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Why is a solid, so solid?

  1. Oct 14, 2014 #1
    I was wondering this question the other day and have read a few websites but a lot of it goes over my head. I have two questions, can anyone explain in relatively simple terms? I am happy to read the more detailed science behind the concepts, but need the concepts to be stated pretty plainly.

    What makes a solid, solid?
    Why, if I place one solid on another, do they not merge? In other words, what maintains the individual identity of two blocks of lead even when placed together? A fluid of the same kind of course does merge.
     
  2. jcsd
  3. Oct 14, 2014 #2

    russ_watters

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    Solids are held together by chemical bonds.
     
  4. Oct 14, 2014 #3
    Yes, but why do the molecules in one lump of lead bond to each other, but not to the adjacent lump of lead? What makes the boundary so distinct and persistent?
     
  5. Oct 14, 2014 #4

    A.T.

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    They might, given a clean smooth surface:
    http://en.wikipedia.org/wiki/Cold_welding

    Surface impurities and lack of consistent contact due to uneven surfaces.
     
  6. Oct 14, 2014 #5
    Well, I'll be... I was actually going to pose the question how my lumps would go if placed on one on top of the other in a vacuum. I hadn't thought of the surface irregularities angle, but I figured a vacuum would remove any possibility for air/liquid obstruction. I like this bit: "The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part."
     
  7. Oct 14, 2014 #6

    DrDu

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    The merging of solids is a problem in outer space as metals tend to merge which may destroy joints and the like. The point is that here on earth there are oxide layers which inhibit the fusing of metal surfaces.
    In soldering you typically apply some acids to remove these oxide layers.
    Similar changes occur on other surfaces like that of glass. But there are some surfaces which merge easily, like organic materials (wax),
     
  8. Oct 14, 2014 #7

    sophiecentaur

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    We usually know what we are referring to as long as we don't push the definition too far.
    If you use the naive definition of a solid then there are very few materials that are true solids. The classification of materials into the 'three' states, as taught in elementary Science doesn't take you very far. Most materials will exhibit 'liquid properties' - take the rocks of the Earth's Mantel, for instance, which behave like a viscous liquid under the great pressures exerted on them.
    Classification can lead you up the garden path, if you are not careful.
     
  9. Oct 14, 2014 #8

    the intermolecular force of attraction is the cause for a solid to retain its rigidity... the intermolecular force acts mostly between same material... when you keep two different or same objects they do not merge because to create any bond you need to do some external work...this is the basic reason when you pile up solid they do not merge... and the little work done by you while piling is not enough for them to have the bonds and the forces of attractions...and so retains individual property.
     
  10. Oct 14, 2014 #9

    russ_watters

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    The fact that each has its own chemical bonds holding itself together but they don't share chemical bonds.
     
  11. Oct 14, 2014 #10

    sophiecentaur

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    Two clean surfaces of the same metal could stick together very easily. Metallic bonding is due to the dissociated outer electrons that are attracted to all the positive ion cores in their vicinity. If you start with clean surfaces in a vacuum and can be sure that they are flat enough and under enough pressure to bring a majority of the areas into contact then there is no reason why bonds couldn't exist between both sides. Wrought Iron is made without actually melting the iron; it is just folded and folded and hammered together so that adjacent surfaces actually stick together. That process even works in the presence of a fair amount of Carbon, too.
    It would be a different matter to try to get two crystals of a non metal to stick together unless they were brought very near to melting (or by using a solvent).
     
  12. Oct 14, 2014 #11
    sophiecentaur, elementary science is about as far as my education went... :)

    Your comment about pressures in the earth's mantle is interesting and one I was thinking about after reading the earlier comments. If an object is 'solid' it is because the bonds constrain the atoms from being free to move? So with temperature/pressure, as an object heats (forgive my words here, I don't know the right language) those bonds weaken somehow permitting some range of movement and a solid can change to a liquid? So your viscous rocks are in a liquid state, aren't they? I read somewhere that atoms can vibrate, though I don't know what that means. So with increasing heat/pressure increasing the energy potential, does that make the vibrational intensity increase nand overcome the bond in some way?
     
