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?
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".
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.
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.
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.
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.
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.
