At the top of the can is the valve. The valve is actually quite a complex little thingamajig. The valve or stem is attached to the dip tube, which grants you access to the product inside. The valve can operate in two different ways, but its purpose is the same. Either you press down on a button (actuator) to open the valve, or in the case of whipped cream, you push a stem to the side. Either way, the purpose of the valve is to open up the can to release the pressure being asserted on the liquefied propellant/product solution. It is that change in pressure that reacts with the propellant and ultimately pushes the product out of the can.
See the diagram below for more details.
So What Happens When I Press The Valve Button?
1) Inside the can, there is a steady amount of pressure being forced upon the liquid product and propellant.
2) When you press the button, the valve opens and releases gas from the can, changing the delicate balance of pressure.
3) Since the pressure inside the can is higher than outside the can, the product is pushed down, sending it up the dip tube and out though the valve.
4) Outside the can, with the pressure relieved from it, the liquefied propellant returns to a gas state, carrying the product along with it in a fine mist. This is how you can spray your body with bug spray or get a steady stream of paint out of an aerosol can.
5) In the case of a mousse or a whipped cream or any sort of foam product, instead of the propellant turning into a gas and carrying the product with it to the outside, the propellant creates bubbles that make the product expand in your hand or on your strawberry shortcake.
