Showing That Water is a Polar Molecule
Posted on: August 28, 2008
Let’s start off with the theory. Water has more of a positive charge on one side and a negative charge on the other. This is because of the way the molecules bond. The picture on the right shows the Mickey Mouse structure of water molecules. Oxygen has six electrons on the outer shell and wants to have eight. So it uses the electrons from the Hydrogen atoms to complete its shell. I’m not a chemist, I don’t know why the molecules make a Mickey Mouse and don’t spread out onto opposite sides, but they don’t. The result is a net positive charge near the ears and a negative charge on the other side.
There is a really cool and simple demonstration to show the polar nature of water. Turn on the faucet so that you have a laminar stream of water coming out. Alternatively, you could set up a simple apparatus that controls the flow of water from a flask. You want the stream to be a small as possible but still laminar (flowing in a parallel stream with no turbulence). Now charge up a plastic rod or a comb or anything that will hold a static charge (even a balloon rubbed on your hair will work. Put the charged object near the water and the stream will deflect. This freaks the kids out. I love freaking the kids out.
13 Responses to "Showing That Water is a Polar Molecule"
October 25, 2009 at 1:57 pm
Water has more of a positive charge on one side and a negative charge on the other.This is because of the way the molecules bond. In the above picture of water molecule, it has net positive charge near the ears and a negative charge on the other side. This is your statement
Now I have a very simple question to put here. the oxygen atom in water molecule, is bigger and negatively charged, Now can you please explain because of which charging on water molecule, the deflection in water stream is occuring, after the charged up comb or plastic is brought near to stream, Because of positive charge near ears or negative charge on oxygen atom.
October 25, 2009 at 2:09 pm
The answer to your question is a good one. You need to look up something called the “Triboelectric Series.” The series is typically ordered from highly positive charged objects at the top to highly negatively charged objects at the bottom. Hair is way at the top, and the plastic comb is at the bottom. So the act of combing your hair makes the comb strongly negatively charged.
After looking over the triboelectric series, you can experiment with other objects and see if you get different results. Does one object attract the water while the other repels it? Does the amount of attraction or repulsion change if you us other objects towards the middle of the series?
October 27, 2009 at 7:29 am
I referred to mr. trefor’s comments above , september, 07, and really wanna discuss this issue that what is the appropriate time for kids to start having an idea about atom and molecule. I guess at the age of 14 years in school, the children should be told about natural chemistry i.e. the natural things happening around them. For instance in the above demo, we are seeing water being attracted to charged up comb or plastic, after studying the water in molecular form. now if a kid asks ” what is a molecule ” we say ” it is the minust particle of a compound. so I guess, at schools we must design courses, so that a school student must have complete idea about atoms and molecules, now to introduce an atom, which element occuring naturally should be made an example for kids to understand atom, , that it is made up of minute particles called atoms, but not molecules.
October 27, 2009 at 6:49 pm
Here in the states, we introduce the idea of atoms and molecules in grade school. I can’t tell you exactly when, but the children are probably seven or eight. I taught a 7th grade (13 yr-olds) honors physical science a few years ago, these kids had heard about atoms many times before that. They had even learned about electron shells and the charges. I would question why you are waiting? Science isn’t Santa Claus, we give them the truth from when they are very small and go deeper into the subject as they get older and can understand more.
January 4, 2010 at 5:12 pm
I’m a little late in posting a response, but maybe there will be another straggler like me 🙂
I have used this demo to show polarization when doing electrostatics by showing the students the demo with a negativing charged rod and then asking “what will happen with a positively charged rod?” Of course most students immediately respond with “REPEL!!!” but then seeing that it also attracts helps them visualize how molecules orient themselves when an objected is polarized.
January 5, 2010 at 8:53 pm
Hey Marcie,
They beauty of teaching is that there is no late. You’re just early for next year’s kids. Actually, in my physics classes, I don’t get to magnetism until late spring, so you are early.
I would have said repel also, thanks for teaching me something. Why is the water attracted in both cases? Certainly Coulomb’s law doesn’t care. I’m at a loss to explain the effect.
January 7, 2010 at 6:48 pm
It has to do with how the water molecules align themselves. If you bring a negatively charged rod near the stream, the positive side of the molecule will orient itself closest to the rod causing the water to attract. If you bring a positively charged rod near the stream, the negative side of the molecule will orient itself closest to the rod causing it to….attract again!
*On a side note: Thanks for keeping this blog. As a fairly new teacher, additional resources and tips are always appreciated, but also I often find myself wanting to add an “AMEN!” to your narratives…it’s nice hear about others classroom experience and know that I’m not alone in my frustrations, concerns, and struggles. Thanks so much!*
June 21, 2012 at 10:03 pm
Opinions are free-learning for website publishers. Those who feel they have no more to know can turn them off.
March 18, 2013 at 1:30 pm
I just wanted to just take a couple of moments and
let you know that I enjoyed the article. I really don’t think generally people know how many hours of work that goes into producing a good post. I know that this will be kind of random but it really bothers me occasionally. At any rate nice blog.
March 18, 2013 at 1:32 pm
Thank you.
On Mon, Mar 18, 2013 at 2:30 PM, Physics & Physical Science Demos, Labs, & Projects for High School Teachers
Physics & Physical Science Demos, Labs, & Projects for High School Teachers 
September 7, 2008 at 7:43 pm
I did this demo last year in class but refused to tell them (at first) how it worked. We got a really good discussion going about how it could possibly be doing what looked like a trick. The kids can surprise you sometimes with how creative and innovative they can be. It felt a lot more useful to use this as a jumping off point for the lesson rather than just showing it to them and explaining it away. I’ve been trying, lately, to get the kids more excited about this stuff. They get uncomfortable when you don’t just tell them the answer right away and it can wake them up. Though, admittedly, I have also had classes where that fails completely and they just sit staring at you.