Science Buddies: "Ask an Expert"

Hi. I'm a 7th grader and I'm doing a science project that has to do with water. I am already done with it, but I'm having trouble explaining exactly what's going on. This is what I did so far:
1.) I took a plastic jar and poked 5 holes in the lid and 1 hole on the side near the bottom.
2.) I filled the jar up more than halfway with water but with my finger on the hole in the side so that no water could leak out.
The thing is I don't know exactly how to explain it in scientific terms. All I do is that after I filled the jar with water, I turn the jar upside down so that the lid side is facing down and I still have my finger on the hole in the side. The idea is that when I turn the jar upside down with my finger still on the hole, no water will escape even though there are 5 holes on the lid. But when I take my finger off the hole, the water will start pouring out. I want to know how to explain what happens when my finger is on the hole and no water escapes even though there are 5 holes through which the water could get out. I also want to know this in a way that is easy to understand for a 7th grader. My project is due on Monday December 12,2005. Please help me explain this, and if not please tell me some websites that might help me explain my project. Thank you!

Hi. I'm a 7th grader and I'm doing a science project that has to do with water. I am already done with it, but I'm having trouble explaining exactly what's going on. This is what I did so far:
1.) I took a plastic jar and poked 5 holes in the lid and 1 hole on the side near the bottom.
2.) I filled the jar up more than halfway with water but with my finger on the hole in the side so that no water could leak out.
The thing is I don't know exactly how to explain it in scientific terms. All I do is that after I filled the jar with water, I turn the jar upside down so that the lid side is facing down and I still have my finger on the hole in the side. The idea is that when I turn the jar upside down with my finger still on the hole, no water will escape even though there are 5 holes on the lid. But when I take my finger off the hole, the water will start pouring out. I want to know how to explain what happens when my finger is on the hole and no water escapes even though there are 5 holes through which the water could get out. I also want to know this in a way that is easy to understand for a 7th grader. My project is due on Monday December 12,2005. Please help me explain this, and if not please tell me some websites that might help me explain my project. Thank you! :)PLEASE HELP!!!!

Hi Romi100,

I'm sure that there will be more knowledgeable folks out there who should be able to answer your question more thoroughly, but here's my interpretation of what you are observing:

The reason that the water fails to exit through the holes at the bottom of the jar (that is, the lid once it has been inverted) is due to the creation of a partial vacuum. Essentially, as the water vacates the 'top' of the jar due to gravity, and your finger is over the top hole, atmospheric pressure prevents the water from pouring out. That is, the atmospheric pressure present all around us, exceeds the pressure created by the water at the drain holes. Since your finger is over of the top hole, no air can enter this partial vacuum. Once you remove your finger, air enters this space, allowing the pressure above the water to equal the atmospheric pressure below, which is holding the water 'up'. As the two pressures equalize, the water is no longer held up by gravity, and pours out of the holes.

The surface tension of the water also has an effect on the phenomenon observed. You will notice that if you vary the size of the holes in the lid, you will reach a point where this surface tension is overcome. At this point (and above), the water will pour out as air enters. You see this in examples such as: if you take the cap off a 2 liter bottle of water and invert it. Air bubbles enter through the relatively large opening and the water pours out.

The volume of water also has an effect, too. You should find that there is a maximum volume (or mass) of water at which point the water will pour out, where it didn't with a smaller volume. The reason for this is that the mass of water is exerting a pressure due to gravity. Once this pressure exceeds atmospheric pressure, the water will pour out of the container. As long as the water pressure is less than atmospheric, it will remain in the jar - until you remove your finger from the top hole, that is!

I had a quick search on Google with "Partial Vacuum" and found quite a few valuable sites. I'm sure if you try using other terms mentioned above, you should be able to find a lot more, too

Anyway, like I said, this is just my two cents - but I hope it helped!

Good luck with your research, and please let us know if you have more questions.

Geoff.

I don't know if this will be in time, but the answer has to do with vacuum. Lets see if I can explain:

In your experiment, you have:

A liquid, which is mostly bounded by a solid object.

Gravity trying to pull the water out of the container, through the 5 holes in the bottom.

A hole in the side, that can let air in.

Air around the bottle, which has a certain force of its own. (this is based on the "weight" of the air....think about how much air there is between the surface of the Earth and outer space. This exerts a force, similar to the one that you'll feel if you dive deep underwater and feel the water pushing into your ears, making you uncomfortable). It may seem strange, but the air pressure where humans can live is much stronger than the force gravity can exert on something the size of the liquid in your container.

