Hot water is heavier than cold water.

  1. Hot water is heavier than cold water.

    If density is decreasing means its cover area is also increasing, and weight is irrelative to the area body occupies. Isn't it?

    W=mg, to vary the weight we have to cop with its mass and gravity.
    As gravity is constant then is it the mass of the water that is varying??? :surprised
     
  2. jcsd
  3. If you tried to measure it with typical scales and instruments you'd find hot and cold water weigh the same. But all energy has an equivalent mass which participates in gravitatational attraction. Therefore the heated water is a very tiny bit heavier than cold water.
     
  4. Liquid water is most dense at 4 C.
    so your statement "Hot water is heavier than cold water." if you mean a certain volume of water is true only from 0 C to 4 C.

    This is also true,
    W = density x volume x g, where density x volume = mass

    So you can state either of the following:( increasing temperature above 4 C )
    and neglecting the very tiny energy increase as per Antiphon
    1. I start with a container of volume V containing water. Increasing the temperature will decrease the density of the water. Therefor, as I increase the temperature of the water, the water will expand and less mass of water will fill the volume V, due to the density decrease, and the water in volume V will now weigh less than before the temperature increase.
    2. I start with a container of volume V containing water. Increasing the temperature will decrease the density of the water. Therefor, as I increase the temperature of the water, the water will expand and I need a larger volume V2>V to contain the water, which has the same mass and thus the same weight as before the temperature increase.
     
  5. Water is made of tiny pieces called molecules. Heat makes molecules excited. They jump around and move quickly! They need space to do this. Cold, on the other hand, makes molecules slow down and it can make them nearly stop. They don’t need as much room to move around and the space between them shrinks, It would seem that cold water would have to be denser than warm water
     
  6. mfb

    Staff: Mentor

    This is true above 4°C. Below that, water forms more and more hydrogen bonds, which increase the volume and therefore reduce the density.

    Only if you want to include relativistic effects, which are extremely small here.

    However, if you put real hot and cold water (of the same amount) on a very precise scale, you might measure a smaller value for hot water, as it needs more volume and therefore gets more buoyancy due to the air.
     
  7. sophiecentaur

    sophiecentaur 13,547
    Science Advisor
    Gold Member

    We should really get this thread in proportion I think.
    It starts off with a very ill defined statement "Hot water is heavier than cold water." and, instead of dealing with that, the thread leaps into relativity. I think that the OP may need to be expanded and clarified; it really doesn't mean a lot of sense to me, as it stands. Could we start walking before we try to run?
     
  8. If we look to what Einstein uncovered where mass is proportional to energy we can see that as we heat the cold water to a higher temperature giving the water a higher thermal energy this would give us a larger mass in theory and in experimentation. But you would need a very precise measuring system because the difference in mass would be extremely small compared to the overall weight. So yes, it makes sense that hot water is heavier than cold water but in normal/every day practices it's not very practical to even consider it or take it into consideration.
     
  9. Yes, I was thinking the same. But then you realize hit water evaporates. Shouldn't that affect the mass? Even if you put a lid on top of the hot water container, the condensation that occurs means that some energy is lost in that transition.

    p.s. I don't think I need to clarify, but I'm talking about mass not weight.
     
  10. mfb

    Staff: Mentor

    If you remove water, the mass goes down, of course.
    Evaporation->condensation is just a re-distribution of energy and water, it does not change the total energy content.
     
  11. my logic is this: for water to evaporate it needs to be heated up, energy needs to be added. we've already done that by heating up the water to make it hot for our experiment. so that amount of energy is no longer relevant because we only care for the water after it became hot. however, when water condensate it loses energy, doesn't it? As far as I remember vapor becomes liquid because it cools down. Shouldn't that cooling down affect the overall energy level of the water?

    On second thought I think I'm overly complicating things when I consider the water's interaction with the environment. Because I have to consider the effects of the environment on the cold water as well.

    So, to recap: if we take relativistic effects into consideration (and I see no reason not to) hot water has more mass than cold water. even if that difference is undetectably small.
     
  12. D H

    Staff: Mentor

    Those relativistic effects are *tiny*. You are ignoring the classical effects which are multiple orders of magnitude larger. You only take relativistic effects into account when you have already accounted for everything else.
     
  13. mfb

    Staff: Mentor

    Well, it releases heat to the environment, the energy is not lost.

    To give an idea about the effect size: To bring 1kg of room temperature (20°C) water to the boiling point, you need an energy of ~320kJ. This leads to a mass increase of 3.6ng or 0.0000000000036 kg.

    At the same time, if pressure is constant, the volume increases by 41.6cm3, replacing air with a mass of 53mg or 0.000053kg. Buoyancy is 7 orders of magnitude stronger than relativistic effects.
     
  14. thank you for these calculations :) I didn't mean "lost" like destroyed. I meant it in the sense you mean it. And I thought lawyers were picky about words :biggrin:
     
  15. If we repeat this experiment with volume as constant i.e. putting same amount of water at same temperature in two closed containers so that they are full without any air gap, then heat one of those. After the heated one has gained enough energy, its mass will be more, even though its weight on earth is same?? (If it is correct, can we generalize that objects having same weight on earth can have different mass?)
     
  16. No we can't. The energy added to a water would add to stronger gravitational attraction, therefor weighing more
     
  17. mfb

    Staff: Mentor

    Right, increasing the mass also increases the weight.


    By the way: those 7 orders of magnitude for buoyancy effects would put unrealistic constraints on the stiffness of the container.
     
  18. Not true. Both liquid water and ice will undergo net vaporization whenever the overlying vapor pressure is less than the equilibrium pressure. The process is completely spontaneous. You don't have to add heat. If no heat is added, the enthalpy of vaporization will be provided by the water (or ice) itself, thereby reducing its temperature.
     
  19. TumblingDice

    TumblingDice 463
    Gold Member

    Please pay closer attention. This is 'June' thread. Launched in June 2012, sustained in June 2013, and now you've messed everything up by posting in December!

    Edit by NSA: Please ignore comment by TumblingDice. There is nothing out-of-the-ordinary, unusual, or otherwise significant about the timing and content of the posts in this thread. Carry on!
     
  20. It's wrong to say that cold water is heavier than hot water, it's just heavier per unit volume, but both have same mass so according density=mass/volume, mass is constant therefore when density increases the volume will decrease, Cold water has a higher density and less volume but same mass and same weight, in another way W=pvg there is an inverse relation between p and v when mass is constant so weight will always stay the same

    this is based on very simple physics,
    At an advanced level you can say that hot water is greater in mass "According Einstein" as there is a direct relation between energy and mass but that isn't practically taken in consideration as it has a mere effect
     
    Last edited: Dec 29, 2013
  21. mfb

    Staff: Mentor

    There is nothing new coming, just repetitions of old posts, so it is pointless to continue this thread.
    Closed.
     
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