Magic Sand By Harvey Black and David Robson When we play at the beach, we build sandcastles and dig moats to protect them. In the process we also learn about the properties of water and beach sand. * Water wets (adheres to) beach sand and flows freely between the grains. * Beach sand is denser than water, and sinks. * Beach sand mixed with a lot of water acts more like a fluid than a solid?this mixture slides, slumps, and resists construction attempts. But there is another type of sand, called magic sand that doesn't behave this way. Magic sand is sold as a toy. The instructions suggest putting water in a large glass bowl and sprinkling in a small amount of magic sand. Instead of sinking, like beach sand, the magic sand will float and form a "sand raft". By sprinkling more and more magic sand onto the sand raft, the raft can be made to plunge to the bottom. If you hold beach sand in one hand and magic sand in the other and lower both hands into water, the beach sand clearly shows individual grains. The magic sand, however, appears to be surrounded by a silvery layer looking like plastic film. When you lift your hands out of the water, the beach sand is wet, with its grains clumping together. The grains of magic sand are not clumped together?they are perfectly dry. This is because a large air bubble surrounds the magic sand?the silvery layer is the curved surface of the bubble. What is magic sand and why does it behave so strangely? The ins and outs of sand Beach sand is mostly silica (mineral quartz) broken into tiny pieces or grains. Silica consists of silicon and oxygen, covalently bonded in a three-dimensional network of billions of atoms. The interior of each grain of sand contains twice as many oxygen atoms as silicon atoms, which can be represented by the formula SiO2. However, the surface of the grain contains oxygen atoms that are covalently bonded to hydrogen atoms. These are polar covalent bonds, like the O?H bonds in water molecules. This means that both the surface of the sand and the water molecules carry partial positive and negative electrical charges and, consequently, water is attracted to the sand. A drop of water will spread out on a sand grain instead of gathering into a bead. Beach sand grains are hydrophilic?water loving. Magic sand is ordinary beach sand coated with tiny particles of pure silica, which have been exposed to a special chemical treatment?vapors of a silicon compound called trimethylhydroxysilane, (CH3)3SiOH. When the beach sand grains are exposed to trimethylhydroxysilane, a reaction takes place resulting in the formation of water and in the bonding of the trimethylsilane compound to the silica particles. Following this treatment, the exteriors of the sand grains contain ?CH3 groups that are insoluble in water or hydrophobic?water hating. When a few grains of magic sand are sprinkled on water, the polar water molecules attract other polar water molecules so strongly that they prevent the grains of magic sand from breaking through the surface until the layer of sand becomes rather thick. When the magic sand finally sinks, the same surface tension effect also keeps it dry. The air between the grains cannot be forced out because the water molecules will not flow between the hydrophobic grains. However, oil will readily flow between the grains, and magic sand can absorb a surprising quantity of oil. Magic at work When magic sand was originally developed, workers expected it might be useful for cleansing water of oily contamination. It was also suggested that magic sand might be useful for trapping petroleum spilled from oil tankers in coastal waters. When sprinkled on floating petroleum, magic sand would mix with the oily material, add weight, and make it sink. This would prevent the petroleum from contaminating marshes and beaches and, theoretically, permit the petroleum to be later dredged from the bottom. If you have ever raised potted plants, you know how difficult it is to water them correctly. If you underwater the plants, they wilt; overwater, and the roots rot. The problem is that the roots of most plants need to be exposed to both water and air. If you give the plant too much water, you will displace the vital air. When magic sand is added to potting soil, the hydrophobic grains permit air to flow between them, but not water, and thus maintain open-air channels to the surface. Magic sand has also been tested by utility companies in the Arctic. The utilities bury electric and telephone wires to protect them from the harsh weather. However, if something needs repair during the winter, digging through frozen Arctic soil normally requires hours of work with power tools. To speed underground repairs, utility companies can cover electrical junction boxes with magic sand and cap the sand with just a few inches of soil. Rainwater flows around, not through, the magic sand and, when the soil freezes solid, the magic sand remains dry and loose. It is easy to break through the frozen cap, then shovel away the loose magic sand. The future of magic sand The applications described above have all been tested successfully, but are not being used today. Some our readers have thought of new and innovative application ideas for the future of magic sand. Holly Barbaccia of Vineland High School, NJ suggested that magic sand could be useful as a surface for horse racetracks. The tracks would not become muddy when it rained. She sprayed water on a layer of magic sand and observed that the water formed beads and ran off the layer. She reasoned the water could be vacuumed up, but she wasn't able to test the idea. Erik Sprague and Jessica Mehlinger, also of Vineland High School, explored the possibility of using magic sand in golf course sand traps. Unlike ordinary sand, it wouldn't clump together in wet weather and make it more difficult to hit the ball with just the right force. Jeff Hall from Seaman High School in Topeka, KS suggested using magic sand in sandboxes. The magic sand wouldn't form clumps after it got wet. The bottom of a sandbox could be lined with a mesh filter to let the water, which would flow freely through the magic sand, drain out. The mesh would have to be small enough so that the sand would not escape. Josh Sales also from Seaman High School proposed that magic sand could be used to aid in the drainage of water from walls and foundations of homes built below ground or into hillsides. A 15-cm layer of sand could be placed next to the foundation, so that water will not pool and infiltrate the foundation, flow into basements, or rot wood. Josh recognizes that such a large quantity of magic sand would be expensive, but reasons that if this use were widely adopted, the cost would decrease and become more reasonable. Shaw Lynds of Paso Robles, CA suggested that a mixture of magic sand and normal sand could be used as a base under asphalt roads. This application would let rainwater drip into the ground through the asphalt, but would prevent oil from cars and asphalt from draining into the groundwater. Magic sand would trap the oil. He tested the idea with three experiments. By trying various ratios of magic sand and regular sand, Shaw arrived at a suitable mixture (1:6). He next compared the speed with which water went through the 1:6 mixture and just ordinary sand. Finally, by pouring varying ratios of oil and water into the 1:6 mixture, he found how much oil would be captured and how much water would pass through, and determined how much magic sand would be needed to capture a cubic centimeter of oil. Shaw also thought that magic sand could be used to waterproof fabrics. References Vitz, E. Magic Sand: Modeling the Hydrophobic Effect and Reversed-Phase Liquid Chromatography. J. Chem. Educ.1990, 67 (6), p 512. Hoffman, A. B. A Demonstration Model for Immiscibility. J. Chem. Educ.1982, 59 (2), p 155.
