77

Tungsten's melting point of 3422 °C is the highest of all metals and second only to carbon (3550 °C) among the elements. This is why tungsten is used in rocket nozzles and reactor linings. There are refractory ceramics and alloys that have higher melting points, notably $\ce{Ta4HfC5}$ with a melting point of 4215 °C, hafnium carbide at 3900 °C and tantalum ...


47

Sorry, can't comment here, but I wanted to more directly answer your question. Blacksmiths avoid melting their forges because the "heat" that can melt or oxidize iron and steel is actually contained in a ball in the center of the coal. In fact, maintaining coal "structure" is an important skill in blacksmithing. To clarify better, imagine a hollow in the ...


32

It's a very general statement, but it's not always true. I'll explain why it's often true, and give a counter-example at the end. Your majority component B and the impurity (let's call it A) form a binary system. In most cases, such binary mixtures exhibit a solid–liquid phase diagram as follows: (image taken from these lecture notes). This binary phase ...


29

Some factors were hinted, but let me put them in an order of importance and mention some more: metals generally have a high melting point, because metallic interatomic bonding by delocalized electrons ($\ce{Li}$ having only a few electrons for this "electron sea") between core atoms is pretty effective in those pure element solids compared to alternative ...


27

Good question. Let's assume the container is infinitely strong, non-deformable, and constant in volume. Let's also assume that cooling the water is an equilibrium process -- that way, we won't have any supercooling. At equilibrium, the first tiny bit of ice that freezes will take up more volume than the water it froze from. This will raise the pressure ...


25

Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions than $\ce{PCl3}$ with its mere dipole-dipole attractions, hence the higher melting point. If not for that fact, you deduction should have worked just fine.


22

Diamond (carbon) does not melt at 1 atm. It sublimes to vapor. Using carbon's theoretical phase diagram below (from Wikimedia), "liquid diamond" could be achieved at about 10 GPa (99 thousand atmospheres) and 5000 K (4700 °C). Edit: In fact, heating up diamonds at 1 atm turns them into graphite first. Only further heating would cause sublimation, as ...


22

Honey is indeed a complex mixture containing more than hundred compounds. As for Wikipedia and depending on the point of view it is a supersatured liquid solution a viscous supercooled liquid (in the sense that it can get so viscous as to appear solid, without affecting its status of being a supersatured solution, and undergoes glass transition). https://...


19

Thermodynamically, you're considering the chemical potentials ($\mu$) of the liquid and solid(s), specifically the temperature where they're equal. In a mixture, the potential is lower as the disorder (entropy) has increased, so all things equal, it will favor the liquid over a purer solid where there can be more disorder. Chemical systems seek to lower ...


19

The melting and boiling points of noble gases are very low in comparison to those of other substances of comparable atomic and molecular masses. This indicates that only weak van der Waals forces or weak London dispersion forces are present between the atoms of the noble gases in the liquid or the solid state. The van der Waals force increases with the ...


19

We use an levitating furnace to heat samples of refractory ceramics up to approx $3000~^\circ\mathrm{C}$. Its for research purposes, so the samples are small (2 mm) beads. These are balanced on a jet of argon and heated with $\ce{CO2}$ lasers. Here is a paper which talks about the technique: D. Langstaff, M. Gunn, G. N. Greaves, A. Marsing, and F. Kargl, ...


19

Wolfram company doesn't conduct any experimental determinations of physical constants for chemical compounds and uses literature data sources. The webpage for ChemicalData Source Information lists numerous sources of chemical information used by the company's products, including Wolfram Alpha. Wolfram Alpha Knowledge Database is linked with 87th ed. (2006) ...


18

High symmetry molecules fit into crystal lattices especially well (higher m.p.), but are volatile for having fewer van der Waals interactions (lower b.p.). $$\begin{array}{lrr} \hline \text{Compound} & \text{m.p.}/\pu{°C} & \text{b.p.}/\pu{°C} \\ \hline \text{pentane} & −130 & 36.1 \\ \text{isopentane} & −160 & 27.2 \\ \text{...


17

Generally, the melting point of para isomer is quite higher than that of ortho or meta isomers. This is due to the fact that it has a symmetrical structure and therefore, its molecules can easily pack closely in the crystal lattice. As a result, intermolecular forces of attraction are stronger and therefore, greater energy is required to break its lattice ...


16

Gallium melts at 30 °C but doesn't boil until 2200 °C. If 30 °C is a bit too warm to count as "room temperature" or "normally" for you, I found an old paper that recommends tetralkyl silanes such as tetradodecyl silane as lubricants that are liquid over very wide temperatures. Addendum: Dowtherm A is a eutectic mixture of biphenyl and diphenyl ...


