When gas in a container expands to twice its volume at constant temperature, the pressure decreases to half its original value.

This is due to Boyle's Law, which states that the pressure of a gas is inversely proportional to its volume at a constant temperature.

Mathematically, this can be expressed as:

P₁V₁ = P₂V₂

Where:

* P₁ = Initial pressure

* V₁ = Initial volume

* P₂ = Final pressure

* V₂ = Final volume

Since the volume doubles (V₂ = 2V₁), the pressure must decrease by half (P₂ = P₁/2) to maintain the equality.