Science Buddies: "Ask an Expert"

Hi, I am not in a big hurry but, I am doing some research and would like to know that if you were to take a partical from a substance would the substance fall apart or would it search for that partical, find it and move the partical back to the substance? Thanks

sciencechick55 wrote:Hi, I am not in a big hurry but, I am doing some research and would like to know that if you were to take a partical from a substance would the substance fall apart or would it search for that partical, find it and move the partical back to the substance? Thanks

I'm not sure what you are trying to say, could you please rephrase it, perhaps including an example of what you mean?

Sure. An example of what I am trying to say is if you were to take a hydrogen atom from water would the water search for the missing hydrogen particle? Thanks.

If you try to remove a water molecule from liquid water, there will at first be an attractive force pulling the molecule back. This force is a short range electric force known as a "hydrogen bond" (you can look this term up in Wikipedia for more details). Once you get the test molecule farther away than a few water molecule sizes this force drops off and the molecule is free to roam around. Whenever it runs into another molecule it will bounce off either elastically or inelastically. ("Elastically" means that the molecule's kinetic energy is the same after the collision as it was before; conversely, "inelastically" means that it transfers some of it's kinetic energy of motion into some other form of energy, such as making the two molecules vibrate after the collision.) Sometimes, when it collides with the wall of whatever container the water vapor is in, it will stick due to another type of intermolecular force. If there is enough water vapor and if the walls of the container are "sticky" enough, the walls will get wet. Other times the free molecule may run into the surface of some liquid water, in which case the force of hydrogen bonding (and other, weaker attractive forces) will grab onto it and its days of roaming will be over, the liquid water will have absorbed it. Depending on the temperature and on the amount of water vapor present above the surface of the water, the rate of molecules escaping (evaporating) versus being absorbed (condensing) will determine if the water vapor is 1) in a stable state, 2) in a state where it condenses into the existing water until some fraction of the vapor is gone, or 3) in the reverse situation where some or all of the liquid water will evaporate into the vapor form. The details of this behavior are complex and are studied under the name "statistical mechanics".

The above answer deals with a water molecule. I read the question to ask what happens if you break a water molecule down and remove a hydrogen atom. The water molecule is primarily held togther with ionic bonds. One way to break apart water molecules is to pass an direct electrical current through the water. Typically you have to add [something to] increase the conductivity of the water [typically] by disolving an ionic salt or adding an acid or base. Hydrogen atoms will be attacted to cathode and oxygen atoms will be attracted to the annode. Bubbles of hydrogen gas and oxygen gas will form on the electrodes and bubble out of the solution where they can be captured in an upside down water filled balloon.

Caution: Hydrogen gas is highly combustible and will burn (combining with oxygen from the air).
Caution: If the salt, acid, or base used to increase conductivity contains chlorine, beware of any green colored gas as Cl2 is a highly toxic poison.

A mixture of Hydrogen gas and Oxygen gas is highly explosive and won't react on their own without a catalyst, spark, flame, or sufficiently hot surface.

Craig is right -- I did misread the question. Sorry for any confusion.

When you say "take a hydrogen atom from water would the water search for the missing hydrogen particle" do you mean remove a hydrogen atom, H, from a water molecule, HOH, leaving a molecule OH behind? If that is the case, then what will happen is that as you pull the HOH molecule apart you will pull off just an H+ ion (a proton with no electron) leaving behind an OH- ion (with an excess electron). In a sense the OH- ion will "search" for any positively-charged thing near to it because of the attractive force exerted by the electrostatic field surrounding its (excess) negative charge. The nearest positive ion will probably be the H+ ion you just pulled off, so in this sense the OH- ion could loosely be said to be "searching" for its missing piece. Note, however, that the OH- ion is not picky -- it will attract any positive ion in its vicinity, not just the H+ that formerly was attached to it. If all this is occurring in liquid water, then the situation becomes more complex because the ions become "solvated", meaning they are loosely attached to one or more un-ionized water molecules, but the basic picture remains the same -- the positive and negative ions "want" to recombine.

I hope this Plus Craig's description will satisfy your curiosity, but we'll be here to answer any questions you still have.