Law of Conservation of Mass question...?

Question

the law of conservation of mass states that matt can not be created nor destroyed, only changed from one form to another. this being said, can you think of any scenarios where the Law of Conservation of Mass does not apply?


take a stab at it...thanks

Answer 1

The law of conservation of mass is only approximately true. Strictly speaking, any material system that undergoes a change in energy will change mass. For the energy changes associated with processes we are familiar with in everyday life, that change in mass is effectively unmeasurably small. However, for processes that involve large energy changes (e.g., nuclear reactions) the change in mass can be significant.

What is conserved is energy, as long as one recognizes that mass is simply another form of energy. In relativity, the energy of a system, E, is given by:

E = sqrt( p^2 * c^2 + m^2 * c^4)

where p is the momentum of the system
m is the rest mass of the system
c is the speed of light in a vacuum.

If the system is at rest, the momentum is zero, and this equation reduces to the familiar E = m*c^2.

For processes involving chemical reactions, the energy changes are on the order of a few tens or hundreds of kilojoules per kg of reactants. For instance, the combustion of methane:

CH4 + 2O2 -> CO2 + 2H2O

releases about 800 kJ per mole of CH4 burned. The rest mass of the reactants involved in burning 1 mole of CH4 is ~52 grams. The change in rest mass resulting from the loss of 800kJ would be 800,000 J/(2.998 * 10^8 m/s)^2 = 8.9*10^-9 gms. The fractional mass change is then

8.9*10^-9 gms/ 52 gms = 1.7*10^-10,

or a little less than 2 parts in 10 billion. This is not measurable.

In contrast, about 200 MeV = 3.2 * 10-11 J of energy is released in the fission of 1 nucleus of 235-uranium. The fission of 1 gram of U-235 would therefore release:

(3.2 * 10-11 J/atom) * (6.022*10^23 atoms/mol)/(235 gm/mol) = 8.2*10^10 J

The mass equivalent of 8.2*10^10 J is:

8.2*10^10 J/(2.998*10^8 m/s)^2 = 9.123*10^-4 grams/gram U,

a mass change of a little less than 0.1%. This is definitely measurable.

Answer 2

When you burn some stuff(woods, vegetation etc) you are left with ashes and surely ashes have less mass than the meterial burnt but law of conservation of mass still holds. Actually you have overlooked the CO2 gas escaped in the air. If you could catch and bring that gas back and weigh it with the ashes, you will have exactly the same mass of the original material. Definitely gas can't be caught. Therefore if you burn something in an airtight container and put it on a scale. There will be no change in mass while the combustion will be in progress because CO2 and water vapours released could not escape.

Answer 3

The ONLY time the Law of Conservation of Mass does not apply is in a nuclear reaction. Lavoisier did not know about nuclear reactions, so he thought the law would apply to all reactions. Now we state that it only applies to chemical reactions.

Answer 4

In nuclearreactions a small amount of mass is converted into energy using the equation
E=mc2