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I have learnt that when we heat ice-like substances it changes to water and when I asked my teacher she said that the distance between molecules increases.

When I thought about it a bit more a question arose: Why does the distance between the molecule increase as we raise the temperature?

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    $\begingroup$ With ice and water, in fact, it is quite the opposite. $\endgroup$ – Ivan Neretin Mar 27 '18 at 13:52
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    $\begingroup$ No, real world physical constants are mostly read-only. $\endgroup$ – Ivan Neretin Mar 27 '18 at 14:04
  • $\begingroup$ @IvanNeretin tho' the actual expansion occurs in water as it cools just before freezing into a solid, not at the phase transition IIRC $\endgroup$ – Carl Witthoft Mar 27 '18 at 17:52
  • $\begingroup$ I was expecting the energy vs interatomic distance graph. $\endgroup$ – Eashaan Godbole Mar 27 '18 at 19:22
  • $\begingroup$ Firstly, the generalisation isn't universally true: water below 4°C is denser than ice despite the extra temperature. Secondly, at least for liquids, the density doesn't usually decrease much as temperature is raised. So the generalisation is a poor one. Except for gases where the density at constant pressure changes a lot with temperature as a direct result of the higher kinetic energy of the molecules and the interrelationships of pressure, temperature and volume. Your teacher was fobbing you off with an over simplistic explanation. $\endgroup$ – matt_black Mar 28 '18 at 18:35
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The answer "the distance between molecules increase" is incomplete if not plain wrong.

Temperature is an effect of energy present. Basically, it's an effect of little movements and vibrations of molecules and atoms due to their energy.

In an crystal, the energy of the molecules is so low, that they don't vibrate and move enough to break the structure. The more energy you put into the system, the more the molecules move. At one point, the movement is too much to keep the molecules in place, the crystal structure breaks apart, the ice melts.

A liquid (NOT WATER, IT IS A SPECIAL CASE) has lower density than the crystal because the molecules are moving around a lot and "need more space".

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    $\begingroup$ It might be worth adding that it isn't true of water anyway - the solid state is less dense than the liquid. $\endgroup$ – Luaan Mar 27 '18 at 18:58
  • $\begingroup$ OK, but for a solid, such as an iron rod in a 1atm environment. Its dimension will change with temperature, say from 0C to 200C; it will remain solid, but will expand with increasing temperature. Same for the alcohol in your average household thermometer over everyday temperature ranges (which is how it works to tell the temperature). What is going on at the atomic/subatomic level to explain this? $\endgroup$ – Anthony X Mar 28 '18 at 3:20
  • $\begingroup$ @AnthonyX vibrations, little movements. $\endgroup$ – Fl.pf. Mar 28 '18 at 16:48
  • $\begingroup$ I think it's also relevant to note that for water and for ice, density still decreases with temperature (for water above 4ºC, where it reaches a density maximum) - it's just that the pretty unique structural effects that take place between 0ºC and 4ºC override the effect of temperature. $\endgroup$ – Vic Lineal Mar 28 '18 at 17:44
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In general, for any substance, molecules in the liquid state are far more free to move as the intermolecular forces of attraction are lesser here (Less-densely packed). But considering water(as pointed out by Ivan), it actually shows the reverse trend ie., density of ice < denisty of water. For more details on this, refer Physics.SE and/or Quora.

Also in general, for any substance, when temperature increases, the total energy (includes kinetic and vibrational) increases, as temperature is a measure of energy of a system. When considering the solid state, since they are tightly packed, the particles can only vibrate about their mean positions and when the vibrations becomes more, it is tougher for the system to hold onto, and hence the phase change occurs. (From solid to liquid state). But you’ve also got to note that some substances don’t exist in liquid state at a particular pressure.

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  • $\begingroup$ Ever seen ice floating in water? $\endgroup$ – Ivan Neretin Mar 27 '18 at 19:21
  • $\begingroup$ Yes, I get that ice is less dense. I referred the second statement to any substance in general. Will edit it to make it more understandable. Thanks. $\endgroup$ – MollyCooL Mar 27 '18 at 23:34
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To followup on other answers, there is no guarantee that particle (molecule or atom) spacing will increase. If you maintain sufficient pressure you can heat things well above their normal phase transition temperatures.

It is true that, in the absence of external pressure, liquids tend to have lower densities than their corresponding solids. The particles no longer have shared orbital bonds as they do in a solid structure, and thus are held together via weaker forces such as ionic. When they are heated to gas-phase temperature, even these forces are overcome and the particles move as far apart as possible -- the iconic "fill the container" behavior of a gas. Due to statistical behavior, the density of the gas tends very quickly to be isotropic within that container.

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I'm going to argue that the distance between molecules does generally increase with temperature for very fundamental reasons, although normal ice (Ih) and liquid water are an exception. At low temperatures, substances minimize their energy, which usually results in ordered crystalline arrangements. At high temperatures, substances maximize their entropy. That is, at high temperatures everything is as random and mixed up as possible.

Assuming that you have some empty space, there are more ways to put two molecules far apart than there are to put two moleucules close together. You can see this in the diagram below. There are more ways to place a second molecule in the blue spherical shell, at a long distance from the central water molecule, than there are to place water molecules in the orange spherical shell, at a short distance from the central water molecule. In fact, the number of ways to place the second water molecule increases with the volume of the spherical shell, that is like 4πR2, where R is the distance between the molecules. So, the state where two water molecules have a long distance between them has a larger entropy than the state where they have a short distance between them.

A central water molecule and a second water molecule added at difference distances.

Simply due to geometry, at high temperatures, molecules will tend to be far apart. Water boils at high temperature simply because there are more ways to scatter water molecules throughout the whole kitchen than to place all of the water molecules in the pot. Most substances become gases at high enough temperatures given sufficient empty space. Most substances expand as their temperature rises.

Adding distance between molecules is not the only way to increase entropy, which accounts for the exception of melting ice. Entropy can also increase by making the arrangement of molecules and their orientations more disordered. Liquid water has more entropy than ice not because the molecules are farther apart, but because they arranged and oriented more haphazardly.

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First you have to understand that molecules are randomly moving in matter.When you heat them,you essentially give them kinetic energy which increases their speed and hence the distance between molecules increases implying that interatomic forces decreases,leading to change in state from solid to liquid.The basic difference between solid,low,and gases are that molecules in gases have more kinetic energy than liquid which have more than solid.

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    $\begingroup$ How heat energy is converted to kinetic energy? $\endgroup$ – Akash. B Mar 27 '18 at 13:53
  • $\begingroup$ First you need to understand what is heat? Supplying heat basically means increasing temperature(provided it is not being utilized for doing work).Now this means we need to understand what is temperature?Temperature is related to average kinetic energy of particles.Supplying heat increases temperature which means AVG kinetic energy increases $\endgroup$ – Yashasvi Grover Mar 27 '18 at 14:00

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