Chlorination basics: At approximately what pressure does chlorine change from its gaseous form to the liquid phase under typical ambient temperatures used for water-treatment storage and dosing?

Introduction / Context:
Chlorine is widely stored and transported as a liquefied gas in water-treatment plants. Knowing the pressure at which gaseous chlorine liquefies at typical ambient temperatures helps operators handle cylinders safely and understand vapor–liquid behavior for dosing systems.

Given Data / Assumptions:

• Ambient plant temperatures around normal room temperature.
• Question asks for an order-of-magnitude pressure used in practice.
• Units are kg/cm² as commonly found in legacy specifications.

Concept / Approach:

Chlorine has a saturation pressure near 6–8 kg/cm² at typical plant temperatures. Cylinders therefore contain liquid chlorine in equilibrium with chlorine vapor at that pressure. This allows a compact source of chlorine to feed gas-phase dosing equipment while maintaining a stable internal pressure envelope.

Step-by-Step Solution:

Verification / Alternative check:

Chlorine physical-property charts used in plant O&M manuals show equilibrium pressures in this range; cylinder pressure gauges corroborate values near 7 kg/cm² at room temperature, varying with temperature.

Why Other Options Are Wrong:

• 5–6 kg/cm²: typically too low at normal ambient temperatures.
• 8–10 kg/cm²: possible at higher temperatures, but 7 kg/cm² is the standard reference point in many texts.

Common Pitfalls:

• Confusing cylinder test/relief pressures with normal saturation pressure.
• Ignoring temperature dependence of saturation pressure.

Final Answer:

7 kg/cm².