Wednesday, July 21, 2010


Magic sand - one of the most rated party tricks of the decade is worth watching.  Just check out the video here.
 Magic sand or Mars sand or moon sand or aqua sand or whatever it is called, is a direct outcome of a special law of Physics - Hydrophobicity! It is never wetted by water.
Hydrophobicity is the property of objects by virtue of which they repel water. (hydro-water, phobic-fear) The substances which repel water are called as hydrophobes or water repellents.

First let's know what the so-called magic sand is made of. It is made up of usual beach sand coated with some hydrophobic substance, usually trimethylsilanol. This trimethylsilanol is an organo-silicon compound. Like any other organosilicon compound, it acts as water-repellent.

How it works?? Let's see.

Usually a water repelling (hydrophobic) substance is a non-polar compound which has a greater tendency to combine within themselves and so, they do not readily bond with other compounds. In other words, their adhesive force is very less than their cohesive force. Not only water, but they repel all polar compounds to different extent.

Water, a polar molecule exists as a liquid due to inter-molecular hydrogen bonding. Since oxygen is highly electronegative, it attracts the shared pair of electrons towards itself and acquires a small negative charge. On the other hand, hydrogen atoms acquire a small positive change. So, hydrogen having a small "+ve" charge naturally attracts oxygen having small "-ve" charge as "unlike charges attracts each other". Thus all water molecules are associated. There is no chance for interaction between water and non-polar covalent molecules as the atoms do not differ much in electro-negativity. They are usually neutral compounds and so, there is no chance for interaction with water molecules which are electric dipoles. At the same time, they are strongly bound to themselves so that they repel water.

Initially this "Magic sand" was developed so as to degrade ocean spills. When it is sprinkled on sea water, it mixes with oil and settle down the ocean floor as the mixture becomes heavier. The point is they are not wetted by water, but can be wetted by oil, where oil is a non-polar compound.

It is widely used as a trick. Under water, they appear shiny, why because, when they are made to submerge in water, they gather a thin film of air around themselves.
Discussion on this article awaited. Please post as a comment.

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