Question

A student is observing two beakers of water. One beaker is heated to \(30^{\circ} \mathrm{C},\) and the other is heated to \(100^{\circ} \mathrm{C}\). In each case, bubbles form in the water. Are these bubbles of the same origin? Explain.

Short answer

No, the bubbles have different origins: boiling at 100°C and releasing dissolved gases at 30°C.

Step-by-step solution

Determine the behavior of water at 100°C

At 100°C, water reaches its boiling point. Bubbles formed at this temperature are due to the phase transition from liquid to gas, specifically water vapor. This is the boiling process, where liquid water changes to vapor and escapes into the air.

Analyze the situation at 30°C

At 30°C, water is well below its boiling point, hence, bubbles are unlikely to be the result of boiling. Instead, these bubbles are typically caused by dissolved air in the water that gets released as the water is heated. This process is not related to the boiling of water.

Conclude the origin of bubbles

Given the conditions, bubbles formed at 100°C originate from the phase change during boiling (liquid to vapor), while bubbles at 30°C result from dissolved gases escaping as the water is heated. Therefore, the origin of the bubbles in the two beakers is different.

Key concepts

Boiling Point

The boiling point of a substance is a specific temperature at which it turns from a liquid into a gas. In the case of water, this transition occurs at 100°C (212°F) under standard atmospheric pressure. At this temperature, the molecules of water have enough energy to break free from the liquid state and become vapor.

• Boiling is a phase transition involving both temperature and energy exchange.
• At and above the boiling point, the liquid water molecules rapidly transition to gaseous water vapor.

When observing bubbles forming at the boiling point, they are typically the result of this water-to-vapor transition. This is an essential concept because not all bubbles in heated water result from boiling, as we will see when discussing dissolved gases.

Dissolved Gases

Dissolved gases in water are gases that have mixed into the water and remain trapped under certain conditions. All natural waters contain dissolved gases, including oxygen and nitrogen, the primary components of air. When water is heated below its boiling point, these gases can become more likely to escape.

• As water temperature increases, the solubility of gases decreases.
• Bubbles formed at warm temperatures (e.g., 30°C) often contain these gases being released.

Therefore, when you see bubbles in water that is not yet boiling, it is likely the result of these dissolved gases escaping the liquid. This phenomenon is distinct from boiling, as no phase transition of water occurs.

Water Vapor

Water vapor is the gaseous form of water, created when liquid water evaporates or boils. Essentially, it is water in a vapor state above its boiling point. In the boiling process:

• Water molecules gain energy and break free from the liquid's surface.
• The process involves high temperatures where water is converted to steam or vapor.

Understanding water vapor is crucial for recognizing how boiling differs from simply heating water. At 100°C, the bubbles in a boiling pot of water are indeed composed of water vapor, unlike the bubbles formed from dissolved gases at lower temperatures.

Liquid to Gas

The transition from liquid to gas is known as vaporization and includes two main processes: evaporation and boiling. Here's a quick breakdown: - **Evaporation:** This occurs at the surface of a liquid and can happen at temperatures below the boiling point. - **Boiling:** This is a quicker process where bubbles form throughout the liquid when a liquid reaches its boiling point. In an experiment observing two beakers of water at different temperatures, it's clear that one undergoes boiling (vaporization from liquid to gas) while the other just exhibits gas release (from gases dissolved in water). This difference is essential in understanding why the formation of bubbles is not always indicative of water reaching its boiling point.