£1000 Prize; Hot Water v Cold Water: Freezing
Discussion
AJI said:
Well here is an interesting graph...
http://olom.info/ib3/iB_html/uploads/post-1-47221-...
Can't vouch for its validity (or the surrounding environment conditions) but it is suggesting that a higher starting temperature liquid spends much LESS time in the transition between liquid and solid. By the look of it, about 1/3 of the time in this particular example.
So one thing that is easily apparent is that the higher starting temperature involves a higher rate of heat loss to the surrounding medium.
Another thing is that both starting temp liquids reach a value above zero whereby they go in to state transition. (ie. the formation of stronger molecular bonds that will define the chracteristics of it as being a solid).
What I can't explain is why the hotter starting temp would mean the transition stage only take about 1/3 of the time of that of the lower starting temp liquid.
(And I guess this is the crux of this topic).
Looking at that graph, the flat section appears to be at around 4C. This is not particularly surprising as water is most dense at 4C so it's a logical place for a temperature boundary of some sort.http://olom.info/ib3/iB_html/uploads/post-1-47221-...
Can't vouch for its validity (or the surrounding environment conditions) but it is suggesting that a higher starting temperature liquid spends much LESS time in the transition between liquid and solid. By the look of it, about 1/3 of the time in this particular example.
So one thing that is easily apparent is that the higher starting temperature involves a higher rate of heat loss to the surrounding medium.
Another thing is that both starting temp liquids reach a value above zero whereby they go in to state transition. (ie. the formation of stronger molecular bonds that will define the chracteristics of it as being a solid).
What I can't explain is why the hotter starting temp would mean the transition stage only take about 1/3 of the time of that of the lower starting temp liquid.
(And I guess this is the crux of this topic).
Maybe below 4C the lowering density is because the crystal lattice of ice is beginning to form and thus energy is used for that rather than lowering the temperature, that would make sense.
Carfiend said:
1L of water at 100C has less matter (molecules are spread out further so takes up the same space with less matter) than 1L at 35C so there is less matter to cool.
Or something.
...and, the vapour coming off of the water has less matter still so perhaps that has something to do with it?Or something.
longshot said:
Carfiend said:
1L of water at 100C has less matter (molecules are spread out further so takes up the same space with less matter) than 1L at 35C so there is less matter to cool.
Or something.
...and, the vapour coming off of the water has less matter still so perhaps that has something to do with it?Or something.
R300will said:
1L of water at 100 degrees has the same matter as 1L of water at 35 degrees it's just less dense. although each molecule it actually heavier
A given molecule always weighs exactly the same amount, regardless of temperature. The lower density of hot water means in a given volume there will be less molecules, and thus less mass. So 1l of water at 100C has less molecules than 1l at 35C.
If you wanted to have the same amount of water in each case you'd be better off weighing it. 1kg of water always has the same number of molecules, regardless of temperature.
Flibble said:
R300will said:
1L of water at 100 degrees has the same matter as 1L of water at 35 degrees it's just less dense. although each molecule it actually heavier
A given molecule always weighs exactly the same amount, regardless of temperature. The lower density of hot water means in a given volume there will be less molecules, and thus less mass. So 1l of water at 100C has less molecules than 1l at 35C.
If you wanted to have the same amount of water in each case you'd be better off weighing it. 1kg of water always has the same number of molecules, regardless of temperature.
Finally the mass of water will always be the same if you have 1L of it but the density of the water may change with the temperature if the volume isn't sealed.
based on A-level chemistry
R300will said:
Not true. E=MC^2 so if you heat the water molecules up you impart energy into them and therefore mass. Also that last bit didn't really make sense, 1Kg of water = 1L of water. Also if you're sealing the volume then there will always be the same no. of water molecules present but the pressure will increase instead especially when it starts to boil.
There's no mass-energy conversion involved in heating water (or any other substance). Heat is a measure of the internal energy of the bulk substance, not individual molecules, and typically measures kinetic energy of the molecules. In a liquid it basically states how quickly they slide over each other (for gas it's speed of motion of molecules, for solids amount of vibration in the lattice).R300will said:
Finally the mass of water will always be the same if you have 1L of it but the density of the water may change with the temperature if the volume isn't sealed.
