Water that doesn’t wet

Procedure

• Fill the plastic container with water until slightly more than halfway full.
• Add talcum powder across the entire surface of the water.
• Gently bring the fingers of one hand close to the surface and immerse them gently into the water. Observe what happens.
• Now try wetting your fingers with liquid soap (dishwashing liquid) and immerse them again in the water. Observe what happens.

Theoretical explanation

This experiment demonstrates a characteristic property of liquids that is very interesting in water: surface tension. In general, in liquids, each molecule is attracted by the molecules closest to it in all directions and with the same intensity. However, on the surface molecules, since there are no molecules above them to also attract them, there is a resulting force directed downward that tends to bring the molecule into the interior of the liquid, thus reducing this surface to a minimum. The result is that the liquid appears as if it is surrounded by an elastic membrane, the surface tension, which is responsible for the resistance offered by the free surfaces of liquids to their rupture.

When gently introduced in water, this surface acts as an elastic membrane previnting our fingers of getting wet. Therefore, when we remove them, the water surface is not pierced, the talcum powders continue to cover the entire surface, and the fingers come out dry. In contrast, soap has the property of breaking surface tension, so when we introduce soapy fingers, the soap prevents the molecules from attracting and reassembling the talcum powder film covering the water, and it gets pierced.