Problem 8 - Chemistry: The Central Science in SI Units, Global Edition

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 first friend is most likely to be correct, as the first bubbles observed were probably dissolved gases (oxygen, nitrogen, etc.) coming out of the solution due to the decreased pressure above the water. The second batch of bubbles was likely caused by the boiling of water due to the reduced boiling point under vacuum.

Step 1: Understanding the effect of vacuum on water

When water is put under a vacuum, the pressure above the water decreases. This causes a decrease in the boiling point of water, allowing it to boil at a lower temperature.

Step 2: Dissolved gases in water

Certain gases, particularly oxygen and nitrogen, have the ability to dissolve in water. These gases can be released from the water when the conditions change, such as when the pressure above the water decreases.

Step 3: Identifying the cause of the first bubbles

Given that the water was at room temperature and in contact with air initially, it is likely that there were dissolved gases (oxygen, nitrogen, etc.) in the water. When the vacuum was applied, the pressure above the water decreased significantly, causing the dissolved gases in the water to come out of solution and form bubbles. This means the first friend, who claimed that the first bubbles were gas molecules from the air that were dissolved in the water, is mostly likely to be correct.

Step 4: Identifying the cause of the second batch of bubbles

As the vacuum continues to be applied, the boiling point of water decreases even further. At this point, the temperature of the water is equal to or above the new boiling point, causing the water to boil and produce water vapor bubbles. This means that the second batch of bubbles is likely caused by the boiling of water due to the reduced boiling point under vacuum. In conclusion, the first friend is most likely to be correct, as the first bubbles observed were most likely dissolved gases, particularly oxygen and nitrogen, coming out of the solution. The second batch of bubbles was most likely caused by the boiling of water due to the reduced boiling point under vacuum.

Key Concepts

Dissolved Gases

When water is in contact with air, it has the potential to dissolve various gases from the atmosphere, such as oxygen and nitrogen. These are called dissolved gases. The nature of this process allows the gases to integrate into the liquid, forming a situation where they are trapped within the water.
This entrapment occurs because the gases are attracted to the water molecules, which leads to their solubilization.

• Oxygen and nitrogen are the primary gases that dissolve in water.
• The amount of dissolved gas in water is influenced by various factors such as temperature and pressure.

When the pressure above the water is suddenly decreased, as in the case of applying a vacuum, these dissolved gases are encouraged to leave the water.
This is why, when a vacuum is applied, you will typically see bubbles form and rise to the surface initially—these are largely the dissolved gases escaping from the liquid due to decreased pressure.
Understanding dissolved gases is crucial in processes such as carbonation in soft drinks, where CO2 is dissolved into the liquid at high pressure and released upon opening.

Vacuum Pressure

Vacuum pressure plays a pivotal role in changing the physical properties of water. By definition, vacuum pressure is the condition in which the pressure within a space is lower than the atmospheric pressure. Applying vacuum pressure to water causes a reduction in the pressure above the liquid's surface.

• This reduction in pressure is significant because it lowers the boiling point of water.
• The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure.

When vacuum pressure is applied, boiling can occur at much lower temperatures than usual. For instance, at standard atmosphere, water boils at 100°C, but under a vacuum, it can boil at room temperature.
Initially, the vacuum allows dissolved gases to escape, but with sustained vacuum, it reduces boiling point enough for the water itself to begin boiling at lower temperatures.
This principle is extensively utilized in industries, such as in vacuum distillation, where substances are separated at temperatures lower than their standard boiling points.

Water Vapor

Water vapor is the gaseous phase of water and is an integral part of the earth's hydrological cycle. When water transitions from liquid to gas, it forms water vapor. This transition can occur at temperatures lower than the typical boiling point when conditions such as decreased pressure are applied, as seen with vacuum pressure.

• Water vapor is invisible; the bubbles seen when water "boils" are actually gaseous water forming within the liquid.
• The process of boiling under vacuum explains why water can evaporate more easily under certain conditions.

In the case of applying a vacuum to water, after initial bubbles of dissolved gases are released, the boiling point reduces. This allows the water to finally vaporize, creating more bubbles.
In atmospheric conditions, water tends to vaporize slowly through evaporation, but applying vacuum pressure speeds up this process significantly by reducing the boiling point.
Notably, water vapor plays a crucial role in the climate and weather systems, acting as a medium for heat transfer within the atmosphere.