（1）Container: a sealed metal can, usually made of aluminum or steel, capable of withstanding high pressure.
（2）Contents: Liquid or suspended particles to be ejected (e.g. paint, alcohol, pharmaceuticals, etc.).
（3）Propellant: A gas or liquefied gas that provides pressure (e.g. propane, butane, carbon dioxide or nitrogen).
（4）Valve system: nozzles and spring-loaded valves to control ejection.
2. Core scientific principles
（1）High-pressure environment:
The tank is pressurized and the propellant is stored in a coexisting liquid and gaseous form. According to the ideal gas law (PV=nRT), gas pressure is inversely proportional to volume at constant temperature. When the valve is closed, the system is in dynamic equilibrium: the liquid propellant evaporates to maintain the gas phase pressure.
（2）Trigger injection:
When the nozzle is pressed, the valve opens, the pressure inside the tank plummets, and the liquid propellant rapidly vaporizes and expands (phase change heat absorption), propelling the contents out of the nozzle at high velocity.
（3）Atomization Mechanism:
As the contents pass through the narrow nozzle, the flow rate increases dramatically (based on Bernoulli's principle), while the kinetic energy generated by the vaporization of the propellant breaks the liquid into tiny droplets (atomization), forming an aerosol.
3. Role of propellants
（1）Liquefied gas propellant (e.g. LPG):
Stored as a liquid at room temperature, it expands hundreds of times in volume when vaporized, providing continuous pressure. This type of propellant mixes with the contents and is ejected together.
（2）Compressed gas propellants (e.g., CO₂, N₂):
Existing only in the gaseous state, the contents are propelled by compressed gas pressure, but the pressure gradually decreases with use.
4. Key Laws of Physics
(1) Henry's Law: the solubility of a gas in a liquid is proportional to the pressure (explains that propellant dissolves in the contents).
(2) Raoul's Law: The vapor pressure of each component in a liquid mixture affects the gasification process.
(3) Adiabatic Expansion: Heat is absorbed when the propellant is vaporized, which may lead to frosting on the surface of the tank (e.g., antiperspirants).
5. Safety and design considerations
(1) Pressure-resistant design: the tank is designed to withstand 4-8 times atmospheric pressure (approximately 0.5-1 MPa).
(2) Explosion protection: High temperatures cause a dramatic increase in pressure (Charles' Law), so tanks are labeled “Avoid High Temperatures”.
(3) Evolution of environmental protection: In the early days, Freons (CFCs) were phased out due to the destruction of the ozone layer, and in modern times hydrocarbons or compressed gases are used.
6. Examples of applications
(1) Hair spray: propellant is mixed with alcohol and the propellant evaporates quickly after spraying, leaving a styling polymer behind.
(2) Fire extinguishers: Compressed gas is utilized to instantly release dry powder or flame retardant.
Summary
The essence of an aerosol can is to store a mixture of propellant and contents under high pressure, using pressure differentials and phase changes to achieve a controlled spray, combined with fluid dynamics to achieve atomization. This design perfectly balances the principles of chemistry, physics and engineering, making it one of the classic technologies for modern, convenient living.
