Chemistry Help

[mageslayer]

When a marble is dropped into a beaker of water, it sinks to the bottom. Which best explains why it happens? Justify your choice and for the choices you did not pick explain what was wrong with them.

-the surface area of the marble is not large enough to be held up by the surface tension of the water

-the mass of the marble is greater than that of the water


-the marble weighs more then an equivalent volume of the water

I picked this one because then the density of the marble would be greater than that of the water causing it to sink.

-the force from dropping the marble breaks the surface tension of the water


-the marble has a greater mass and volume then the water


I need help explaining what was wrong with the other choices, especially choice 1.

 

[nismotigerwvu]

It isn't necessarily the total surface area that is the issue, it is the area of contact between the surface of the water and the marble. When the marble comes in contact with the surface of the water, essentially only only one tiny point will be pressing down with the entire weight of the marble. Surface area itself would encompass the whole outer "shell" of the marble. So for answer 1 to be reworded to be correct you could replace "surface area" with "area of contact" or something along those lines. To think about it more reasonably, you can have a lilly pad float on the water, but if you crumble it up into a ball it will sink.

 

[mageslayer]

thanks that helped alot

 

[Cogman]

Originally posted by: nismotigerwvu
It isn't necessarily the total surface area that is the issue, it is the area of contact between the surface of the water and the marble. When the marble comes in contact with the surface of the water, essentially only only one tiny point will be pressing down with the entire weight of the marble. Surface area itself would encompass the whole outer "shell" of the marble. So for answer 1 to be reworded to be correct you could replace "surface area" with "area of contact" or something along those lines. To think about it more reasonably, you can have a lilly pad float on the water, but if you crumble it up into a ball it will sink.

I don't agree with this post. The density of an object is almost always the determining factor for why it floats or not. What you explained is why a marble would break surface tension, not why it doesn't float.

If you take a marble and hollow it out, it will float (Ping pong ball) If you take that same hollow marble (or ping Pong ball) and add some lead to it, it will sink. As long as an object is less dense then the water it will float or sink.

Think of a submarine. The shape of the submarine doesn't significantly change when it goes for air to when it dives under water. So what changes? They have ballast tanks that they fill/empty water into. By doing that, they change the density of the sub which in turn allows them to control how far the sub sinks down to.

Your initial answer was correct.

 

[Gibsons]

Originally posted by: Cogman

Originally posted by: nismotigerwvu
It isn't necessarily the total surface area that is the issue, it is the area of contact between the surface of the water and the marble. When the marble comes in contact with the surface of the water, essentially only only one tiny point will be pressing down with the entire weight of the marble. Surface area itself would encompass the whole outer "shell" of the marble. So for answer 1 to be reworded to be correct you could replace "surface area" with "area of contact" or something along those lines. To think about it more reasonably, you can have a lilly pad float on the water, but if you crumble it up into a ball it will sink.

I don't agree with this post. The density of an object is almost always the determining factor for why it floats or not. What you explained is why a marble would break surface tension, not why it doesn't float.

If you take a marble and hollow it out, it will float (Ping pong ball) If you take that same hollow marble (or ping Pong ball) and add some lead to it, it will sink. As long as an object is less dense then the water it will float or sink.

Think of a submarine. The shape of the submarine doesn't significantly change when it goes for air to when it dives under water. So what changes? They have ballast tanks that they fill/empty water into. By doing that, they change the density of the sub which in turn allows them to control how far the sub sinks down to.

Your initial answer was correct.

With some care, you can make things that are more dense than water float, due to surface tension. Text

 

[mageslayer]

Originally posted by: Cogman

Originally posted by: nismotigerwvu
It isn't necessarily the total surface area that is the issue, it is the area of contact between the surface of the water and the marble. When the marble comes in contact with the surface of the water, essentially only only one tiny point will be pressing down with the entire weight of the marble. Surface area itself would encompass the whole outer "shell" of the marble. So for answer 1 to be reworded to be correct you could replace "surface area" with "area of contact" or something along those lines. To think about it more reasonably, you can have a lilly pad float on the water, but if you crumble it up into a ball it will sink.

I don't agree with this post. The density of an object is almost always the determining factor for why it floats or not. What you explained is why a marble would break surface tension, not why it doesn't float.

If you take a marble and hollow it out, it will float (Ping pong ball) If you take that same hollow marble (or ping Pong ball) and add some lead to it, it will sink. As long as an object is less dense then the water it will float or sink.

Think of a submarine. The shape of the submarine doesn't significantly change when it goes for air to when it dives under water. So what changes? They have ballast tanks that they fill/empty water into. By doing that, they change the density of the sub which in turn allows them to control how far the sub sinks down to.

Your initial answer was correct.

I realize my initial answer was correct, however, nismo simply helped me disprove the first statement by pointing out that increased surface area is not the same as increased area of contact.

 

[Cogman]

Originally posted by: Gibsons

With some care, you can make things that are more dense than water float, due to surface tension. Text

Hence the reason I said almost all.

 

[CycloWizard]

For something to sink when placed on water, it should have a force balance such that

buoyancy force < gravitational force + surface tension.

If we assume the marble is made of standard glass, then the marble is very hydrophilic and the surface tension contribution will be small or even negative. Thus, the only significant term for a standard marble is the buoyancy term.

In more exotic cases, where the glass may have a superhydrophobic coating and has a very low density (e.g. a "porous" glass), then surface tension effects could prevent it from sinking if placed on the water. However, dropping the marble into the water adds the additional kinetic energy term due to the inertia of the marble, making flotation even harder. Obviously it can still be done if we approach the problem correctly.

 

[Eeezee]

Originally posted by: Cogman

Originally posted by: nismotigerwvu
It isn't necessarily the total surface area that is the issue, it is the area of contact between the surface of the water and the marble. When the marble comes in contact with the surface of the water, essentially only only one tiny point will be pressing down with the entire weight of the marble. Surface area itself would encompass the whole outer "shell" of the marble. So for answer 1 to be reworded to be correct you could replace "surface area" with "area of contact" or something along those lines. To think about it more reasonably, you can have a lilly pad float on the water, but if you crumble it up into a ball it will sink.

I don't agree with this post. The density of an object is almost always the determining factor for why it floats or not. What you explained is why a marble would break surface tension, not why it doesn't float.

If you take a marble and hollow it out, it will float (Ping pong ball) If you take that same hollow marble (or ping Pong ball) and add some lead to it, it will sink. As long as an object is less dense then the water it will float or sink.

Think of a submarine. The shape of the submarine doesn't significantly change when it goes for air to when it dives under water. So what changes? They have ballast tanks that they fill/empty water into. By doing that, they change the density of the sub which in turn allows them to control how far the sub sinks down to.

Your initial answer was correct.

He also needs to explain why the other answers are not correct. Basically, the point of contact is very small and the marble exerts a lot of force on that point of contact, thus it easily breaks the surface tension.

There are instances where surface tension becomes a large factor, but it's uncommon. Typically buoyancy is the only factor you need to consider.