  13. Oct 14, 2014 #12

    sophiecentaur

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    Reading around can take you way beyond that. Good news - you found PF:). My point is that crude classification can let you down terribly. There's no such thing as an irresistible force or an immovable object but elementary Science often uses such concepts.
     
  14. Oct 15, 2014 #13
    Talking of rocks becoming viscous under great pressure... That pressure is the weight of the crust above. Given that a solid, say my lump of lead, is composed of tightly bound atoms, how does gravitational force get expressed through that medium? The top layer of atoms/molecules will have weight which bears on the next layer and so on cumulatively, but what is it that bears and transmits the weight? Wikipedia says "Metallic solids are held together by a high density of shared, delocalized electrons, known as "metallic bonding". In a metal, atoms readily lose their outermost ("valence") electrons, forming positive ions. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud."

    So if at the submicroscopic level we have atoms bound together by electrostatic interactions, what transmits the gravitational force? Or is the effects of this force a macro level effect for large objects?

    In this case I don't mind how detailed you get, I can chase down the meaning of that by doing some searching and reading if I have the right pointers.
     
  15. Oct 15, 2014 #14
    It might be worthwhile to note that "solid, liquid, gas" are legacy terms coined before suspensions, colloids, amorphous solids, etc. were understood. Additionally the concept of solid originally assumed physical contact of solid components rather than electrical field repulsion. Dragging legacy into the present is a large, annoying problem of "inertia".
     
  16. Oct 15, 2014 #15
    enorbet, can you expand on what you mean by "Additionally the concept of solid originally assumed physical contact of solid components rather than electrical field repulsion"?
     
  17. Oct 15, 2014 #16

    sophiecentaur

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    Chemistry and the properties of material is all to do with electric Forces. This is because the Electric force is far stronger than the gravitational force. Electric forces are always fairly localised because there are always equal numbers of positive and negative charges around and the forces 'cancel out' once you are a short way from each electron or proton in a material. But there are no 'negative' gravitational forces so they just add up and add up, all over an object, pulling the whole thing downwards,. When the Electric forces on a molecule in a substance can manage to withstand the gravitational forces, it will stay solid. If they are too weak, it will break or 'flow (liquid or plastic behaviour).

    Electric forces connect all the particles together and 'transmit a force on one part to another (if the object is rigid.)
     
  18. Oct 15, 2014 #17
    Sure. My apologies if I was vague but trying to generalize about many cultures over many centuries of time tends to get that way. Without going into what each and every culture thought things were made of, before humans understood anything about the real nature of matter and even well after the concept of atoms was conceived by some and well up into the Middle Ages, people assumed, because of intuition, that whatever elements they believed in (such as Earth, Air, Fire, Water and Aether) could actually come into physical contact with each other. They didn't realize that all matter is well over 99% empty space.

    In actuality what we feel is the repulsive field of electrons. This is one way that we can "see" how gravity is so weak by comparison. As is often joked, "it isn't the fall that kills you, it's the sudden stop" :D
     
  19. Oct 16, 2014 #18
    OK, I can see that some kind of electrostatic bonding (again sorry if my terminology is incorrect) binds the atoms and molecules together. But this is an attractive force, and a force is not a thing. If gravity is acting upon, presumably, the atoms, and they are bonded by an attractive force, what actually propagates the gravitational force on each atom such that the object weighs cumulatively more as we progress from 'top' to 'bottom' of the object? And how does that work for gasses which presumably do not enjoy the same attractive qualities (or at least, their repulsive qualities overcome what attractive qualities they have).
     
  20. Oct 17, 2014 #19
    Anyone? or even a pointer where I'd go or what to search on. How does the weight of an object accumulate through the object? Take my lump of lead resting on a table. Presumably the first layer of atoms bears on the next layer and so on, until the force is expressed on the table's surface. But how exactly does the 'weight' propagate through the molecular structure? Given that the lump of lead is largely space, and that the atoms are joined together by electrostatic bonds, what is the conduit for the expression of weight cumulatively? Or is that effect only a macroscale effect? Or am I just completely failing to understand how mass and gravity interact?
     
