Bending Water with a Charged Rod: A Polarity Demo

Few demonstrations get a louder reaction for less effort. Rub a plastic rod with a cloth, hold it near a thin stream of water, and the stream swerves sideways towards it. It’s often used to “show” that water molecules are polar. That claim is only partly true, and working out what is really going on is a better lesson than the demo itself. This guide gives a safe method, the expected results, and an honest account of the physics and chemistry involved.

Purpose

To observe how a thin stream of water is deflected by an electrically charged rod, to test whether the sign of the charge matters, and to evaluate how far the result can be used as evidence for polar molecules.

Background: what “polar” means

In a water molecule, oxygen is much more electronegative than hydrogen, so the shared electrons in each O–H bond sit closer to the oxygen. Oxygen carries a small negative charge (δ−) and each hydrogen a small positive charge (δ+). Because the molecule is bent rather than straight, these bond dipoles don’t cancel. The whole molecule has a permanent dipole: one end slightly negative, the other slightly positive. The details are in polar vs non-polar molecules and dipole moments.

A non-polar liquid such as hexane is made of molecules whose C–H bonds are only very slightly polar and are arranged so the small effects cancel out. Hexane molecules have no significant permanent dipole.

Equipment

• A burette on a stand, or a plastic bottle with a small hole near the base, or a slowly running tap
• A large beaker or sink to catch the water
• Rods or objects to charge: a polythene (polyethene) rod, an acetate or Perspex rod, a plastic comb, an inflated balloon
• Cloths to rub them with: a dry woollen or synthetic cloth, and a dry paper towel or silk
• A ruler held behind the stream (or a sheet of squared paper) to judge the deflection
• Paper towels for spills
• Eye protection

Method

1. Fill the burette with tap water and set it over the beaker. Open the tap until you get a thin, smooth, unbroken stream a few millimetres wide. A thin stream works far better than a thick one.
2. Rub the polythene rod briskly with the woollen cloth about ten times. Polythene usually becomes negatively charged.
3. Bring the rod slowly towards the stream, about 3–5 cm below the burette tip, without touching the water. Watch the stream from the side against the ruler.
4. Record the direction of bending and estimate the sideways shift of the stream just above the beaker.
5. Wipe any water off the rod, dry it and recharge it. Repeat twice.
6. Now rub the acetate or Perspex rod with a dry paper towel or silk. This usually leaves it positively charged. Repeat steps 3–5.
7. Try a comb run through dry hair, or a balloon rubbed on a jumper.
8. Test an uncharged rod as a control.
9. Optional: move the charged rod closer to and further from the stream and describe how the deflection changes.

Static charging works best in dry air. On a humid day the rods lose charge quickly, so recharge between every trial.

Risk assessment notes

Hazard	Risk	Control
Water on the floor	Slips	Catch the stream in a large beaker or sink; mop spills immediately
Water near electrical equipment	Damage or shock	Keep mains sockets and equipment away from the demo area
Glass burette	Breakage, cuts	Clamp securely; don’t lean on the stand
Rubbing rods	Very low hazard	None needed beyond normal care

The charged rods carry only a tiny amount of static charge. There is no mains electricity anywhere in this activity.

Expected results

Object	Rubbed with	Usual charge	Effect on water stream
Polythene rod	Wool	Negative	Bends clearly towards the rod
Acetate or Perspex rod	Paper or silk	Positive	Bends clearly towards the rod
Balloon	Jumper or hair	Negative	Bends towards the balloon
Plastic comb	Dry hair	Negative	Bends towards the comb
Uncharged rod	Nothing	None	No visible bending

The deflection grows as the rod gets closer. Occasionally, if the stream touches the rod, it picks up the same charge and can then be pushed away, or it breaks up into a spray. That is a sign you went too close.

What the results actually tell you

The most important observation is that the stream bends towards the rod whichever charge the rod carries. That rules out a simple explanation such as “water is positive, so it’s attracted to a negative rod”. Water as a whole is neutral. So why does it move at all? Two effects work together.

1. Charge induced in the stream

The water in the burette is connected, through the liquid, to a large reservoir, and tap water contains dissolved ions that let charge move. When a negative rod is held near the stream, it repels negative charge back up the stream and draws positive charge towards the part of the water nearest the rod. As the stream leaves the nozzle and breaks into droplets further down, some of those droplets carry away a small net positive charge. A charged stream near an oppositely charged rod is attracted, strongly. A positive rod does the same in reverse, inducing negative charge. Either way, the water ends up with the opposite charge to the rod, and attraction follows.