Mass = Density * volume. If you heat an unsealed container the density does indeed change, and so does the volume to compensate, assuming you're not losing any mass by evaporation of course. However, 1l isn't a measure of mass, it's a measure of volume. So as you heat it the volume will increase until you have more than 1l of water.
A given mass of a substance always has exactly the same amount of molecules, regardless of it's density or the volume it occupies. This was discovered by Avogadro and is what the Avogadro constant relates - 1 mole of a substance has by definition Avogadro's constant atoms in it and weighs a specific amount. This is independent of the temperature and state of the substance.
As such, 1 kg of water always has exactly the same number of atoms, no matter what temperature. Thus as it is heated and the density decreases the volume must increase, thus 1l of hot water has less atoms (and also less mass) than 1l of cold water.
based on undergraduate chemistry
Edited by Flibble on Friday 6th July 01:04
Flibble said:
R300will said:
Not true. E=MC^2 so if you heat the water molecules up you impart energy into them and therefore mass. Also that last bit didn't really make sense, 1Kg of water = 1L of water. Also if you're sealing the volume then there will always be the same no. of water molecules present but the pressure will increase instead especially when it starts to boil.
There's no mass-energy conversion involved in heating water (or any other substance). Heat is a measure of the internal energy of the bulk substance, not individual molecules, and typically measures kinetic energy of the molecules. In a liquid it basically states how quickly they slide over each other (for gas it's speed of motion of molecules, for solids amount of vibration in the lattice).R300will said:
Finally the mass of water will always be the same if you have 1L of it but the density of the water may change with the temperature if the volume isn't sealed.
Mass = Density * volume. If you heat an unsealed container the density does indeed change, and so does the volume to compensate, assuming you're not losing any mass by evaporation of course. However, 1l isn't a measure of mass, it's a measure of volume. So as you heat it the volume will increase until you have more than 1l of water.
A given mass of a substance always has exactly the same amount of molecules, regardless of it's density or the volume it occupies. This was discovered by Avogadro and is what the Avogadro constant relates - 1 mole of a substance has by definition Avogadro's constant atoms in it and weighs a specific amount. This is independent of the temperature and state of the substance.
As such, 1 kg of water always has exactly the same number of atoms, no matter what temperature. Thus as it is heated and the density decreases the volume must increase, thus 1l of hot water has less atoms (and also less mass) than 1l of cold water.
based on undergraduate chemistry
Edited by Flibble on Friday 6th July 01:04
And you're thinking of one mole of water having the same number of atoms, not kilograms. 6.02x10^34 if i remember rightly.
And if you've got a sealed container of 1L of water then the density or volume will not change and therefore the pressure will, which was my point. If the volume is not sealed then the density will remain uniform throughout the container but the volume will increase slightly.
1L of hot water weights slightly more than 1L of cold water. And it's the same as two magnets weigh slightly less when they are stuck together than when you have pulled them apart because you have had to impart energy into them in order to separate them and this is turned into mass. A very very very very small amount of mass but still.
Bedazzled said:
R300will said:
E=MC^2 so if you heat the water molecules up you impart energy into them and therefore mass.
Heating up water makes its molecules jiggle about more, i.e. increases kinetic energy, not the mass. E = mc^2 + 1/2 mv^2 (at non relativistic speeds)
Bedazzled said:
Only as a result of their increased KE, we're talking about water molecules moving at about 600m/s at room temperature, maybe 670m/s at boiling point, so the mass of each molecule increases by.... wait for it... an infinitesimal 0.00000000000000000000000000000000000001 grams, that's 1E-38g, or 15 orders of magnitude less than their rest mass.
It's all about the jiggling.
Of course it is really i was just being a bit pedantic because it's not true to say that they don't get a bit heavier. It's all about the jiggling.
R300will said:
It is a fundamental law of physics that if you impart energy into something you make its molecules heavier.
It's irrelevant as we're not talking about molecular properties, we're talking about bulk properties of a sample of water.R300will said:
And you're thinking of one mole of water having the same number of atoms, not kilograms. 6.02x10^34 if i remember rightly.
