What is the melting point of a substance?

[treestar]

I've only had high school and college freshman chemistry.

But melting point and freezing point are identical.

So how does a substance "weaken" before it is heated to its melting point? It seems that it would only start to weaken once once it is at the melting point. Before that, it would not be weakened at all.

People I have debated on this say to imagine a stick of butter getting soft. Exposed to room temperature it may get softer without actually melting. Or is it that room temperature is the melting point and that it is a matter of time (it only melts faster if exposed to an even higher temperature).

[rman]

1. what are you asking?

Do you want to know the physics behind the weakening of a material due to heating, or do you question whether or not a material does weaken at all before it melts?

Simple experiments can demonstrate that materials do weaken with increasing temperature.
My position is that it can be accepted as fact without exactly understanding the physics that is involved.

[treestar]

4. When does a material start to weaken?

One would think there would be a way to know what temperatures it would be exposed to when it starts to weaken.

Water must not start to even weaken until it gets to 32 degrees. If you have a snowpile big enough, it won't be gone until it has been over 32 degrees for several days or even weeks.

[rman]

7. each (solid) material

has a temperature range where it has (near) maximum strength. There needs to be some optimum balance between elasticity and stiffness of the material for it to be able to resist some external mechanical force. When steel gets very cold it will become brittle and actually fracture instead of bend, stretch and sheer - under a much smaller force then when it is not so cold. If it gets above some temperature it will bend, stretch and sheer much easier. That's no good either.

The range of temperature where there is actual weakening of the material with increasing temperature, is different for different materials. Water (frozen water) has a very narrow range; it is solid ice and maintains whatever strength it has right up to the temperature where it completely loses all strength and becomes liquid water.

For all i know most of the numbers that have been cited about the weakening of the steel of the WTC are correct. Claims regarding this that support the OTC usually don't mention numbers; they're usually a non-quantified "weakening of the steel due to heating by fire".

[hack89]

8. Everything you ever wanted to know abut steel and fires ...

http://www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/TestData/default.htm

Some good reference material here.

[treestar]

10. OK, I'm getting that, but what is this phenomena

Say a blacksmith in a forge - he heats the metal to where he can bend it. It is not yet at the melting point.

But wouldn't science have a definition for the point at which the substance would be pliable?

[sabbat hunter]

15. elastic point

at a blacksmiths forge it depends what he wants to do with the metal. the more he wants to bend it, the hotter he needs to make it. at a lower temperature it will be brittle and crack, higher temps it becomes more elastic.

every substance has a different point and range for this. mercury is liquid at room temperature.

iron/carbon steel used by a blacksmith. higher carbon steel have a lower melting point.

as steel heats up it loses its strength and hardness. it doenst have to get to its melting point 1370 degrees C (2500°F), to lose its strength. that is when it becomes a liquid.

it starts to lose its strength far before then. as you can see from a blacksmith forge, the metal is no where near its melting point, but it becomes hot and softer and far easier to work with. the blacksmith in general wants to get it as hot as possible so he has to use less force to shape it.

from my speaking with blacksmiths at ren faires, the iron/steel needs to be around 1300-1600 degrees F to be worked with effectively. much higher and the metal can actually be too soft, splattering instead of shaping when hit with a hammer. too low and it becomes brittle and cracks.

the highest temps a blacksmith will work steel at is 2400 when he wants to join to pieces of steel togehter (the welding temp).

but if a blacksmith wants at least 1300 degrees so he doesnt have to work to hard to shape the metal, that doesnt mean the steel hasnt lost much of its strength at a far lower temperature.

[LARED]

2. Latent Heat

Latent heat is the energy needed to convert a solid to the liquid or visa-versa. As energy is transferred between the two states the temperature stays the same. Also, changes in the strength and other physical characteristics during this transition are different for different materials.

For instance water changes from a solid to liquid with nearly no observable phenomena during the transition or heating up. Steel changes color, and gets weaker in a non-linear fashion as temperature raises.

[TheWraith]

3. Materials can weaken before they melt.

"So how does a substance "weaken" before it is heated to its melting point? It seems that it would only start to weaken once once it is at the melting point. Before that, it would not be weakened at all. "

That's not the case. Due to the nature of materials, most things have different strengths at different temperatures. The same way that something becomes brittle if it freezes, it can do the same if it becomes too hot. For instance, steel melts at around 2500 degrees. However, it loses its loadbearing capability at just 600 degrees. It varies from material to material, but in general it's not as simple as something only weakens when it melts. It's similar to the butter--it won't melt if left at room temperature, but it does soften. It gets harder if you put it in the refridgerator, and even harder still if you freeze it. Room temperature isn't enough to melt it, but it does lose all tensile strength.