This induced charge is widely regarded as a major part of the effect, not a small side issue. The stream falling near the nozzle is where it matters most, which is why the demo works best with the rod held close to the tip.

2. Polarisation of the liquid

The rod’s electric field is strongest near the rod and weaker further away. Any material placed in such a field becomes polarised: its charges shift slightly so that the side facing the rod takes on the opposite sign. In water, part of this polarisation comes from permanent dipoles turning to line up with the field, which water does very readily. In a non-uniform field, a polarised object is pulled towards the region of stronger field, whichever way the field points. This also produces attraction towards both positive and negative rods.

Water responds unusually strongly to electric fields. Its relative permittivity (a measure of how much a material polarises in a field) is about 80 at room temperature, compared with about 2 for hexane. That large value is linked to water’s polar molecules and hydrogen bonding.

So is it proof that water is polar?

No. It is consistent with water being polar, but it doesn’t prove it. Much of the bend comes from charge induced in the stream and carried by droplets, which depends on water containing some mobile ions and being connected to its reservoir. Even a non-polar liquid would be slightly attracted by a charged rod, because any material can be polarised to some extent. The evidence that water molecules themselves have a permanent dipole comes from measurements such as dipole moments and from the bent shape found by spectroscopy, which you can read about in bond polarity.

The non-polar comparison

The interesting comparison is with a non-polar liquid such as hexane or cyclohexane. Run through a burette in the same way, a thin stream of hexane is deflected much less than water. Hexane has a low permittivity and conducts electricity so poorly that very little charge can be induced in its stream.

This comparison should not be carried out as a class practical. Hexane and cyclohexane are highly flammable, and their vapour can be ignited by a static spark, which is exactly what a charged rod can produce. The safe way to include it is as a video or data comparison: many teaching resources show water and a hydrocarbon stream side by side, and students can compare the measured deflections from still frames. Discuss why the difference exists without anyone pouring flammable liquid near static charge.

Questions

1. Describe what happened to the water stream with the negative rod and with the positive rod.
2. Explain why the result with the positive rod shows that the water stream is not simply “charged” on its own.
3. Draw a water molecule and label the δ+ and δ− ends. Explain why the dipoles don’t cancel.
4. Explain, in terms of induced charge, why a negative rod attracts the stream.
5. A student writes: “This proves water is polar.” Give two reasons why this conclusion goes too far.
6. Predict how a hexane stream would behave compared with water, and explain your prediction.
7. Why must a hexane comparison never be done with a charged rod in an open lab?

Answer pointers

1. Both bent the stream towards the rod.
2. If the stream carried its own fixed charge, it would be attracted by one sign and repelled by the other. Attraction to both shows the charge separation is induced by the rod.
3. Oxygen is δ−, both hydrogens δ+. The molecule is bent (about 104.5°), so the two bond dipoles add to give a resultant dipole.
4. The rod pushes negative charge away through the connected water, leaving the nearby stream and droplets with a net positive charge, which is attracted to the negative rod.
5. Much of the effect comes from induced charge, which needs mobile ions rather than polar molecules; and any liquid can be polarised to some extent, so attraction alone doesn’t prove a permanent dipole.
6. Much smaller deflection: hexane’s molecules are non-polar, it has a low permittivity, and almost no charge can be induced in it.
7. Hexane is highly flammable and a static spark from the rod could ignite its vapour.

Sources of error and limitations

• Humidity. Damp air discharges the rods quickly, so deflections shrink with time. Recharge before every trial.
• Charge varies. Different rubbing gives a different amount of charge. Comparisons between rods are only rough.
• Distance. A few millimetres’ change in rod position changes the deflection a lot. Hold the rod at a marked distance, or use a clamp.
• Stream thickness and speed. A fast, thick stream bends less. Keep the burette tap setting the same.
• Judging deflection by eye. Use a ruler or squared paper behind the stream, or film it and measure from a frame.
• Water type. Tap water contains ions and usually shows a clear effect. Distilled water also bends, but results can differ slightly.

Key takeaways

• A thin water stream bends towards a charged rod, whether the rod is positive or negative.
• The effect comes largely from charge induced in the stream and carried by droplets, plus polarisation of the liquid, which includes permanent dipoles lining up with the field.
• Water’s polar molecules help explain why it responds so strongly, but the demo is consistent with polarity rather than proof of it.
• Non-polar liquids like hexane deflect much less; show that comparison with video or data, never with a charged rod near flammable liquid.
• For stronger evidence of polarity, look at electronegativity differences, molecular shape and measured dipole moments; see electronegativity and bond type.