Understanding the Boiling Process of Pure Water

When pure water is heated, its temperature increases. As the temperature reaches the boiling point (100°C or 212°F at standard atmospheric pressure), a significant physical change occurs. The liquid water transitions into its gaseous state, which is known as water vapour (or steam).

Vigorous boiling indicates that this phase change is happening rapidly throughout the bulk of the liquid, not just at the surface (which is evaporation). The heat energy supplied overcomes the forces holding the water molecules together in the liquid state, allowing them to escape as a gas.

Composition of Bubbles During Vigorous Boiling

During vigorous boiling, the bubbles that form within the liquid and rise to the surface are pockets of this gaseous water. They are essentially filled with water molecules that have gained enough energy to become a gas at the boiling temperature.

Let's analyze the given options:

• Air: While water can contain dissolved gases like air, these gases are typically driven out of the water as it is heated, well before it reaches vigorous boiling. The bubbles at the point of vigorous boiling are primarily composed of the substance that is changing phase, which is water.
• Hydrogen: Water is made of hydrogen and oxygen atoms (H₂O). However, boiling is a physical process, not a chemical one like electrolysis, which would break water down into separate hydrogen and oxygen gases. Boiling changes the state of water molecules, not their chemical composition.
• Hydrogen and oxygen: Similar to the hydrogen option, boiling does not chemically decompose water into hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)). The \(\text{H}_2\text{O}\) molecules remain intact; they simply move further apart and enter the gaseous phase.
• Water vapour: This is the gaseous form of water. At the boiling point, liquid water turns into water vapour. The bubbles observed during vigorous boiling are indeed made up of this water vapour escaping from the liquid.

Why Water Vapour Forms the Bubbles

Boiling is defined as the phase transition from liquid to gas that occurs when the vapour pressure of the liquid equals the surrounding environmental pressure. At the boiling point, water molecules gain enough kinetic energy to overcome intermolecular forces and transition into the gaseous phase, forming bubbles of water vapour within the liquid. These bubbles then rise to the surface and escape into the atmosphere.

Therefore, when pure water boils vigorously, the bubbles are composed primarily of water vapour.

Revision Table: Key Concepts of Boiling Water

Additional Information on Water Phase Transitions

Water exists in three common states or phases: solid (ice), liquid (water), and gas (water vapour or steam). Transitions between these phases occur at specific temperatures and pressures.

• Melting/Freezing: Transition between solid and liquid. Occurs at the melting/freezing point (0°C for water at standard pressure).
• Boiling/Condensation: Transition between liquid and gas. Occurs at the boiling/condensation point (100°C for water at standard pressure).
• Sublimation/Deposition: Direct transition between solid and gas, bypassing the liquid state (e.g., dry ice).

Boiling is a form of vaporization that occurs throughout the liquid when the vapour pressure is sufficient. Evaporation, another form of vaporization, occurs only at the surface of the liquid and can happen below the boiling point.