

How it works:

• With reduced pressure, water can boil at a temperature lower than its standard boiling point.
• In a vacuum, the boiling point can drop below room temperature.
• This phenomenon explains why additional bubbles form as the pressure decreases further in the experiment.

Here's the takeaway: those second bubbles are water molecules transitioning to vapor, showcasing reduced boiling point due to the low pressure.

Vapor Pressure

Vapor pressure is a concept that links temperature and the tendency of particles to escape from the liquid phase to the vapor phase. At any temperature, water molecules have a certain kinetic energy, allowing some to break free and become vapor. The pressure they exert in this phase is the vapor pressure.

Why is it important?

• For water to boil, its vapor pressure must equal the external pressure applied by the atmosphere.
• In a vacuum, where the external pressure is much less, even warm water suffices to achieve this equilibrium.
• This dynamic explains how lowering the surrounding pressure can enable boiling at lower temperatures.

So, in the vacuum-sealed setup, once the vapor pressure of water exceeds the ambient pressure, boiling initiates, forming vapor bubbles.

Solubility of Gases

The solubility of gases in water is an intriguing topic that affects everything from the taste of your carbonated drink to the oxygen levels in lakes. This concept boils down (pun intended) to how well a gas dissolves in a liquid, influenced heavily by both pressure and temperature.

Under normal conditions, gases like oxygen and nitrogen dissolve in water. However, as pressure drops, the capacity of the liquid to hold these gases diminishes, prompting them to escape:

• Increasing temperature or decreasing pressure lowers gas solubility.
• The initial rapid bubble formation in a vacuum results from gases becoming less soluble.
• Once the pressure decreases enough, dissolved gases exit as bubbles.

Understanding solubility helps explain why the first bubbles aren't from boiling water but from the degassing process happening due to lower solubility at reduced pressures.