Here's the breakdown:
* Mass: The amount of oxygen gas in the tank is what truly defines "empty". If half the oxygen gas is gone, the tank is half-empty by mass.
* Pressure: The pressure gauge on an oxygen tank measures the pressure of the gas inside. This pressure is related to the density of the gas, which is how tightly packed the oxygen molecules are.
Why it's tricky:
* Temperature affects pressure: As temperature increases, the gas molecules move faster and collide more frequently, leading to higher pressure even if the mass of gas remains the same.
* Pressure decreases as oxygen is used: As oxygen is used, the mass decreases and the pressure drops. This makes it seem like the tank is half-empty when it might actually be more than half full by mass.
So, can a tank of oxygen gas ever be half-empty?
Yes, it can be half-empty by mass. But you can't rely on the pressure gauge to tell you that. To know if it's truly half-empty, you'd need to know how much oxygen was originally in the tank and how much has been used.