[treestar]

5. But at what point do they start to weaken? At the melting point?

Or at some other point?

What backs up the statement: Steel melts at 2500 degrees, but loses its load bearing capability at 600 degrees.

[William Seger]

6. Empirical testing

The load carrying capacity of steel at various temperatures is determined by simply testing it.

"It seems that it would only start to weaken once once it is at the melting point. Before that, it would not be weakened at all."

No, steel does not behave like water; it behaves more like plastic. At 600^oC, structural steel loses 40% to 50% of its load carrying capacity due to increased plasticity.

[TheWraith]

9. It depends on the material, but often well before the melting point.

Some materials, such as wood, will burn before they melt. Steel behaves more like plastic, in the sense that it will become weak and malleable long before it actually melts. Untempered glass does something similar, albeit at different temperature ratios.

[treestar]

11. OK, so what are the conditions for these substances and how

are they different from water/ice which doesn't have this interim period?

Although there are ice sculptures, so there may be some point at which ice can be bent. Unless those are made by freezing in a mold.

[jberryhill]

12. Ice Sculptures Are Carved

Ice does not appreciably bend, nor do most polar covalent crystals. Nobody makes stone sculptures by bending stone.

Not all solid materials are the same. There are amorphous solids, like glass, and there are crystalline solids, which may be ionic, covalent, or polar covalent crystalline materials, whose behavior under different conditions is dependent on the type of bonding involved in holding it together. Think about, say, comparably thin pieces of glass and quartz. They are both composed of SiO2, but you are going to get a lot more bend out of the amorphous glass than you are out of the crystalline quartz having the same dimensions. If you remember bending glass tubing or rods in your chemistry class, then you might appreciate that you can't do that with a rod of ice.

Then, there are large scale features of various materials - crystalline grain structure, etc. - that determine how a big chunk of the material is going to behave. Additionally, most crystalline solids have several different ways in which they arrange themselves into crystals depending upon temperature and composition. Steel is a solid solution, that may have a variety of different crystalline phases depending on its composition and how it was formed and worked.

The word you are looking for, in reference to how "bendable" something is, is "ductility" which, for metals, is a function of temperature. The "melting point" of a substance is the temperature at which, for a given pressure, a liquid phase and a solid phase may coexist. It has utterly no relevance to whether a steel beam supporting a load will bend or break.

Going back to your blacksmith working a piece of metal, think about this:

What is the temperature at which coal burns?
What is the melting point of steel?

And then ask yourself how it is possible for a blacksmith to work steel using a coal fire. There are people here who will tell you it is not possible for a blacksmith to do what he does because coal doesn't burn anywhere near the melting point of steel - which is entirely irrelevant to what is going on in a forge.

Of course, a blacksmith's forge has a bellows which drives a supply of air up through the coal bed. You can get quite a hot fire going - well above the temperature at which a material ordinarily burns - if you have a chimney type arrangment drawing air upward into where the material is burning. That can happen naturally, for example, if you have an arrangement with a number of vertical shafts beneath where the fire is, so that the fire itself draws air up the shafts and the increased heat provides positive feedback to suck even more air up the shafts. If you've ever tended your fireplace on a really cold day, you'll notice that there's a lot more going on with your chimney than just providing a tube for the smoke to get out, and that there's a reason why your fireplace has a metal contraption for holding the wood above the bottom of the fireplace.

[treestar]

13. Under what conditions would a steal beam bend or break?

In the WTC, was it bent or broken?

[wildbilln864]

14. Well, all the pieces were....

small enough to be carted off as quickly as was possible! So how did every steel columb in that building simultaneously get rendered into pieces small enough to be loaded onto dump trucks?

[sabbat hunter]

16. during the clean up

there were people welding and cutting pieces up. they didnt all disintergrate. there were many large pieces that needed to be broken up.

[jberryhill]

17. What makes you think that?

Note the size of the dump truck relative to the much larger steel columns:

[wildbilln864]

18. Regardless....

I'm sure 911 was an inside job! These three buildings should not have fallen completely down!
You believe whatever you want!

[MervinFerd]

19. BRILLLIANT argument! Simply BRILLIANT.

And encapsulates Truther logic in three sentences:

1, I am sure of it.
2, I am sure of it.
3, And don't bother me with facts.