Understanding Boiling Water at Room Temperature

The question asks under which pressure condition water can be boiled at room temperature. Room temperature is typically around 20-25°C, which is significantly below the standard boiling point of water, 100°C.

What is Boiling?

Boiling is the process where a liquid turns into a gas throughout the bulk of the liquid, not just at the surface (which is evaporation). This happens when the vapor pressure of the liquid becomes equal to the external pressure applied to the liquid surface.

The Relationship Between Pressure and Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding atmospheric pressure. The vapor pressure of a liquid increases with temperature.

• If the external pressure is high, the liquid needs to reach a higher vapor pressure to boil. This requires a higher temperature. So, high pressure increases the boiling point.
• If the external pressure is low, the liquid needs to reach a lower vapor pressure to boil. This happens at a lower temperature. So, low pressure decreases the boiling point.

Mathematically, this relationship is described by the Clausius-Clapeyron equation, which shows how vapor pressure changes with temperature.

Analyzing Conditions for Boiling Water at Room Temperature

We want water to boil at room temperature (e.g., 25°C). This means the boiling point needs to be reduced from the standard 100°C down to 25°C or lower.

• Option 1: At atmospheric pressure
  At standard atmospheric pressure (about 1 atmosphere or 101.3 kPa), water boils at 100°C. Room temperature is much lower than 100°C, so water will not boil under normal atmospheric pressure at room temperature; it will simply evaporate slowly.
• Option 2 & 4: At very high pressure / At high pressure
  Increasing the pressure above atmospheric pressure would increase the boiling point of water. This would make it even harder to boil water at room temperature, requiring a temperature much higher than 100°C.
• Option 3: At low pressure
  Decreasing the pressure below atmospheric pressure lowers the boiling point of water. If the pressure is lowered enough, the boiling point can drop to room temperature or even below. For example, at a pressure of about 3.2 kPa (approximately 0.03 atm), water boils at 25°C.

Conclusion: Boiling Water at Low Pressure

To boil water at room temperature, the external pressure must be significantly reduced. This is because lowering the pressure lowers the boiling point of water. When the pressure is low enough, the water's vapor pressure reaches the external pressure at a much lower temperature than 100°C, causing it to boil.

Revision Table: Boiling Point of Water vs. Pressure

Additional Information on Vapor Pressure and Boiling

The vapor pressure of a liquid is the pressure exerted by its vapor when the vapor is in equilibrium with the liquid phase at a given temperature. As temperature increases, more molecules have enough energy to escape into the gas phase, increasing the vapor pressure.

Boiling is a specific type of vaporization that occurs throughout the liquid when its vapor pressure overcomes the external pressure. At temperatures below the boiling point, vaporization only occurs at the surface (evaporation). The rate of boiling depends on the pressure and the heat supplied.

Experiments demonstrating boiling at low pressure are often done using a vacuum pump to reduce the pressure in a sealed container holding water. As the pump operates, the pressure drops, and the water can be observed to boil even without heating.