  21. Oct 17, 2014 #20

    sophiecentaur

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    There's your problem. The electric forces will both attract and repel, depending on how you stress the object. Remember, the electrons are attracted to the positive nuclei but they will repel each other and the nuclei will also mutually repel, if you try to push them together. The equilibrium shape of the object is it shape when the internal forces, alone. are at work. At the bottom of your object on a table, the gravitational forces all add up to the total weight and the object will have distorted a finite amount, as a result.
    Imagine, as a model for this idea, a heavy and weak coil spring, which can only just support its own weight on a table. The coils at the top will only have a small weight acting on them from above, so they will be widely spaced. The ones at the bottom will have more weight on them so they will be close together. The spring will not actually be uniform.
     
  22. Oct 17, 2014 #21
    Thanks Sophiecentaur. I am not sure that clarifies things for me. Electric forces attracting and repelling are all very well, but it seems to me neither effect is a gravitational one? The gravitational force is a separate force. Are you saying (in simplified form) that as gravity pulls the topmost atom down, it exerts a repelling force on the one below it, which now has both the repelling force pushing down and gravity pulling it down. Those summed forces are then applied to the one below that and so on?

    I can see that, sort of, but that depends on the relationship between the atoms in the solid. That is, they are bound by an attractive electric force but their electric forces repelling each other are how gravity expresses through the solid. I am not sure I see how that works for a gas.

    A gas is not a bound thing is it? One of the properties of a gas is that it expands to fill its container hence I assume you can have a very low density gas. A low density gas in a large container would have its atoms relatively homogeneous at equilibrium wouldn't it? And the distance between atoms relatively large. Do those electric forces work over those scale? I thought the idea was that the electric forces are strong but only locally.

    Given our gas molecules are free to zip around in that container according to their temperature and density, there is no actual 'bond' between them. So how does gravity affect the gas using the model of weight you describe for a solid?
     
  23. Oct 17, 2014 #22

    sophiecentaur

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    Why should they be gravitational? The gravitational forces are between each of the atoms, individually and the nearby huge mass of the Earth. The g forces between the individual atoms are more or less zero. If you took the thing out into deep space, the same internal forces would be there. If you stretched it or compressed it with your hands, it would still distort with no measurable external gravitational influence. You have asked why a solid ' resists' the effects of gravitational forces. It is because it has other forces at work inside it. Both sets of forces are involved. The electric forces are strong but 'local' and the gravitational forces are very weak but all add together because there is only one sign of the force. With a solid, the forces all establish equilibrium by the object taking up a modified shape.
    BTW, this all happens for an object that's suspended - but the other way round, with the maximum stretch being at the top and zero stretch right at the bottom.

    The gas thing is another issue but the basics must apply. The molecules at the bottom of a container will be ever so slightly closer together because of the fact that pressure is greater - due to the weight of gas on top. Think of the atmosphere getting lower and lower pressure as you go up. The AP grips to a half, every 5000m of altitude or so (very roughly and ignoring all the other effects, such as temperature) due to the 'weight of gas above'. The g field hardly changes at all between ground level and 100k but the AP drops very low by that height. Have you read about the Gas Laws yet?
     
  24. Oct 25, 2014 #23
    OK, I've had a read of the gas laws, and while I generally grasp the concepts, the detail is well beyond me. It hasn't really helped me with my question though.

    With the solid, it retains its shape because the local forces overcome the gravitational forces. I understand - speaking very simplisticly - these local forces are composed of several forces. The intramolecular forces which holds the atoms together, and the intermolecular forces which hold the molecules together. The intermolecular forces are weaker than the intramolecular forces. And some kind of 'repulsive' force that stops them getting too close together (or is this repulsion an aspect of the inter and intra molecular forces?).

    So that's why a solid is solid.

    But I still unclear on the propagation of gravity. If gravity attracts the atoms, then each atom is pulled 'down'. I assume it then pushes on the atom below it which must push on the one below it etc. This 'push' must be via either of the attractive forces I describe above with the repulsive force acting to retain the solid's shape under this pressure. Obviously the pressure can overcome the local forces and cause the solid to deform.

    Now, given all the atoms/molecules are bonded together, the force of gravity must propagate sideways as well, internally? While gravity is acting in a downwards direction, the bonds are between all atoms/molecules and thus the force would seem to me to spread throughout the structure.

    In the same way that gas pressure inside a container is equal at all points, would not the pressure inside our solid be equal at all points?