Same number of molecules, and same mass. See http://en.wikipedia.org/wiki/Molar_mass . And yes, that's Avogadro's constant, which I already mentioned.R300will said:
1L of hot water weights slightly more than 1L of cold water.
1l of hot water weighs less than 1l of cold water due to the lower density of hot water. Mass = Density * Volume remember.According to this: http://en.wikipedia.org/wiki/Properties_of_water#D...
Water at 100C has a density of 958.4 kg/m^3, which means a litre of it weighs 958.3 grammes. At 30C it has a density of 995.6502 kg/m^3 so a litre weighs 995.6502 grammes. 958.4g is definitely lighter than 995.6502g.
If you took 1l of cold water and heated it to 100C it would marginally increase in weight. However it would also increase in volume, so you would have more than 1l of water. Based on the above densities, 1l of water at 30C heated to 100C would have a volume of 1.0389l.
From the molar mass formula above it is known that the number of molecules of a substance is directly proportional it its mass. Given the mass of the hot water is less it follows that there are less molecules of the hot water as I stated before.
Edited by Flibble on Friday 6th July 19:48
Flibble said:
R300will said:
It is a fundamental law of physics that if you impart energy into something you make its molecules heavier.
It's irrelevant as we're not talking about molecular properties, we're talking about bulk properties of a sample of water.R300will said:
And you're thinking of one mole of water having the same number of atoms, not kilograms. 6.02x10^34 if i remember rightly.
Same number of molecules, and same mass. See http://en.wikipedia.org/wiki/Molar_mass . And yes, that's Avogadro's constant, which I already mentioned.R300will said:
1L of hot water weights slightly more than 1L of cold water.
1l of hot water weighs less than 1l of cold water due to the lower density of hot water. Mass = Density * Volume remember.According to this: http://en.wikipedia.org/wiki/Properties_of_water#D...
Water at 100C has a density of 958.4 kg/m^3, which means a litre of it weighs 958.3 grammes. At 30C it has a density of 995.6502 kg/m^3 so a litre weighs 995.6502 grammes. 958.4g is definitely lighter than 995.6502g.
If you took 1l of cold water and heated it to 100C it would marginally increase in weight. However it would also increase in volume, so you would have more than 1l of water. Based on the above densities, 1l of water at 30C heated to 100C would have a volume of 1.0389l.
From the molar mass formula above it is known that the number of molecules of a substance is directly proportional it its mass. Given the mass of the hot water is less it follows that there are less molecules of the hot water as I stated before.
Edited by Flibble on Friday 6th July 19:48
If you are measuring water at 100C then due to its lower density it will weigh less than water at 30C yes.
However if you took two separate litres of water, both at 30C then heated one to 100C it would weigh slightly more than the water at 30C and take up slightly more volume as well.
R300will said:
No. The density of the water is less. Not the mass.
If you are measuring water at 100C then due to its lower density it will weigh less than water at 30C yes.
However if you took two separate litres of water, both at 30C then heated one to 100C it would weigh slightly more than the water at 30C and take up slightly more volume as well.
Did I not just say that? I'm pretty sure I said exactly that and now you've said I didn't... Ok.If you are measuring water at 100C then due to its lower density it will weigh less than water at 30C yes.
However if you took two separate litres of water, both at 30C then heated one to 100C it would weigh slightly more than the water at 30C and take up slightly more volume as well.
Flibble said:
R300will said:
No. The density of the water is less. Not the mass.
If you are measuring water at 100C then due to its lower density it will weigh less than water at 30C yes.
However if you took two separate litres of water, both at 30C then heated one to 100C it would weigh slightly more than the water at 30C and take up slightly more volume as well.
Did I not just say that? I'm pretty sure I said exactly that and now you've said I didn't... Ok.If you are measuring water at 100C then due to its lower density it will weigh less than water at 30C yes.
However if you took two separate litres of water, both at 30C then heated one to 100C it would weigh slightly more than the water at 30C and take up slightly more volume as well.
When in actual fact if you take both litres at 30C and then impart heat into one to get it to 100C the density will change but the mass of the hotter water will increase not decrease.
Gassing Station | Science! | Top of Page | What's New | My Stuff