How does a polar covalent compound affect melting points?
957540
You have three types of bonds (essentially)\n\nYou have ionic, non polar covalent, and polar covalent.  Each has a strength in bonding.\n\nIonic is the strongest.  These will have the highest melting points (just try to melt salt for instance).  The non polar covalent bonds tend to be liquids or gases at room temperature (very low melting points).  Examples of these would be Methane , Benzene, ethane etc.\n\nPolar covalent bonds have a stronger pull on the electrons and therefore tend to be higher in melting point than non polar and lower than ionic.
729609
In covalent bonds, elements actually lose and gain electrons to other elements in the compound.  Polar bonds are covalent bonds that don't require hybridization, and Non-Polar bonds do require hybridization.  Ionic bonds occur when elements share an electron but do not lose it.  Therefore, you can tell that NaCl is ionic because Sodium has an extra electron and Chlorine needs one, so Sodium shares it's electron with Chlorine, forming a very strong bond.  Ionic bonds, however are broken by Polar-Bonded liquids such as water, which is why NaCl dissolves.
244868
melting points are mostly determined by the bonding between atoms and the polarity of the atoms in the rest of the molecule (if any).  In metals, it's all about the bonding.  All metals have metallic bonding; not all metallic bonds are created equal.
1094405
A covalent bond is a bond in which the atoms making up the resulting molecule "share" the electrons in the form of covalent bonds.\nAn ionic bond is an extremely one sided "sharing" of electrons, so one sided, one atom is said to have completely stripped of an electron from another atom.\n\nWhat determines if the bond is covalent or ionic is the difference in the electronegativities of the two atoms bonding.  If the electronegativity difference is high enough, the bond is said to be ionic.  If the difference in electronagativities is significant, but not quite to the level of a full ionic bond, the bond is said to be polar (which may or may not mean that the resulting molecule is polar, this depends on several things).\n\nLooking at a chart of [Pauling] electronegativities,\nhttp://www.psigate.ac.uk/newsite/reference/plambeck/chem1/p01244a.htm\none can see that most elements are somewhere in the 1 to 2 range, with only a few elements getting much lower or higher than this value.  Unless the difference is high enough (usually a difference of about 1.5 is considered ionic), then the bond will just be polar, or less so.\n\nSo, all elements have the potential to theoretically bond ionically, but only in certain combinations with other elements.\nFor example, you will never see an ionic bond between Carbon and Hydrogen since the difference is just .4, but an ionic bond between Lithium and Fluorine is easy to make with a huge difference of 3.0.
809957
The most polar bond will be between the atoms with the greatest difference in electronegativity.\n\nF2 has bond that is not polar since the electrons are equally shared between the two identical atoms. The electronegativity series is As<S<Br<F so the biggest difference is between As and F and the most polar bond is in AsF3.\n\nAs belongs to the same family as N. Thus in AsF3 it forms 3 single bonds and also has 1 lone pair. So the electron-group arrangement is tetrahedral but since there are only 3 identical bonds there will be a net dipole moment.\n\nI already explained why F2 has no dipole moment\n\nBrF3 has trigonal bipyramid electron group arrangment (3 bonds+2 lone pairs) but not corners are occupied by identical atoms (2 have lone pairs of Br) so the dipole moments do not cancel out and the molecule has a net dipole moment\n\nSF4 has again trigonal bipyramid electron group arrangement(4 bonds + 1 lone pair) with one lone pair at one of the corners so it has a net dipole moment\n\nSF6 has octahedral electron group arrangement (6 single bonds)\nand since all corners have F atoms the dipole moments of the bonds cancel each other and the molecule has no net dipole moment.
792713
In covalent, atoms share electrons, but in ionic, one atom gives them to the other.  So in an ionic compound, there is a positive ion and a negative ion.  They ions can separate when put in water, and then the electricity can pass through.  Covalent compounds don't have a positive pole and a negative pole, and they don't ionize in water.  Also, the ionic bond is kinda stronger, so usually the compound is solid at room temperature (salt is ionic, oil is covalent).  You have to know that there is a third category, a polar molecule (like water).
952034
polar refers to bonds and molecules where the electrons tend to be closer to one atom than another because of the "strength" with which the atoms "pull" the electrons.  This "strength" is usually measured by a number called "electronegativity".  There should be a chart or list of these numbers in your text book.\n\n"Covalent" is a type of chemical bond where the electrons are shared between atoms, rather than transferred (ionic bond).  Usually covalent bonds form between two non-metals.\n\nFrom your question it looks like the first two terms (polar and nonpolar) are referring to individual chemical bonds.  Find the two atoms attached to either end of the bond, look up their electronegativities and see if one atom "pulls" on the electrons stronger than the other.  This would produce a polar BOND.\n\nThe last two (nonpolar covalent; polar covalent) could refer to both a bond or a molecule.  For the molecule you need polar bonds,  and then you have to know the shape of the molecule since it is possible for the individual bond "pulls" to cancel each other - kind of like a tug of war.  Generally if a molecule is symmetrical (triangle, tetrahedron or linear triatomic) the "pulls" cancel.  Although the BONDS may be polar covalent, the entire MOLECULE would be nonpolar covalent.
