Understanding States of Water During Heating

When water is heated at a constant pressure, it goes through different phases or states depending on its temperature. Starting from a low temperature, water is initially in the liquid phase. As heat is added, its temperature increases until it reaches a specific point where it is ready to change phase into vapor (steam).

Exploring Different States of Water

Let's look at the different states mentioned in the options:

• Sub-cooled liquid: This is water in its liquid phase at a temperature below its boiling point for the given pressure. It needs more heating before it can start boiling or vaporizing.
• Saturated liquid: This is water in its liquid phase at the boiling point temperature for the given pressure. At this state, any additional heat added will start converting the liquid water into vapor without increasing the temperature (at constant pressure). This is the state where the liquid is "saturated" with thermal energy and is about to vaporize.
• Saturated vapor: This is water in its vapor phase (steam) at the boiling point temperature for the given pressure. At this state, the vapor is in equilibrium with the liquid phase, and removing heat will start condensing the vapor back into liquid.
• Super-heated vapor: This is water in its vapor phase at a temperature above the boiling point for the given pressure. All the liquid has already turned into vapor, and further heating increases the vapor's temperature.

Water About to Vaporize

The question asks about the state of water when it is "about to vaporize". Based on the definitions above, this state occurs when the liquid water has reached its boiling temperature for the given pressure and is ready to start converting into vapor as soon as more heat is supplied. This specific state is known as the saturated liquid state.

Imagine heating a pot of water on a stove. Initially, it's sub-cooled. As it heats up, its temperature rises until it reaches 100°C (at standard atmospheric pressure). At 100°C, the water is saturated liquid. Any more heat causes bubbles to form, and the water starts boiling and turning into saturated vapor. If you continue heating the vapor after all the liquid is gone, it becomes super-heated vapor.

Therefore, the state where water is just about to begin vaporizing is the saturated liquid state.