We are regularly asked why the weight of the contents of an aerosol can is not numerically equal to its volume. In other words, why the contents of a 520 ml can weigh less than 520 g. Does that mean the can is filled with less than it should be?
What are volume, weight and mass?
Let us sort this out. Volume and weight are different physical quantities. Volume is measured in cubic meters, liters and milliliters and shows how much space a solid or a liquid takes up. Weight is measured in newtons and shows the force with which a body presses on its support or suspension — in other words, how heavy a body or a substance is. Weight and mass, by the way, are not the same thing. Mass is a measure of the inertia of a body and is measured in kilograms. If a body moves with acceleration, its weight differs from its mass. Weightlessness is a clear example: astronauts in orbit weigh nothing, they need no support and can float in the air. Their mass, however, stays exactly what it was on Earth. This is easy to confirm — if one astronaut pushes off a wall and hits another at a fair speed, both will feel that their kilograms have not gone anywhere, even though a scale in weightlessness would read zero.
How are volume and weight (mass) related?
The common belief that 500 ml must always weigh 500 g is wrong. That equality holds for water only. Imagine the same half-liter container filled with molten lead. Will it still weigh 500 g, since it is still half a liter? And what if the container is filled with helium instead of lead? Will the scale still show half a kilogram? The volume has not changed, so the weight should stay the same, should it not?
Nothing of the kind: the lead "half-liter" will sink, and the helium one may well fly away. It is all about the density of the substance, which is exactly what tells you how much a unit of volume weighs. The density of lead, for example, is 11.3 g/ml, that of water 1 g/ml, and that of helium 0.000178 g/ml. Lead is 11.3 times heavier than water, and helium is 5,618 times lighter. To find out how much a given volume of a substance weighs, multiply that volume by the density. So 500 ml of lead weighs 5,650 g, 500 ml of water weighs 500 g (because its density equals one), and half a liter of helium tips the scale at just 0.089 g.
Why is the weight of an aerosol can not equal to its volume?
The contents of aerosol cans are usually lighter than water: the density of various aerosol lubricants, paints and cleaners is in the range of 0.6 to 0.9 g/ml. That is why their weight is numerically lower than their volume.
The weight of the contents of an aerosol can is also reduced by the propellant. To spray out the entire product, a certain pressure has to be maintained inside the can. The propellant is present in the can in two states, liquid and gaseous. The formula is designed so that while the product is being sprayed, the propellant turns from liquid into gas and keeps the pressure constant until the product has been sprayed out completely. This is why at least 30% of the volume of the can is taken up by the propellant, leaving no more than 70% for the product. With a can volume of 520 ml, that means no more than 364 ml of liquid can be filled in. Multiply that volume by a density of, say, 0.7 g/ml, and the net weight comes to only 255 g.
So the weight of the contents of an aerosol can is numerically lower than its volume for two reasons: the can is filled with a substance that is lighter than water, and it is filled to only 70% of its volume, because the rest is taken up by the propellant.
How an aerosol can is built
An aerosol can consists of the body (1), the valve (2), the dip tube (3) and the spray head (4). It is filled with paint (5) and propellant (6). The propellant maintains the pressure inside the can, and that pressure pushes the paint out through the spray head as a spray when the valve is pressed. During long storage the pigment contained in the paint settles at the bottom of the can. The can therefore has to be shaken thoroughly for several minutes before use, and shaken regularly while spraying. To help the material mix better while shaking, two small metal balls (7) are placed inside aerosol cans.
