Expansion/Contraction

In general, materials expand when heated because of the increased kinetic energy of their particles at higher temperatures. Higher kinetic energy results in higher velocities (E k = 1/2mv 2 , where E k = kinetic energy, m = mass and v = velocity). When the particles strike one another at higher velocities they bounce further apart. This fact accounts for the expansion exhibited by most materials when heated. Likewise, the decrease in kinetic energy and correspondingly lower velocities account for the contraction of most materials as they cool.

Temperature measuring devices (mercury, alcohol, gas, and bimetallic strip thermometers) rely upon the regular expansion and contraction of materials during heating and cooling.

Aircraft once were hot riveted because hot metal is more malleable. As hot rivets cool, they contract and fit loosely in the holes in the metal frame. This fact was not a problem until aircraft became supersonic. During the buffeting of passage through the sound barrier, the loose fitting hot rivets led to vibrations and cracking of the airframe.Several aircraft and pilots were lost as a result of this ÒslackÓ problem. Supersonic aircraft are now assembled through a cold riveting process. As these cold rivets warm to normal flight temperatures, they actually expand to fit tightly into the holes in the metal frame.

Liquids when heated expand at faster rates than do solids. If one fills an automobile gasoline tank (solid) to the brim with gasoline (liquid) on a cool summer evening, and doesnÕt drive the auto until the hot part of the next day, gasoline will be flowing out of the filler tube and onto the driveway. The liquid, gasoline, and the solid, gas tank, have both expanded, but the liquid has expanded more. Thus an amount of liquid flows out of the warmed tank.

The fact that materials (including glass) expand and contract due to temperature changes is the reason that volumetric flasks are accurate only at one designated temperature.

Pyrex glass can withstand rapid temperature changes better than normal (crown) glass because it expands and contracts at about 1/3 the rate of normal glass. If one pours cold liquid into a hot crown glass container, the inner surface cools and contracts rapidly relative to the still hot outer surface. The stress is normally too much for the forces within the glass to withstand, and it cracks. With Pyrex Ò glass, given the same temperature differential between the inner and outer surfaces, the contraction rate is only 1/3 as great. This lower stress is within the limits which the bonding structure can tolerate, and the Pyrex Ò container resists this cold liquid shock.

Hydrogen Bonding in Water

The hydrogen bonding structure in water produces a very open hexagonal crystal lattice which requires that water molecules move farther apart in order to solidify into ice. Thus, water goes against the normal trend and expands as it freezes. As a result of this unusual behavior, ice is less dense than water and floats on water. If not for this uncommon property, ice would form on the bottom of lakes and oceans in the higher latitudes of the earth. Freezing would occur from bottom to top and aquatic life would find it impossible to survive.

Another material which exhibits this unusual expansion when freezing is type metal. If type metal contracted as it became solid, it would pull away from the type mold, and ÒchunksÓ of the lettersÕ surfaces would be missing. The resulting print would be fuzzy and difficult to read. Type metal expands upon freezing to fill every nook and cranny in the type mold.

Normally, substances contract when they enter the solid state. Placing enough pressure on such substances in the liquid state will cause them to become solid. Water, which expands upon freezing, exhibits reverse characteristics. Solid water (ice) when pressurized will return to the liquid state. This is why those icy footprints appear on your sidewalks if pedestrians get to them on a winter morning before you can scoop the snow. This phenomenon leads to the glacial ÒflowÓ that is characteristic of large bodies of ice.

The open structure of the crystal lattice of ice is both a blessing and a curse. The expansion of ice is an indispensable aid in the weathering process. Freezing water expands in small fissures in rocky surfaces and these cracks grow larger. After a fashion rocks are reduced to the soil necessary to sustain life. ThatÕs small comfort to the home owner who must pay several thousands of dollars to replace sidewalks and driveways which have been converted to rubble by this same weathering process.

Continued

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