Physical properties of salt

Consider some of the physical properties of matter: physical state, melting and boiling points, crystal structure, electrical conductivity. Physical state of matter is determined by the force of attraction between its constituent molecules and temperature. The solid molecules strongly enough adhered to each other and their movement is limited oscillations about fixed positions. With increasing temperature, the energy molecules increases, fluctuations are becoming more intense and eventually molecules acquire enough energy to make a reciprocating motion. This solid melts (if the intermolecular interaction is strong enough) or sublimating, turns into a gas whose molecules are moving chaotically. Melting point (solidification) - is the temperature at which a solid turns into a liquid (or liquid turns into a solid). Melting point of water is 0 ° C (Celsius) or 32 ° F (Fahrenheit). Since the amount of melting of the body changes slightly, the pressure has little effect on the melting point.However, it is under high pressure exerted by the skid skate, ice melts and the rider slides easily thereon. Boiling point - the temperature at which the liquid is converted to steam (gas). It depends on the pressure, so in the mountains, water boils at a lower temperature than at sea level. The boiling point of water at a pressure of 760 mm Hg. Art. (A "standard" pressure approximately equal to the pressure at sea level) is 100 ° C (or 212 ° F).

Crystalline and amorphous materials. The solids are amorphous and crystalline. In amorphous molecules are arranged randomly. As an example, amorphous glass can be reduced. As with other similar substances, glass has no definite melting point: when heated, it gradually softens until finally becomes liquid. On the contrary, the molecules (or ions) of crystalline substance are arranged strictly ordered. For crystalline substances include sand, salt, sugar, diamond, graphite, etc. All they melt at a certain temperature (unless when heated undergoes no chemical change, as it happens with the sugar). Many ionic compound (e.g., salt NaCl) crystal form, in which each ion is surrounded by oppositely charged ions; as a result we can not say that any particular molecule forms a pair of ions. Owing to the mutual attraction of the ions in the crystal of sodium chloride (NaCl) is a substance melts at a higher temperature (801 ° C). Each ion NaCl is surrounded by six nearest neighbors having an opposite charge. The unit cell of the crystal of sodium chloride - a cube whose corners and at the center of each face are arranged sodium ions. A cell of this type is called a face-centered cubic. Cubic forms and have large crystals of salt.

The crystal lattice of salt. Small balls - sodium ions, large - chlorine ions. All crystals of salt have the same cubic shape.

       Crystalline diamond, in which each carbon atom covalently linked to four neighboring carbon atoms, are also characterized by a face centered cubic unit cell. Diamond - a very solid, having a high transition temperature.

Diamond crystal lattice. Carbon atoms are enclosed in a rigid crystalline lattice, making the diamond is extremely hard.

         Quite differently arranged carbon atoms in graphite. There they form layers, not very firmly bonded to each other. Each layer of "lined" hexagons of carbon atoms analogous to the benzene ring. Since the adhesion between the layers is rather weak, graphite is soft. The layers easily slide relative to one another, so that graphite is a good lubricant. Different crystalline forms of the same element, such as graphite and diamond, called allotropes.

Graphite crystal lattice. Carbon atoms in graphite  
form layers. They are connected with each other is not very durable and can slide relative to one another.

           Substances whose molecules are held together by weak forces of attraction and not by covalent or ionic bonds, melt at relatively low temperatures seldom exceed 400 ° C. This is the most organic compounds and inorganic covalent. In the Examples water and benzene their melting considerably below room.

The electrical conductivity. Metals - excellent conductor of electricity. Electric current carriers are electrons in them freely "floating" in the crystal lattice between the metal ions occupy a fixed position in the lattice. These electrons compensate the mutual repulsion of positive ions and stabilize the whole structure. If the metal potential difference is applied, electrons will move to the positive pole and the electric current occurs.

source  dic.academic.ru