    This still doesn't help with the gas though. If the bonds are how the gravity or 'weight' is propagated, whatever happens with a gas? The molecules of a gas are not bound to each other. The intermolecular bonds are too weak to overcome the rapid motion of the molecules, which are free to move more or less at random.

    I understand that the effect of gravity on gasses is to change the average distribution - more molecules lower in the atmosphere, or alternatively fewer as you go higher and urther from the earth.

    The effect is then higher pressure at lower altitudes with the highest pressure at sea level (ish).

    But that pressure is not actually 'weight' in the way that weight is expressed by a solid, IF my description above is correct.

    What's really going on?
     
  25. Oct 29, 2014 #24
    I've come back to this one and it reminds me that I generated a long thread here a while back about atmospheric pressure. The end result was that I learned that the weight of the atmosphere is essentially equal to surface pressure and that 'weight' was transferred through the vertical column by way of the energetic collisions of the air molecules. That still bothers me though.

    Now, given what I have learned on this topic, I offer the following pretty simplistic explanation of how a solid is solid. As I summarised above from what other commenters have offered, a solid is solid because attractive forces hold the molecules/atoms together. I assume that the resistant forces of the particular composition of the solid determine the density of the object - that is, the closer together and more molecules/atoms there are per volume then the denser the object.

    Liquids are different and deform more readily than a solid. I think the reason is that the bonds are less strong and so the molecules in the liquid tend to be bonding and unbonding rapidly as the molecules move around.

    A gas is much more energetic and the molecules/atoms do not bond at all. As a result a gas tends to expand to fill its container in contrast to a solid or a liquid which are more tightly bound.

    I am still left with the question of weight. Is weight propagated in a solid as I imagined above? That is, gravity pulls the atoms down and each atom then pushes on the atom below it which must push on the one below it etc. This 'push' must be via either of the attractive forces I describe above with the repulsive force acting to retain the solid's shape under this pressure. Obviously this accumulating pressure can overcome the local forces and cause the solid to deform.

    But from my earlier discussion around gasses, I learn that weight in a gas is propagated via the collisions of the molecules applying force cumulatively from top to bottom. But I still struggle to grasp how these collisions constitute both pressure and weight. I posed the question last time of a safe in a vacuum, and I don't think this was ever resolved as the conversation branched off to other details.

    The equation PV=nRT describes the derivation of pressure, but this makes no use of mass or gravity. This equation tells me that pressure depends on number of molecules, volume and temperature to give rise to pressure, and my reading tells me that pressure in a container is equal on all walls. And the pressure on the walls is the impact of molecules per unit area of surface. So what particular component of that pressure is just pressure arising from temperature, volume and number of molecules and which is from weight?

    Going back to my safe in a vacuum - if I transport a safe containing normal old air at 1 atmosphere into a vacuum and maintain internal temperature, will the pressure change or stay the same? I assume the same, but if so, what component of that pressure is derived from the 'weight' of the overlying atmospheric column which is no longer present?

    I just cannot get my head around this question of weight of an object versus a gas. And a liquid too, come to think of it.
     
  26. Oct 29, 2014 #25

    sophiecentaur

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    Where is the difference? The weight of the air in a column happens to be spread evenly over the floor but you could achieve this with care, if the column were solid. The force on a piece of the floor is due to the sum of weight forces of sections of the stuff above, transmitted by direct action via the inter molecular forces in a solid or by change of momentum during the collisions of the molecules in an ideal gas. The force on a horizontal element of a vertical bar / column is due to the weight above and the same goes for gas in a vertical cylinder. It increases as you go down because there is more stuff above. This force plus the weight of the section acts downwards on the next lower section and so on . . .

    Btw, in an ideal gas, it is assumed that there are no 'bonds' between the molecules - or, in reality the attractive or repulsive energy involved is significantly less than the kinetic energy of the individual molecules. When they collide, they lose no energy (elastic collisions) and their Momentum is exchanged. There is increasing pressure as you go down (density increases) so there are more collisions per second. That's a circular statement, I know, but if you think of the effect of gravity over a thin horizontal slice will be to accelerate the molecules downwards so they will transfer more momentum downwards than upwards, resulting in a net upwards reaction force on the slice. This difference will just balance the total weight of the molecules in the slice, acting downwards.
     
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