“Do not incinerate even when empty.” This is a common warning found on the labels of aerosol cans. Why can’t we burn empty aerosol cans?
An empty aerosol can is not really empty. It contains some amount of air which when heated might cause the can to explode. To understand this, let us consider some familiar properties of air.
Air is really a mixture or gases. However, the physical properties of air are just like those of a single gas. In fact, all gases show remarkably chemical identity. For example, one mole of carbon dioxide, helium, and oxygen gas each occupies 22.4 liters at 0 Celsius and 1 atm pressure.
The physical properties common to all gases are as follows. Gases have mass. Try pumped up. You will note an increase in mass! Gases can be easily compressed, unlike solids and liquids. The easy compressibility of a gas makes it a good shock absorber in the form of safety air bags in automobiles or car tires.
Gases expand to fill their containers completely. The oxygen gas in a tank does not concentrate in any one portion of the tank- it is evenly distributed throughout the tank. Balloons are able to keep their shape because the gas inside exerts pressure in the inner walls of the balloon.
When you open a bottle of perfume, the pleasant scent immediately permeates the whole room. This is because gases diffuse readily. Diffusion is the tendency of particles to move toward areas of lower concentration until the concentration is uniform throughout the system.
Increasing the temperature increases the pressure exerted by a gas. A tire blow-out is more likely to happen on a hot summer day because the pressure of the air inside the car tires can get dangerously high. Conversely, as the weather gets cold, the pressure inside the tire decreases and tires may flatten.
Another interesting property of gases is their low density. Some gases have very low density that they can be used to inflate balloons and blimps. In order for a balloon to rise, it must be filled with something whose density is much, much less than the density of air. This is to offset the mass of the materials making up the balloon, as well as the valves, fixtures, and the human passengers whose densities are definitely much higher than that of air.
Two principal lifting gases are hydrogen (H2) and helium (He). Hydrogen is easy to produce but it is a highly flammable gas. It burns fast, and large amounts of heat are released in the process. This property of hydrogen makes it an ideal fuel for rocket ships but not a safe lifting gas for dirigibles. In fact, the dramatic explosion of the airship Hindenderg in 1937 when it caught fire and crashed was due to the hydrogen that was used to fill the dirigible. This incident led to the use of helium in modern dirigibles.
Helium has less lifting power than hydrogen because it is denser. It is also rare and expensive. However, being a noble gas, it is quite nonreactive and has no tendency to burn. This inertness of helium makes it a very safe gas to lift balloons and airships.
Raising the temperature decreases the density of a gas at constant pressure. This is the principle behind hot-air ballooning which has become a popular recreational activity. A propane gas fitted at the bottom of the balloon heats the air inside the bag of the balloon. Producing hot air is less expensive than helium and definitely safer than hydrogen. Although the lifting power of hot air is much less than that of helium, it is sufficient to lift the balloon because only light loads are involved.
So why should an empty aerosol can explode when thrown onto a fire? Inside the can is a propellant which is really a gas kept at high pressure. The air outside the can has a lower pressure. When the push button is pressed, the high pressure gases inside the can forces the product out through the opening in the valve.
Even if the product has been used completely, there is still gas inside because the can is sealed. When the can is thrown onto a fire, the high temperature raises the pressure of the gas inside. The internal pressure keeps on increasing until eventually; the container can not withstand the pressure. When this happens, the can ruptures and explodes.
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