Middle School Science Question: Conductivity of Water

[cj]

My 8th grade students are doing a comparison of ionic and covalent compounds, comparing their electrical conductivity. We see that water is not a good conductor. So why is it so dangerous to have electricity around water?

 

[Kurdt]

Pure water is an insulator but if the water is not pure and especially if it has ionic solutes in it then it will conduct electricity.

 

[wysard]

Most water is not pure but even pure water passes electricity. (Somewhere around 245,000 ohm meters at STP). The hardness or softness in tap water for instance is a measure of the dissolved mineral content in water. The ions are what carry a charge in water. Bottom line, the more stuff you dissolve in water that creates ions the lower the resistance of the water. Sea water for example is a very good conductor (as far as water goes that is)

 

[noagname]

what experiment were you doing
have you tried boiling the water
just so you know i am an eight grader

 

[TVP45]

In addition to the good points that Kurdt and wysard made, water is particularly dangerous because it makes such an easy, good contact.

 

[hongkongrubbish]

Pure water is an insulator but if the water is not pure and especially if it has ionic solutes in it then it will conduct electricity.

Pure water is actually a conductor, since the H2O molecules will dissociate themselves into ions, although it is of very low concentration

 

[Kurdt]

Pure water is actually a conductor, since the H2O molecules will dissociate themselves into ions, although it is of very low concentration

I was aware of this. The resistance of pure water is so high however that for the purposes of this question it would act as an insulator.

 

[wysard]

I was aware of this. The resistance of pure water is so high however that for the purposes of this question it would act as an insulator.

Yup. As I mentioned somewhere around 245k ohm meters at STP.

Noagname: sorry, sometimes the adults miss what you asked. To answer your question: Yes I have boiled water. Sometimes even to make soup. But I think you mean with regards to resistance. Hot water IS less resistant to electricity than cold water. But not because you made the water hot. Most of the reason is because you made the ions IN the water more energetic. But most of that won't show up on a middle school experiment lab.

But if you boil the water a while and a pile of water leaves as steam so say the pot or beaker is only half full of water, I bet there are almost the same number of free ions in the water, but only half the water. So the concentration of ions is higher in the water that is left. So the resistance would go down. Does that make sense to you? If not, just ask why, or what experiment you are doing and we will help out.

 

[noagname]

if you have boiled water is that considered pure then would that mean more ohms can go through it

 

[dst]

if you have boiled water is that considered pure then would that mean more ohms can go through it

Ohms are a unit of resistance. If you want to talk about Xes of electricity, you would say coloumbs (the total amount of electrons involved).

The purer the water, generally, the higher the resistance. As said, conduction in water comes from the presence of ions. Less ions = more resistance.

 

[TVP45]

The question, as I understand it, is why is electricity dangerous around water. The answer to the dangerous part has to do with how electricity kills. Aside from large currents which turn you into a crispy critter, the danger comes from the current that flows through your heart (all that blood and fluids in your body are pretty good conductors). So, these are general rules only. Don't try this to see if it's OK.

If a current greater than 5 mA flows through your heart, it will (usually) put your heart into irregular beats (fibrillation) or (less often) stop it all together. Both kill you, fibrillation a little more slowly.

Dry skin resistance can be as high as 100,000 Ohms (and that depends on the actual voltage and changes with the current, etc but isn't important here). Wet skin resistance can be as low as 1000 Ohms (or much less depending on your skin). And that doesn't much depend on whether you have spring water, recently deionized water, boiled water, etc.

So, if you're in the bathtub, your skin resistance is about 1000 Ohms, and you're using the 120VAC electric shaver and drop it in the water, the current will be about 120 mA and it has the opportunity to flow through your heart (maybe in the left arm and out the left leg which are both wet - who knows?). This will likely kill you. If you're interested, Google the death of the monk/author Thomas Merton.

Ground Fault Current Interrupter circuits are required in the US wherever you have wet conditions - bathroom, kitchen, basement, outside, etc - and they work by quickly opening whenever the "extra" current (that is, the difference between the currents in the hot and neutral wires) reaches 5 mA.

 

[Sojourner01]

It is extraordinarily difficult to obtain water pure enough that it would not be a significant conduction/electrocution hazard. Simply being in contact with any material surface would, most likely, instantly contaminate it with stray ions to the point that it would become a good enough conductor to pose a hazard - within a very short space of time. I would suggest that the only situation where water can be considered an insulator would be in nanotechnology applications where the quantities are microscopically small and confined in very particular systems.