953050
If the difference in the electronegativity between the two elements is less than 1.67, the bond will be covalent.\n\nNote: As to the statement that electronegative atoms don't form covalent bonds, that's nonsense.  ALL atoms are electronegative to some extent.  Just go look up the electronegativity series in any general chem. text.  Or check out this:\nhttp://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.html
674689
It's because of the lower pressure with the higher altitude.  Remember the phase diagram denotes the phase of a particular compound in terms of the pressure and temperature.  If you're sufficiently talented with respect to the maths, refer to a physical chemistry text and research upon chemical equilibrium and dG/dn.\n\nhttp://www.freewebs.com/chemistryhomework\n\nEDIT:  \n\nYou said:\n------------\nThe phase diagram doesnot explain this.\nYou forgot that it takes higher pressure to shift the freezing point higher in the phase diagram, not lower pressure like up in the mountain.\n--------------\n\nIncreasing the pressure makes it easier to melt ice, e.g. the pressure exerted by ice skates result melted ice at the surface, this is a common excerpt in introductory chemistry texts.  Again, also consider dG/dP and chemical equilibrium.
989710
1. impurities in the compound\n2. temperature of the surface thats heated on\n3. old lab furniture.\ncant say more!
327842
I am pretty sure that it is due to polarity.  Since sulfur forms nonpolar S8 molecules, they are nonpolar, where water has a + end and a -end, making it polar (has a dipole moment).  The general rule is like dissolves like.  Molecules that a alike can interact on  the molecular level and form solutions, unlike molecules can not.
665955
it must be polar because the structure is tetrahedral with a lone pair at one corner of the tetrahedron and Cl atoms at the  rest three corners. so the individual As-Cl bond moments are not cancelled bcoz of the asymmetry of the molecule and the net polarity cant be zero.
308551
Xavier is dead wrong. NH4+ is tetrahedral. Most of his answer applies to NH3, which is trigonal pyramidal, and not the same thing as NH4+.\n\nPolar molecules have a large net electrical dipole moment. In a sense, the dipole moment tells you how close you are to becoming an ionic compound. If there is no net dipole moment, the compound is completely non-polar. On the other extreme, if it's an ionic compound, it's completely polar.\n\nLook carefully at your question. NH4+ is an ion, not a molecule, and the terms "polar" and "non-polar" don't really apply to ions. The reason is that ions must be part of an ionic compound, and the ionic compound would be considered polar by definition. For example, (NH4)Cl is an ionic compound and therefore very polar, but asking whether NH4+ is polar is no different than asking whether Cl- is polar.\n\nA couple of good textbooks for further information and examples would be "Organic Chemistry" by Solomons and "Organic Chemistry" by Peter Vollhardt.
331795
your chemistry teacher probably wanted to know the + or - sign, then it should be + because when ice melts, it absorb heat to overcome the van der walls force between its molecules and ice also has high hydrogen bond between its molecules to overcomed. so, ice needs extra energy (heat) and the sign is positive. \nif he/she is asking bout the other sign, then it is the melting part. if heat is released, u can feel the heat on the surface of the material but if heat is absorbed, u can feel the coldness on its surface. if you hold ice in ur hand, it will melt more faster coz the ice absorbs heat from ur hand to overcome its force between bonds ( like i explain above)
461802
The Be2+ cation is smaller in size (being above the rest of the group 2 elements) and hence has high charge density. This enables it to easily polarise the comparatively larger Cl- anion, which results in a covalent characteristic of the compound. As explained clearly in the previous answer, dative bonding occurs in the compound and BeCl2 usually occurs as a dimer i.e Be2Cl4 (polymeric type) based on the linear molecular structure of BeCl2.\n\nPolarisation occurs between a small cation with high charge density and a larger anion. Since the size of the cations of the group 2 elements increases down the group (more atomic radius), charge density around the cation decreases and hence their ability to polarise the Cl- also decreases, resulting in an ionic compound.
883690
The compounds have a high lattice enthalpy, thus much energy is required to break to ions up in solution. The also have very low solubility products, that is the product of the compounds ions in a saturated solution.
613215
Simply put, the salt lowers the freezing point of water.\n\nHow does it do this?  From kinetic molecular theory, the temperature of an object is proportional to the random motion of the molecules of the object.  As water cools down the molecules move slower and stick closer together until the intermolecular forces are stong enough to lock the molecules into a crystal structure (overcome random motion) and water freezes.  The salt lowers these intermolecular forces by interacting with the water molecules: there is some salt-water interaction and thus less water-water interaction.  Thus, a lower temperature is required to lock in the crystal structure (lower melting point) and the ice melts.\n\nThis works to about -30 or -40 C IIRC.