How can a penny hold so much water.?

Question

I did a lab for my science class and what we did was we had an eyedropper and we slowly droped drops of water onto the penny until it would fall over, and see how many drops it took. Some people got up to 45! I know it has something to do with surface tension, molecular attraction and capillarity. I dont really understand these words either! I need to have them explained in laymens terms! If you have any part of this please help! thanks : )

Answer 1

Yes it does have something to do with surface tension. The best way to imagine it is to first ask yourself why the drops would spill across the penny's surface in the first place. The reason is because the water molecules slide past one another towards the Earth, because the Earth's gravity pulls them down. In a zero gravity environment, the water would just hang there.

So you can see there are two forces here at work. Gravity tries to pull the water down, so it flattens the water droplets a lot like a water ballon is flattened when you put it on the ground. But the water's surface has a force, called surface tension, which tends to want to keep the surface water molecules from sliding past each other like the skin of a balloon. If the surface forces are stronger than gravity, then they "win" and the water stays in one big drop without falling off the surface of the penny. But once there are enough water droplets, the extra weight of all that water is greater than the surface tension.

If you want to get even more technical, you can do calcuations for the volume and surface area of a sphere (which is similar to a droplet). The idea is that as the sphere- or droplet- becomes larger, the ratio of the surface to the volume gets smaller and smaller. Since the surface area is where the tension comes from, and the volume is proportional to the weight, that means that there is less of that surface in comparison to the volume.

Imagine a ballon that expands. At low volumes, the balloon will be ok and won't explode. But beyond a certain volume, the surface of the balloon will no longer be able to counteract the internal pressure and it explodes. With the water droplets, the surface is like the surface of the balloon, and the extra added weight is like the air inside the balloon. At some point that surface tension will no longer be able to hold the molecules from sliding past each other.

The surface tension comes from very weak attractive forces between molecules of water. There are also some attractive forces between the water molecules and the surface of the penny (capillarity). Water molecules have a partial positive and a partial negative charge, so as you may know positive and negative forces attract each other. That's the technical reason for the surface tension. It is a sort of molecular-level sticky-ness that is caused by tiny positive and negative charges.

Answer 2

I think it is mostly to do with the bonds and surface tension. Surface tension is the resistance of a liquid towards an increase in surface area (i.e. when you add more water, it makes a bubble instead of overflowing from the penny because it doesn't 'want' to have a larger surface area). This is also due to the strength of the hydrogen bonds between the water molecules.

Answer 3

This is kind of a complicated question, but I'll do my best. You are absolutely right that surface tension, and molecular attraction (also known as cohesion) are what allows so much water to stay on top of the penny. I'm not sure if capillary action has so much to do with it, but this is also another fascinating topic.

First of all lets look at molecular attraction. Thats where is all starts. Molecular attraction is just an easy way to say that certain molecules like to stick together. This is caused by certain delicate electromagnetic forces, but for now lets just state that a lot of similar molecules like to stick together. If you've ever seen cheerios floating in a bowl, you'll notice that they often move toward each other, and stick together. This is caused partly because of molecular attraction in the molecules of the milk, and also by surface tension, which is what I'll discuss next.

Surface tension is caused by molecular attraction. Because molecules like to stick together, under the surface all the molecules are pulling together from all directions. For example, lets say molecule A is under the surface, and molecule B is on the surface. Molecule A has other molecules that it is attracting to above, below, to the right, and to the left, so it moves around freely because the attraction is equal. Molecule B, on the other hand is only attracted to molecules below it, and to the left and right, because there aren't any other molecules above it. This means that it is pulling harder on the left, right, and bottom than it is to the top, so it sticks harder to the molecules to the sides and bottom than do the molecules under it.

What ends up happening is called surface tension. All the molecules on top are pulling on each other side by side more than the molecules under them, so it makes the very top layer of molecules stronger. Thats why cheerios floating on top of the milk stick together, but cheerios under the surface don't.

Now how does this all relate to your experiment with the penny? When you put the water on top of the penny, as the water builds up, the very top layer of water molecules is stronger than the inside molecules, so it acts like a shell around the water. If you notice, the water makes a sort of tiny dome on top of the penny. Think of it like you're filling up a balloon, except instead of rubber, the balloon is made of water. Eventually, the surface tension can't overcome gravity anymore, the outside layer breaks, and the water spills over the sides of the penny, just like a water balloon eventually will break.

The other term you mentioned was capillarity - you cant really see this at work in the penny experiment, but this is what happens when molecules of one substance are more strongly attracted to molecules of another substance than they are to each other. In the penny experiment, this is what keeps the water stuck to the penny, but you can see this at work better if you put the edge of a piece of paper towel into a glass of water. The water "climbs up" the paper against gravity. This happens because the water molecules are more attracted to the paper towel molecules than they are to each other, so they leave the water and move toward the paper. In nature, this is how plants and trees live; water from the ground creeps up into the stems and trunks by capillary action.

Hope this clears a few things up!

Answer 4

The penny has a rim around it. Surface tension is what gives it its "bubble-like" quality as you add drops and they hold together. The rim keeps it from running off and the surface tension lets it stay together and grow.

Answer 5

The penny will carry greater drops of standard water by way of fact the Hydrogen bonds will stay at the same time while cleansing soap is extra to the water the hydrogen bonds can now no longer stay at the same time. for this reason a tumbler of water could be crammed previous the brim or the cup.

Answer 6

water molecules are in a way "sticky" they want to stay together. This causes the water to buble up on the penny. This is how my science teacher explined it.