It will continue to slowly boil as energy from the room enters the cup in the form of heat. Cordless Electric Air Duster, PDGROW Compressed Air Cleaner for PC Keyboard Cleaner with LED Lighting, Powerful 45000 RPM, Rechargeable 6000mAH Battery, Best Replaces Compressed Spray Air Can Duster 170 Limited time deal 4419 63.
So as it boils, the liquid cools until it reaches the equilibrium that is it's boiling point at the pressure of the room (about -25 degrees C). But boiling takes energy, and this energy (think: heat of vaporization) comes in the form of temperature from the surrounding. As such, it begins to boil when it is released from the can. This liquid can be thought of similarly to liquid nitrogen: it is a liquid that cannot exist at room temperature and pressure because it's boiling point at standard pressure is much lower than room temp.
The liquid does not really expand any significant amount. First, unplug and switch off your keyboard before you clean it or remove the batteries. Scenario 2 (what you asked): when upside down, you release the liquid in the can. So, increase volume, you must drop temperature. The gas expands under pressure and cools off in accordance with the ideal gas law Pressure * Volume = mass of gas * adjusted constant * temperature. Scenario 1 (not quite what you asked): When you open the can's valve right-side-up, you release the gaseous difluoroethane at the top of the can. The "gas" (often difluoroethane) is chosen specifically because it becomes a liquid under relatively little pressure within the can.
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