Does Surface Tension Affect the Process of Crystallization?

ABSTRACT The characteristics of crystals have always fascinated man. Man has always been interested in the appearance of them and some of the exotic locations where many crystals may be found. These locations tend to be cold and damp. Usually crystals tend to form under extremely saturated liquids. For example, salt crystals tend to form in very saturated water such as in ocean water. Though one knows that crystals usually form in saturated liquids, but does the property of the liquid affect the rate or formation of these crystals? This experiment has been conducted to prove if that is true or not. INTRODUCTION AND RESEARCH Many scientists have been interested in crystals and the science associated with crystals. Though one knows that certain conditions must be present in order for crystals to form; and if some of these conditions are not present or are not perfect, then the formation of the crystal will be disturbed. One may ask him/herself, how is the formation disturbed if only one of the condition would happen to be modified? Most crystals are formed under water or under liquids with similar characters to water; and tend to form under liquids or solutions containing a maximum amount of solute. If the liquid absorbs additional solute, then the solution is supersaturated.

If a tiny crystal, called a seed crystal, is added to the substance, a chain reaction would occur where the crystal growth will grow dramatically. In the case of the experiment, a staple is used instead of a crystal so that the crystal can be easily removed at the end of the experiment. One may wonder, if a certain characteristic of the solution were to be changed or altered, then would the growth of the crystal change by quantity? shape? In the experiment conducted, the types of liquid have been changed; both the surface tension and chemical properties have been changed. In order to compare on solution to another, water has been used as a control and that the atmosphere associated with the container containing water will also be considered the control. In this case, the surrounding atmosphere will be regulated so that every container will experience the same conditions. Crystals are homogenous portions of matter and has a definite atomic structure. They also tend to have an outward form bordered by smooth, plane surfaces and are symmetrically arranged.

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Scientists have been involve in the study of the growth, shape and geometric characters of crystals. These studies are called crystallography. When the conditions are right, chemicals and compounds tend to crystallize in a certain form. For example, salt crystals tend to form cubic crystals; but garnet, which can also form cubic crystals, tend to form dodecahedron or a twelve sided solid. Though salt and garnet are different in their appearance, they are classified in the same class and system. Theoretically, there are thirty-two classes of crystals and those thirty-two classes are grouped into six crystal systems. The classification of the systems is based on the length and the location of the crystal axis. Minerals in each system have certain details of symmetry, crystal form, and many optical properties. The six crystal systems are very important especially to those such as mineralogists and gemologists. The six crystal systems are isometric, tetragonal, orthorhombic, monoclinic, triclinic, and hexagonal. The classification of the crystals is determined by their axes and the way the axes are positioned. Under the class of isometric, the crystals have three axes that are perpendicular to one another and each axes are all equal in length.

Under the class of tetragonal, thecrystals have three axes that are perpendicular to one another but only two of the axes are equal in length. Under the class of orthorhombic, the crystals have three perpendicular axes but all of the axes are different in length. Under the class of monoclinic, the crystals have three axes that are not equal in length, but two of the axes are not perpendicular to one another but are perpendicular to the third axis. Under the class of triclinic, the crystals have three axes which are not equal in length and are not perpendicular to one another. Under the class of hexagonal, the crystals have four axes. Three of the axes are on a single plane and are symmetrically spaced and are equal in length. The fourth axis is perpendicular to the other three axes and can be any length compared to the other three axis. Though crystals may have the same chemical composition, they may be considered very different in all their physical properties. For example, under the isometric system, carbon crystallizes to form a diamond, and under the hexagonal system, carbon crystallizes to form graphite. Also, though diamond is in the same system as salt and garnet, it is in a different class.

Crystals are produced whenever a solid is formed gradually from a liquid and usually when the liquid is being cooled. A natural occurring process in which crystals are formed is when lava erupts from the earth and cools down. If the lava is cooled quickly, then a glassy rock called obsidian is formed. If the cooling is considerably slower, a rock called felsite is formed. Felsite is a type of crystal but the crystals are too small to be seen with the naked eye. Crystals such as those on felsite are called cryptocrystalline or aphanitic. If the cooling process were to be slightly slower, then a rock called rhyolite is formed; the crystals formed are large enough to be seen. Though these rocks have the same composition, they have different physical properties and so are classified in different system and classes. Felsite, rhyolite and other rocks are not considered to be a single crystal. Theses types of rocks are called crystalline. They consist of many small forms of crystals. Crystal growth may only occur if another seed crystal is present in the solution.