So....if gravity tries to pull the water out of your container through the holes, and there is no way for more air to get inside, what happens is that you have "nothing" in the container - and the scientific word for this is a vacuum.

The problem is, that the air under the bottle will want to rush in and fill the vacuum. If the whole bottom of the container was open, what would probably happen is the water would pour out, and air would rush up (tip a glass of water over quickly...you might see air rush through the water, forming turbulence....a soda bottle works even better).

The reason a small hole won't let water out and a big hole will goes into a concept called surface tension. Briefly, a liquid's molecules are attracted to each other, but don't lock into a crystal structure like a solid. When liquid is on all sides, the molecules don't attract to each other as strongly as they do when the molecules are only on one side. Each liquid has this tendency, but it varies in strength from liquid to liquid. You can think of this almost like a "skin" around the liquid.

The air only exerts so much force. The "skin" will only break if the force applied exceeds its strength...and air can exert more force if it has a bigger hole (more surface) to operate on.

So you can think of this as "The air is pushing on the water, but can't get to the vacuum because the surface tension in the water is strong enough to prevent the air molecules from pushing past the water molecules. The water is being pulled by gravity, but can't get past the air pushing on it.

Now you take your thumb off the hole in the side. Something new happens...air rushes in the side. Now we have gravity pulling the water out, but air replacing the water, so there isn't any vacuum forming in the container. Therefore the air underneath the container doesn't want to rush inside it, and push against the water.

The air coming in the side is still affected by surface tension but it has an easier time getting in (even if it is below the waterline) because there isn't as much water wanting to go in the opposite direction (only the water above the hole would want to "push out" the hole). The geometry of the hole might also matter - a very small hole on the side might not let the water out or the air in.

The best way to let water out of a container like this would have been to make a hole in the top. Then air could enter freely without having to push through the water. The air would push down, adding to the gravity, and cancelling the air that wants to push up.

So to sum up....more small holes in the bottom won't let the water out. One larger hole in the bottom might let the water out. A hole in the side might let the water out, and with a smaller hole than would be needed in the bottom. A hole in the top of almost any size will let the water out.

The difference is how the air around the container interacts with the holes, and how easy it is for the air to replace the vacuum formed by the water pouring out.

[quote="geoffbruton"]Hi Romi100,

I'm sure that there will be more knowledgeable folks out there who should be able to answer your question more thoroughly, but here's my interpretation of what you are observing:

The reason that the water fails to exit through the holes at the bottom of the jar (that is, the lid once it has been inverted) is due to the creation of a partial vacuum. Essentially, as the water vacates the 'top' of the jar due to gravity, and your finger is over the top hole, atmospheric pressure prevents the water from pouring out. That is, the atmospheric pressure present all around us, exceeds the pressure created by the water at the drain holes. Since your finger is over of the top hole, no air can enter this partial vacuum. Once you remove your finger, air enters this space, allowing the pressure above the water to equal the atmospheric pressure below, which is holding the water 'up'. As the two pressures equalize, the water is no longer held up by gravity, and pours out of the holes.

The surface tension of the water also has an effect on the phenomenon observed. You will notice that if you vary the size of the holes in the lid, you will reach a point where this surface tension is overcome. At this point (and above), the water will pour out as air enters. You see this in examples such as: if you take the cap off a 2 liter bottle of water and invert it. Air bubbles enter through the relatively large opening and the water pours out.

The volume of water also has an effect, too. You should find that there is a maximum volume (or mass) of water at which point the water will pour out, where it didn't with a smaller volume. The reason for this is that the mass of water is exerting a pressure due to gravity. Once this pressure exceeds atmospheric pressure, the water will pour out of the container. As long as the water pressure is less than atmospheric, it will remain in the jar - until you remove your finger from the top hole, that is!

I had a quick search on Google with "Partial Vacuum" and found quite a few valuable sites. I'm sure if you try using other terms mentioned above, you should be able to find a lot more, too

Anyway, like I said, this is just my two cents - but I hope it helped!

Good luck with your research, and please let us know if you have more questions.

Geoff.[/quote]


Hi Geoff!
Thank you so much for your help! My teacher was very impressed with my experiment and I got an A grade thanks to your help. Once again thank you very much!

*Romi100*

Hi Romi100!

Thank you for letting us all know how you got on with your project - and I am delighted to hear that you received an "A" for your efforts! Well Done!

It is always so wonderful to hear that we have been able to make even a small difference to someone's work - but please be assured that *you* are the one who did all the hard work. I was very pleased to see that your teacher was impressed with your efforts, too. Well done, once again!

Best wishes,
Geoff.