Water’s remarkable surface tension allows it to resist gravity and clump together in drops rather than immediately spreading out flat. This surface tension comes from water molecules attracting each other. The molecules on the surface don’t have other water molecules above them, so they cling tightly to their neighbors. Their cohesion allows them to form round drops.
Quick Answers
– Water drops can fit on a penny due to surface tension from water molecules attracting each other.
– The water molecules on the surface cling tightly together in drops rather than spreading out.
– Surface tension allows the water to resist gravity and clump in drops.
– This surface tension comes from the water molecules being polar, with slight positive and negative ends.
– The molecules attract each other through hydrogen bonds between the positive and negative areas.
– Enough water drops can fit on a penny to eventually overflow the penny and spill off the sides.
Explaining Surface Tension
Water’s surface tension stems from the molecules being polar. They have a slight positive charge on one side and a slight negative charge on the other. This makes them stick together like magnets. The molecules attract each other at the surface because they don’t have other water molecules above them. Below the surface, the molecules pull in all directions to surround themselves with other water molecules.
At the surface, the unbalanced molecular forces result in the water trying to clump together and minimize its surface area. This intermolecular attraction forms a type of “skin” that holds the water molecules together. This skin-like layer causes drops of water to bead up into dome shapes rather than immediately spreading out flat.
Hydrogen Bonding
More specifically, water’s polar molecules form hydrogen bonds with each other at the surface. These bonds occur when the positively charged hydrogen atom of one molecule is attracted to the negatively charged oxygen atom of another. This electrostatic attraction lets the water molecules stick together while resisting external forces.
The hydrogen bonds have just the right amount of strength – strong enough to accumulate into drops of water on a penny but weak enough to allow movement between molecules. This flexibility still allows the water drops to slightly flatten out over time as the molecules redistribute while clinging to their neighbors.
Cohesion and Adhesion
In addition to hydrogen bonding, water’s surface tension is strengthened by cohesion and adhesion forces between the molecules:
- Cohesion – water molecules sticking to other water molecules
- Adhesion – water molecules sticking to other surfaces like the penny
The combined strength of hydrogen bonds, cohesion, and adhesion allows numerous water drops to curve into domes across the penny’s surface. These forces overcome the downward pull of gravity to let the water group together.
Measuring Surface Tension
Surface tension can be measured by calculating the force needed to break through a water’s surface. This force is measured in dynes per centimeter. Water has a relatively high surface tension of about 72 dynes/cm at room temperature.
In comparison, oils like gasoline have weaker intermolecular bonds and lower surface tension around 20 dynes/cm. Soap molecules can also disrupt water’s surface tension by preventing hydrogen bonds.
Some objects like pins or paper clips can float on clean water due to the high surface tension. However, a drop of soap can break the surface tension and cause the items to sink as the water molecules spread out more.
Capillary Action
Another effect of surface tension is capillary action. This occurs when water climbs up small spaces like paper towels or plant roots due to the adhesive forces along the walls. As the water sticks to the sides, further water gets pulled along by cohesion.
The smaller the tube, the higher water can climb due to having more surface area for adhesion. The height also increases as surface tension increases. Testing different liquids in the same tubes shows that lower surface tension results in less climbing height.
Fitting Water Drops on a Penny
A penny provides the perfect metal disk for demonstrating water’s surface tension in action. The drops curve into domes across the penny until enough accumulate to spill over the sides. With careful placement, optimizing the surface tension allows fitting over 100 drops on a penny before they overflow.
Filling the Penny
Drops initially form rounded domes floating slightly above the penny’s surface. This demonstrates the balance between adhesion and cohesion holding them together against gravity’s pull. As more drops get added, they gradually flatten out while still clinging together.
Each dome spreads out when another drop lands nearby, merging together while maximizing surface tension from their hydrogen bonds. The drops flatten more and more as they fill up the penny’s surface. With good spacing, optimal size drops can coat the penny before spilling over the edge.
Technique
Proper technique is key to fitting the most drops on a penny:
- Use clean or distilled water since impurities lower surface tension
- Fill an eyedropper or pipette with water to control drop size
- Gently lower drops right above the penny without hitting too hard
- Aim for narrow spots between existing drops to fill gaps
- Spread drops evenly around the penny for efficient filling
With practice, over 100 drops can be layered before one has to bridge the edge and spill off. This overflow represents reaching the maximum for that water sample’s surface tension under those conditions.
World Records
The world record for most water drops on a penny is currently 263, achieved by The Super Mario Brothers in 2020. Their technique involved precisely layering tiny drops from a 30ml dropper bottle.
Other top attempts include 248 drops by Flip It Guy and 229 by the Happy Family Channel on YouTube. Many experiments aim for the most drops without spilling over the sides.
In addition to pennies, records exist for the most water drops on different items. These include 177 drops on a tablet screen, 167 drops on a sticky note, and 145 drops balanced on a coin on top of a shell.
Conclusion
Water’s unique surface tension enables incredible feats like accumulating numerous drops on a penny. The electrical attraction between polar water molecules allows them to dome up into beads and resist gravity’s pull to spread out. Careful technique can optimize properties like hydrogen bonds and contact angle to maximize water drops fitting on the penny before overflowing.
Surface tension also produces many other water phenomena like capillary action and floating paper clips. The molecular forces involved can even be measured in units of dynes per centimeter. While water’s behavior is complex at the molecular scale, beautiful macroscopic results like clinging drops showcase the power of surface tension.
| Forces | Description |
|---|---|
| Cohesion | Water molecules sticking to other water molecules |
| Adhesion | Water molecules sticking to other surfaces like penny |
| Hydrogen bonding | Positive hydrogen and negative oxygen atoms attracting between molecules |
| Surface tension | Net force of molecules clinging together at the surface |
| World Records | Number of Drops on Penny |
|---|---|
| The Super Mario Brothers | 263 drops (2020) |
| Flip It Guy | 248 drops |
| Happy Family Channel | 229 drops |
