Law of Conservation of Mass: Explaining the Confusion

[MRCHEM]

it is said in our book that when matter undergoes changes whether physicaly or chemically, the atoms are merely rearranged. no atom is created nor destroyed. (the law of conservation of mass)

but isn't it that we are made of atom then if I am made up of atom an atom is created when I am born. I am confused please help me

 

[star.torturer]

it isn't created when you are born, they just are assembled from existing atoms from the mother, and anyway when your are born you are fully formed anyway, and even the sperm and egg is huge compared to molecular terms.

 

[Danger]

It appears that this book still adheres to the idea of the law of conservation of mass as separate from the law of conservation of energy. That is outdated. The law of conservation of mass-energy allows for the transformation of one to the other, as long as the overall quantity is conserved. Burning something, for instance, is a chemical reaction that converts part of the matter into thermal and EM energy.

 

[rcgldr]

Burning something, for instance, is a chemical reaction that converts part of the matter into thermal and EM energy.

I was under the impression that all chemical reactions involve a change in the energy states of electrons, not a change in mass, (unless relativity effects on the speeds of the electrons are chaning their relative mass).

 

[fargoth]

I must agree with Jeff Reid.
the energy you get from burning, or any other chemically exothermic process, is due to the atom's energy change (an energy change that occures from different angular momentum, and "orbit" (usually marked n) of the electrons, oh and energy from the defined space of the electrons.. -because of pauli's exlusion principle).

EDIT: hmm... i just remembered, a friend told me that atoms in higher energy levels actually have more mass... so danger may be right too. (i think i'll look it up)

another EDIT:

There are two other interesting phenomenons in the chemical reactions are, slight mass gain in the endothermic reactions and slight mass losses in the exothermic chemical reactions even though it is only measurable if the reactants are in much quantity.

though i haven't checked this guys credibility... it was found here:
http://www.physics-edu.org/Chemical_Energy.htm

 

[Danger]

Again, Jeff, I'm not a scientist... but...
Take a fire as in the last example. There are definitely changes in the energy states, as you said. Some of that is strictly kinetic, expressed as heat, but the light and infrared radiation come from the electrons emitting photons. That involves dropping an energy level, after having been pumped up by (I assume) the kinetic gain. Even conduction of heat away by contact with another body requires that such is lost by the initial system. That might be a misinterpretation on my part, but it certainly seems to me as if that energy comes from the partial conversion of the fuel mass.

 

[chroot]

The truth is the products of a combustion reaction (smoke particles, CO2 gas, water vapor, ash, etc.) all together weigh slightly less than original reactants (fuel, oxygen, etc.). The energy released by the combustion reaction is indeed the conversion of some mass into energy -- although it's an unbelievably tiny fraction of the total rest mass of the molecules involved. The difference in mass is so small that's essentially not even measurable, but this is the theory.

This means, of course, than an excited atom actually has more mass than a relaxed atom.

- Warren

 

[fargoth]

The truth is the products of a combustion reaction (smoke particles, CO2 gas, water vapor, ash, etc.) all together weigh slightly less than original reactants (fuel, oxygen, etc.). The energy released by the combustion reaction is indeed the conversion of some mass into energy -- although it's an unbelievably tiny fraction of the total rest mass of the molecules involved. The difference in mass is so small that's essentially not even measurable, but this is the theory.

This means, of course, than an excited atom actually has more mass than a relaxed atom.

- Warren

thank you for clearing that up, it seems I had the right guess

 

[Johnson]

it is said in our book that when matter undergoes changes whether physicaly or chemically, the atoms are merely rearranged. no atom is created nor destroyed. (the law of conservation of mass)

but isn't it that we are made of atom then if I am made up of atom an atom is created when I am born. I am confused please help me

You start out as a single-celled organism. Sperm and Egg meat, and start adding new atoms to your embryo, thus increasing in size and mass. As your mother matures in pregnancy, she is constantly feeding you through ambilical cord. Therefore atoms are continuously being added to yourself, through the foods your mother eats.

And once your born, you start eating, thus increasing your mass.

~Andrew

 

[loom91]

it is said in our book that when matter undergoes changes whether physicaly or chemically, the atoms are merely rearranged. no atom is created nor destroyed. (the law of conservation of mass)

but isn't it that we are made of atom then if I am made up of atom an atom is created when I am born. I am confused please help me

I'm speechless! What a question. The fact is that nothing is created when you are born. No atom is created when you are born, and if someone told you this they were criminally wrong. Already existing atoms from the body of your mother (and a few from your father) combine to form the first parts of you, a cell (known as a zygote). Even this is not one atom, but billions of them. You will learn about the details when you study biology and chemistry in higher classes.

As for actual conservation of mass in chemical reactions, I think rest mass is conserved usually. That is, elementary particles don't change into others except in nuclear reactions. The loss of mass is relativistic or inertial mass, which depends on the state of motion of the particle. When energy is radiated, this energy is lost from the inertial mass.