16

Melting and dissolving are all the same when you look at mixtures close to saturation. You can say water lowers the melting point of the sugar, or that the solubility of sugar increases with temperature. Different description, same fact. What makes this seem different from e.g. a salt water solution is that the molten (i.e. non-crystalline) sugar is fully ...


15

source One way to raise the melting point of water is to increase pressure beyond about 635 MPa. By raising pressure you could get the melting point to be even greater than the normal boiling point. A second way is to lower the pressure, but this can only increase the melting point by 0.01 K. A third way would be to added enough of a high melting point ...


14

Yes . You are right that structural symmetry comes into play . Boiling point depends upon intermolecular interactions which over here is more in cis due to its net dipole moment . The dipole moment enables electronic interactions which hold molecules together . This shows some general factors of boiling point . Also the below link to the google book ...


13

I don't believe there is an equation that you can use for melting points of a general substance as a function of pressure (since the melting phase transition has a lot to do with the geometry of the molecule and the structure of the solid), but there is one for the boiling point of any pure substance when you are not near the critical point. The liquid-...


13

Obsidian This is a volcanic glass, formed when rhyolitic lavas cool too quickly for crystals to form. From a chemistry point of view, it's a mixture of silicon dioxide, aluminium oxide, with sodium ,potassium, calcium and iron oxides in various quantities. These lavas will also contain significant volatiles (water and $\ce{CO2}$) held in solution by ...


12

One could melt them floating on a pool of high-boiling point denser metal, or in space where they can readily be contained. Or one could create a thick actively cooled shell and melt them inside it, melting part of the shell as well. Finally, it's probably not very practical, but one could use an air jet to keep then suspended away from other matter and then ...


11

The answer is given in Faraday’s own work, Experimental Researches in Electricity, Volume 1, freely available thanks to The Project Gutenberg: 399. Other substances, which could not be melted on glass, were fused by the lamp and blowpipe on platina connected with one pole of the battery, and then a wire, connected with the other, dipped into them. In this ...


11

The polar aprotic solvents (for example dimethylformamide, mp −61 °C, bp 153 °C, or hexamethylphosphoramide, mp 7 °C, bp 230–232 °C) would be a place to start. Silicone oil is often used in heating baths – one product in the Aldrich catalog is advertised as having a working range of −40 °C to +230 °C.


11

Now that's a great question indeed! Evidently, at 0K all elements except helium are solids, at 10000K they are all gases, so someplace in between the number of liquids must reach a maximum; what and where might that be? Well, there is no formula or theorem that says liquid hydrogen must boil at 20K, nor is there such a thing for any other element, so this ...


10

I don't really like parts of this explanation (was hoping to find a better one here actually), but it's the best I know. I'll build on suggestions by @michielim and @vrtcl1dvoshun. (Note: I think most of this argument can be transposed to the orbital overlap picture of bonding, but it might be slightly trickier to describe.) From the physicists' "electron ...


10

You've figured out one half of the Hall-Heroult process. Just melting alumina on its own won't liberate the oxygen. The reason is difficult to explain without getting into chemical thermodynamics - one way to think about it is that the lowest energy state is one where oxygen is bound to alumina - where the electrons from alumina have been "taken" by the ...


10

The strength of the ionic bond depends on Coulomb's law for the force acting between two charged particles where larger force translates to a stronger ionic bond. The equation is $$F = \frac{-k\cdot q_1 \cdot q_2}{r^2}$$ $k$ is a constant; $q_1$ and $q_2$ are the charges on the ions and $r$ is the distance between the ions. So the larger the charge the ...


10

With regard to organic compounds, you might be interested in group contribution methods, e.g. the Joback method. These methods divide a molecule into functional groups or similar small building blocks and estimate the melting point (and other thermodynamic properties) based on the contributions of the individual groups.


10

Melting point depends not only on the structure of the molecule itself, but also on the structure of the solid (crystalline) phase and molten/liquid state of the substance. That is a much bigger computational problem than calculating energetics for a small molecule or ensemble. It is not easy to predict how a molecule (especially a complex one) will ...


10

Sure. Even discounting non-equilibrium conditions, any substance which expands upon freezing will by symmetry melt under pressure, even if the pressure is caused by supporting its own weight. There's a simple example: water! A sufficiently tall self-supporting block of ice standing on a flat surface will create enough pressure at its bottom that it could ...


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