Weight of water

[10ClarionSt]

6yr old granddaughter: 'Is cold water heavier than hot water?'

Granddad: 'Er...er...'

6yr old 'If it is, why is it?'

Granddad: 'Er...er...do you want to watch Barney?'

[steve @ home]

Equal amounts of water would weigh the same whatever the temperature. But I suspect your granddaughter might be referring to the density of water, just not having the vocabulary to express this. ( I teach 6 year olds at my school and often get questions like this in science lessons) Water is a strange substance really and its density plays odd tricks. Usually, the colder a substance gets, the denser it becomes but ice floats! I hope someone comes in with a more scientific explanation.

[shammydodger]

Warmer water is generally less dense than colder water, except under about 4 degrees Celsius. Thus, if you were to cool water from room temperature down to freezing point, the water would get more dense until it reached approx 4 degrees Celsius. As it cooled further, the water would become less dense. Eventually, at zero degrees Celsius, it will freeze. The ice formed is indeed less dense than the water at zero degrees from which it is formed, which explains why ice floats on water.

The reasoning behind this is explained by the changing intermolecular forces within the water. Hydrogen bonding will be involved somewhere, which will help to explain this "odd" behaviour of the density of water when compared to the behaviour of most other liquids.

[mjd]

I really don't understand how you can be teaching science, at any level, if you don't understand this basic behaviour of water. Perhaps it's my problem in not understanding how the education system works now.

[mjd]

I'm sorry if my answer seemed blunt but it was directed at steve @home

[shammydodger]

mjd

Hopefully he's not a teacher! And if he is he has my profound sympathy.

[shammydodger]

And I don't think he ever said he was teaching.

[Buenchico]

Why's Mjd getting at Steve?

As I read it, he's got a good understanding of the behaviour of water but simply stating that the reasons behind such behaviour are complex (and certainly beyond the understanding of 6 year old children).

Teaching science, at any level, can be a tough job - partly because of the difficult questions which children ask. The reason I joined AB, many thousands of posts ago, was to seek an answer to a question asked by a 12-year-old pupil, in a Science lesson, over 30 years ago. Despite the help of several ABers (plus hundreds of people I've asked over the years), I still haven't found the 'perfect' answer to that one 'simple' question! :
http://www.theanswerbank.co.uk/Science/Questio n140971.html

I say 'Well Done' to Steve for his prompt (and accurate) reply to the original question.

I offer an even bigger 'Well Done' to 10ClarionSt for understanding that children's questions should be taken seriously.

Chris

[shammydodger]

Chris

Hear hear!

Can I buy you a pint?

[shammydodger]

And mjd

Yes I think it is your problem.

[ugly_bob]

mjd - What?

If you fully understand the behaviour of water and the reasons behind it, I will happily stand and applaud you.
Anyway, 6 year old science lessons aren't exactly astrophysics, and the requirements for a teacher at this level do not include an in-depth knowledge of molecular chemistry

[jake-the-peg]

I think we've all got the concept that generally speaking the colder something is the denser it is.

There is an oddity with water as has been said that it reaches it's densest at about 4 degrees Celcius.

Water is two hydroden atoms with an oxygen atom in the middle. The Oxygen atom in the middle attracts the electrons that bind up the parcle so the whole thing is a bit bent (like a boomerang) with a small net negative charge on the Oxygen atom and a small net charge on the hydrogen atoms.

This means the hydrogens on one atom will weakly attract the Oxygens in another ( The hydrogen bond ).

As watre cools this means the atoms attempt to arrange themselves with the hydrogens against Oxygens which takes more room and hence is denser.

A slightly better worder explanation with pictures here:

http://www.chemistryquestion.com/English/Questions/ChemistryInDailyLife/5c_Water_Ice.html

[gazzawazza]

A simpler way to explain it might be that water expands by 30% (volume) when it freezes, therefore one litre of frozen water will "weigh" less than the water underneath it.

Hmm, maybe it wasn't a simpler way after all.

[qapmoc]

Its a good job that ice does float or the worlds oceans would be a solid mass of ice from top to bottom, because once ice formed at the surface it would sink to the bottom and never melt again and eventually the oceans would have filled up completely.