This is why a staple is used in the experiment; it is used to begin the process of crystal growth. If the seed crystal is not present in the solution, then crystallization may not take place and the solution is just considered supersaturated. Crystallization may not also take place if the viscosity of the liquid is too high. For example, water has a low viscosity and crystallization is more likely to occur than honey, which has a high viscosity. If the substance is easy flowing and not thick, then crystallization has a higher chance of occurring. The viscosity of a liquid is related to the surface tension of a liquid. Surface tension is a state which exists at the free surface of a liquid; it is similar to the properties of an elastic skin under tension. Liquids with high surface tension tend to want to become as small as possible. Liquids with high viscosity tend to have a high surface tension, or low cohesion. For example, alcohol has a low viscosity and a low surface tension or high cohesion; it isn’t as thick as liquid soap, which has a higher surface tension and a lower cohesion. In the experiment, water was used as the control. Water is generally used as a control for most experiments since its properties may be considered a normal or neutral liquid.

Water is not very reactive to most chemicals; because of this, water can be considered neutral and used as a control. Four more solutions were chosen as variables; alcohol, bleach, dishwashing soap, and corn oil. Most of the variables have different properties than water except for alcohol. Alcohol and water have very similar chemical properties and structure. Also, bleach and the dishwashing soap have very similar physical properties and corn oil is not closely related to any of the solutions used. These solutions were chosen to prove if surface tension affects the growth of crystals. Different liquids of similar properties were purposely chosen to see if they would react the same way as one another. For example, bleach and dishwashing soap were chosen because their properties were similar to one another. If the results of the bleach came out different than the dishwashing soap, then the outcome of the experiment would result in that surface tension does not have any affects in the growth of crystals. In this case, it may seem that surface tension would have an affect on the growth of crystals; it would probably reduce the speed of the growth of the crystals or would cease the growth if the surface tension is too great.

MATERIALS 5 clear plastic cups 5 x 100 ml beakers 1 cup of water 1 cup of alcohol (isopropyl-rubbing alcohol) 1 cup of bleach 1 cup of corn oil 1 cup of dishwashing liquid (brand name “Dawn”) 5 x 1/4 cup of table salt 1 small portable stove 1 thermometer 1 stiring rod 5 straws 5 strings (15 cm in length) 5 staples scale which measures in milligrams PROCEDURE 1) label cup for each of the following liquids: -water (control) -alcohol -bleach -corn oil -dishwashing soap 2) label beakers for each of the following liquids: -water (control) -alcohol -bleach -corn oil -dishwashing soap 3) turn on stove 4) measure temperature of stove using thermometer and regulate the temperature at 110 degrees F 5) measure one cup of each liquid, and pour into beakers with the correct name of the liquids 6) pour 1/4 cup of table salt into each beaker 7) heat the beakers on top of the oven for ten minutes 8) stir each beaker using the stiring rod (be careful to clean stiring rods before placing in different liquids) 9) locate string, straw, and staple-tie the straw to one end of the string and the staple to the other 10) repeat 4 more times 11) measure weight of straw, string and staple using a milligram scale 12) pour each liquid into their corresponding cups

PROCEDURE(Continued) 13) place the straws horizontally above each cup, letting the staple dangle within the liquids (see fig. 1) 14) let stand for 24 hours in room temperature environment 15) then place in refrigerator for another 24 hours 16) measure weight of the straw, string and staple with residue for each liquid using scale 17) record all 18) repeat experiment CONCLUSION By carefully observing the data, one may notice that the results turned out as expected. It seems that surface tension does affect the rate of growth of crystals. Solutions which have a high surface tension tend to want to keep the solvents as small particles.

This is why solutions such as soap have a high surface tension. Soap is used to break down particles such as oil by making the oil particles smaller. In this case, solutions which have a high surface tension are doing just that. They are preventing the salt particles from coming together to form crystals. As was mentioned under the Introduction and Research, the solutions were chosen for a specific reason. It was mentioned that water and alcohol have similar characteristics and bleach and dishwashing soap have similar characteristics. When their results are compared together, they are similar. The weight of the salt crystal of water was similar to that of alcohol; and the weight of the salt crystal of bleach was somewhat similar to that of the dishwashing soap. One may conclude that surface tension does affect the process of crystallization.

BIBLIOGRAPHY

Chalmers, Bruce, The Structure and Properties of Solids, Philadelphia, Heydon, 1982. “Cohesion”, Encarta 95, Microsoft, Seattle, 1995. “Crystals”, Encarta 95, Microsoft, Seattle,1995. http://www.hamptonresearch.com/crystalgrowth.htm “Surface Tension”, Encarta 95, Microsoft, Seattle, 1995. Smoot, Robert C., Richard G. Smith, Jack Price, Chemistry, McGrawl-Hill, New York, New York, 1998.

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