PHYSICAL PROPERTIES AND INTERMOLECULAR BONDING IN SOLIDS

PURPOSE

DESCRIPTION

TIME REQUIRED

MATERIALS

Chemicals:

iron*

copper*

lead*

petroleum solvent*

ascorbic acid *

potassium chloride*

p-dichlorobenzene*

paraffin*

sucrose*

hydroquinone*

sodium nitrate*

copper-zinc alloy*

silicon carbide*

carbon *

silicon dioxide*

Equipment:

evaporating dish*

spatula

small test tubes with solid rubber stoppers

9-V conductivity tester

hot plate

ring stand and ring

wire gauze

wash bottle with water

crucible tongs

crucible (demo)

clay triangle (demo)

Fisher burner (demo)

HAZARDS

MODIFICATIONS/SUBSTITUTIONS

1. All chemicals used in this experiment may be obtained locally. Common names and possible sources are:
   
   

   Chemical Name	Common Name	Store
   iron	steel wool or iron nails	hardware
   copper	copper pipe	hardware
   lead	lead sinkers	sports
   petroleum solvent	mineral spirits	hardware
   ascorbic acid	vitamin C	grocery or drug
   potassium chloride	salt substitute (Adolf's)	grocery
   p-dichlorobenzene	moth crystals	grocery
   paraffin	candles or canning wax	grocery
   sucrose	sugar	grocery
   hydroquinone	photo developer	photo
   sodium nitrate	nitrate of soda	garden
   copper-zinc alloy	brass	hardware
   silicon carbide	carborundum	hardware
   carbon	graphite lubricant	hardware
   silicon dioxide	sand	hardware

   
   
2. Metal jar lids 5 to 10-cm in diameter may be substituted for evaporating dishes.

PROCEDURE

1. Obtain an evaporating dish and a small test tube. Using a spatula place a pea-sized sample of a solid into the dish (1.0 grams max). Use the bottom of the test tube, carefully, to attempt to grind the solid substance. Use slight pressure at first. Test the volatility of the solid (and thereby its vapor pressure) by cautiously smelling it. Be sure to waft the vapors toward your nose with your hand. Do not directly "sniff" the open dish. Record your observations in each step.
2. Use a 9-V conductivity apparatus to test the solid for electrical conductivity. Do the test by simultaneously touching the bare wire electrodes to the sample while observing the light bulb. Record your observations for each solid.
3. Place about 3 grams of a solid in a small boat made from aluminum foil and set it on the hot plate. (Several samples can be done at one time.) Turn on the hot plate and adjust the temperature to a medium setting. Observe the sequence as each solid melts. A solid which melts very rapidly indicates a low melting point. For any solid that melts, test the conductivity of the melt with the conductivity apparatus. Discard the solids which melted after checking their conductivity.
4. If a substance does not readily melt using the above procedure, increase the temperature of the hot plate to its highest setting. Test the conductivity of any sample that now melts as you did above.
5. If any solid fails to melt at the highest temperature setting, place a 1.0 gram sample of the solid in an evaporating dish and place it on a wire gauze on a ring stand. Using a Bunsen burner flame - gently at first heat the evaporating dish. Note, some solids will not melt even under these conditions. Record all observations.
6. Place about 3 mL of water in a test tube. Use the tip of your spatula to add a pea-sized sample of solid to the water. Stopper the test tube and shake well. Observe the degree to which the solid dissolves in the water. Test the conductivity of the solution. Repeat for each sample. Record all observations.
7. Place about 3 mL of mineral spirits in a test tube. Use the tip of your spatula to add a pea-sized sample of solid to the mineral spirits. Stopper the test tube and shake well. Observe the degree to which the solid dissolves in the mineral spirits. Test the conductivity of the solution. Repeat for each sample. Record all observations.

DISPOSAL

DISCUSSION

Compounds with IONIC BONDS, potassium chloride and sodium nitrate were our examples, will generally show properties of conductivity in the molten state or in water solution, somewhat high melting points, solubility in water (polar solvent), insolubility in mineral spirits (nonpolar solvent), and a low vapor pressure (no odor). These properties are explained by the presence of strong ionic bonds in the compound, formed by the attractions of oppositely charged ions which are very strong over short distances. These bonds can be broken by melting or dissolving, both of which free the ions from their crystalline structure, thus accounting for their conductivity.

Substances with METALLIC BONDS, iron, copper, lead and copper-zinc alloy were our examples, will generally show properties of conductivity in the solid and liquid states, insolubility in both types of solvents, high melting points and low vapor pressures. The atoms in a metal are arranged in a regular pattern or lattice. The metal is held together by the valence electrons that are free to move through orbits which extend over the entire lattice thus accounting for conductivity in both the solid and liquid states.

A compound described as MOLECULAR, can consist of either polar or nonpolar molecules. Our polar molecular examples were ascorbic acid, sugar and hydroquinone. Nonpolar molecular compounds were p-dichlorobenzene and paraffin. The properties of these two types of molecular solids differ because of the difference in their molecular polarity. The nonpolar solids are not soluble in water (a polar solvent), whereas the polar solids are water-soluble. Since there are no charged units in either type of solid they do not conduct electricity. Nonpolar molecular solids usually have high vapor pressures and low melting points. (See remarks about ascorbic acid under Tips.) The vapor pressure of most polar molecular compounds is lower, so most do not have an odor. Nonpolar molecular compounds are held together by very weak London dispersion forces, while polar molecular compounds are held together by the somewhat stronger dipole-dipole forces or hydrogen bonds.

Compounds which are described as COVALENT NETWORK, silicon dioxide and silicon carbide were our examples, generally have very high melting points and very low vapor pressures; they do not conduct electricity. In a covalent network solid, all of the individual atoms making up the solid are held together in a giant lattice by covalent bonds. This structure creates a very stable system.

TIPS

1. Unless students have already studied liquids, it will be important to discuss solubility tests, specifically what happens when water (a polar solvent) and mineral spirits (a nonpolar solvent) are mixed.
2. A simple, inexpensive 9-V conductivity tester can be build as shown below.
   

   Be sure that the wires on the conductivity apparatus are stripped at least 5-8 cm. This will prevent the covering material from melting if it comes in contact with a very hot solid.

3. Potassium chloride melts only under extreme conditions, therefore its melting point and conductivity is best illustrated by the following demonstration. Place approximately 3 grams of KCl in a crucible. Set the crucible on a triangle on a ring stand. Place the hottest part of the flame of a Fisher burner against the bottom of the crucible. Heat the crucible until the KCl melts. (This may take up to 15 minutes.) Conclude the demonstration by testing the conductivity of the liquid. When heating the potassium chloride, black spots may appear. These are the decomposition products of any tartaric acid which might be in the sample. This will have no effect on the result and can be ignored.
4. Hydroquinone (m.p. 285°C) is used as an example of a polar molecule because this compound differs from p-dichlorobenzene only in the type of functional group present. The presence of the two -OH groups leads to hydrogen bonding.
5. Ascorbic acid, commonly purchased as Vitamin C tablets at the drug store, can be used to illustrate the properties of a polar molecular compound. This substance shows the solubility and vapor pressure of this group, but since Vitamin C decomposes when heated it cannot be used to illustrate melting point or conductivity.

REFERENCES

Submitted by Robert Davis, Diana Doepken, Larry Dukerich, Larry Ferguson and Marie Fiedler

Woodrow Wilson Leadership Program in Chemistry lpt@www.woodrow.org
The Woodrow Wilson National Fellowship Foundation webmaster@woodrow.org
CN 5281, Princeton NJ 08543-5281 Tel:(609)452-7007 Fax:(609)452-0066