[Amitaabh]

Lets stay I have a piece of iron and press it against another piece of iron, why don't they bond to form one piece? I am thinking that the piece of iron is essentially many atoms of iron which have formed a lattice, so why wont another identical lattice interface with the first one?

[discodermolide]

Lets stay I have a piece of iron and press it against another piece of iron, why don't they bond to form one piece? I am thinking that the piece of iron is essentially many atoms of iron which have formed a lattice, so why wont another identical lattice interface with the first one?

Have a look at this page and you tell me why they will not bond together. What do you have to do to get this to happen?
http://en.wikipedia.org/wiki/Iron

[Amitaabh]

I still can't tell why not. Why don't any two identical solids combine? Like two pieces of cellulose or anything.

[discodermolide]

I still can't tell why not. Why don't any two identical solids combine? Like two pieces of cellulose or anything.

To get them to combine you would have to break the crystal lattice in both pieces. This requires energy. So if you put enough energy into both bits you will achieve this.
To break the crystal lattice you would need to melt both pieces.

[Amitaabh]

I don't see why the lattice has to be broken why cant it just incorporate the other lattice on its open end. When iron rusts doesn't that break the lattice as well, but no energy is put in right?

[discodermolide]

I don't see why the lattice has to be broken why cant it just incorporate the other lattice on its open end. When iron rusts doesn't that break the lattice as well, but no energy is put in right?

There are still lattice bonds to be made at each face. This needs energy!
Rusting is an oxidation process and does not affect the lattice.

[Amitaabh]

Is it true that no two solids (same or different) will react unless extra energy is added because the lattice of each must be broken to react? Does a potential for bonding not supply enough energy for this?

[discodermolide]

Is it true that no two solids (same or different) will react unless extra energy is added because the lattice of each must be broken to react? Does a potential for bonding not supply enough energy for this?

No this is not true. Now you are talking about chemical processes which are distinct from physical processes.

[Arkcon]

I don't see why the lattice has to be broken why cant it just incorporate the other lattice on its open end.

Its not open.  The lattice is closed.  That's why the piece of iron ends.  All crystals contain defects, which eventually add up, to allow a crystal to end.  Someone asked a similar question here: http://www.chemicalforums.com/index.php?topic=61310.msg218796#msg218796

When iron rusts doesn't that break the lattice as well, but no energy is put in right?

You're sure about that, are you?

[Amitaabh]

I don't understand the distinction between a chemical process and a physical process. Aren't all solids "closed" why do they ever react with anything?

I am guessing the lattice breaks when iron rusts because part of it comes off.

[curiouscat]

The biggest reason why normal metals won't bond like that is surface finish. Typical metal surfaces on a microscale will look like a set of valleys and mountains.



Even a fairly polished surface is exceedingly rough on a microscopic scale. Hence the real metal-to-metal contact happens at very few points, and that is insufficient for  bonding. Of course, any sort of surface oxide or dirt makes this worse.

Surprisingly, there are certain non-everyday situations when what you say actually happens. One situation is "gauge blocks".

http://en.wikipedia.org/wiki/Gauge_block#Wringing

Lay your hand on a set if you know a machine shop. Once put together these blocks bond so well that it is pretty hard to separate them. You've to try it to believe it.





Apparently if you forget to separate them after you are done, in a few months they will bond so well that it is impossible to separate them without damage.

So yes, you can make two lattices bond through adhesion alone; but it isn't easy.

[confusedstud]

When you break the iron bar into 2 pieces, energy is absorbed (endothermic) as the metallic bonds are broken. When you join them together again, since bonds are formed so energy is given out. But since cutting them is not a chemical reaction so they do not have the same energy profile as a exo or endo reaction. So why is energy required? I mean the whole process requires energy to be given out so I'm also unsure about this. In the usual chemical reaction there is a hump. But since this a purely physical change so I guess there are other physical factors prohibiting their reformation as curiouscat said the grooves might prevent their rejoining.

[discodermolide]

Quote "But since cutting them is not a chemical reaction so they do not have the same energy profile as a exo or endo reaction. So why is energy required? "
Cutting is a type of chemical reaction, if I may say this, the energy of the cutting tool puts energy into the system to break the bonds.
Cutting breaks chemical bonds. The energy profile is still similar to a normal chemical process, only the scale must be different.

[confusedstud]

Oh, but since no new products are formed so why would this be considered a chemical change?

[discodermolide]

Oh, but since no new products are formed so why would this be considered a chemical change?

You are forming a new lattice, or exposing the atoms in the lattice to the environment, so you may get a change in crystal structure or an oxidation process starting. Think what happens when you get a scratch to your car. You put energy in damage the lattice and rust starts. Chemical change.