Floating Pin on Water
A small metal pin is floated on the surface of water, and then sinks when detergent is added due to the decrease in surface tension.
- petri dishes for water and ethanol
- small metal pin
- wooden stick
- Petri dish is filled with ethanol
- The pin is attempted to be floated on the ethanol but the surface tension is not sufficient to support it.
- A second petri dish is filled with water.
- The pin is carefully placed onto the surface of the water. You can show that it is floating on the document camera by moving it around with the magnet.
- A small amount of detergent is placed onto the end of a wooden stick. Touching the soapy stick to the water edge causes the pin to sink in the middle. This can also be confirmed with the magnet.
The surfaces of liquids are higher in energy than their bulk. Since all systems will minimize their potential energy if possible, a liquid will always seek the lowest surface area to volume ratio. This is the reason for spherical droplets in the absence of other forces. The liquid in the petri dish conforms to the dish shape because gravity is pulling it down and the interactions of water with the container walls are increasing the interfacial area. The intermolecular forces in water (dominated by hydrogen bonding) resist this expansion of the surface area until all forces are balanced. The surface tension of water is sufficient to resist the pin passing through its liquid-air interface. For the pin to transfer across the interface, the surface area of the water would have to be further increased, and, under the weight of this small pin, water is not willing to let its surface are be increased by that much.
Detergent is a surfactant with a polar and nonpolar end. The polar, hydrophilic end interacts with the water, but the hydrophobic tail does not want to enter the water. In thermodynamic terms, the energy of breaking hydrogen bonds in water is not sufficiently offset by the new interactions (dipole-induced dipole) forces that would be returned if the hydrophobic tails sank into the water bulk. As a result, the detergent spreads across the top of the water. The intermolecular interactions of detergent with other detergent molecules (mainly dispersion forces) are weaker than the hydrogen bonding forces of water, so the detergent coated water has a lower surface tension than water alone. The new surface tension is not strong enough to hold the pin afloat, so it sinks.
Special Safety Notes
- no safety hazards