Molecules of solid can only vibrate about their mean positions.Why?Explain the same for gases and liquids.
Molecules of solid can only vibrate about their mean positions.Why?Explain the same for gases and liquids.
The atoms in a solid are so attracted to each other that they vibrate and don't move past each other. The molecules of a liquid areattracted to each other, but move more freely and past one another
The important point is that the molecules themselves are NOT different. Water is H2O whether it is in solid, liquid or gaseous form (states). It is the ARRANGEMENT and DEGREE of MOVEMENT that is different. What follows is a simplified explanation that would seem to be appropriate to the asker of the question. Yes, there is a lot more to it!
In a typical solid, the molecules are quite close together and there are forces between the molecules (INTERmolecular, not intramolecular as one poster said below, inter means between, intra means inside) that keep them arranged into fairly rigid lattice shapes. Each molecule can vibrate, it is not still, but it doesn't travel. When heat energy is applied, the molecules vibrate more and eventually some will have enough energy to overcome those intermolecular forces and break free of the lattice. This is melting and the solid will turn into a liquid. Typically it happens around the edges of the solid first as these are the first molecules to be bumped out of the lattice arrangement. Molecules in liquids move a bit more freely (so liquids can flow) but still don't travel long distances and don't rush around madly (as sometimes portrayed - that's more like a gas). Even though a change of state has occurred, the substance is chemically the same as before. This is a physical change only.
It is also important to realise that water is NOT TYPICAL of most solids and liquids. That is due to its chemical nature as a polar molecule - the Oxygen end of the molecule is very attractive to the electrons it is 'sharing' with the H atoms (you can think of the oxygen as 'greedy' but it has no thoughts or feelings!), so the electrons spend more time at that end, giving it a slightly negative charge. The H atoms don't get to spend much time with their electrons so the H end is slightly positive. This sets up additional attractive forces between different water molecules - the negative end of one molecule is somewhat attracted to the positive ends of others. Called Hydrogen bonding, do not confuse these cohesive forces with the bonds that bind the 2H to the O to make a water molecule (which are intramolecular). Water as a liquid thus tends to 'stick together' - one reason why plants can pull water all the way from roots to leaves in the very narrow tubes of the xylem vessels. As water is lost from the leaf by evaporation (transpiration) so more water follows along behind it as the water molecules stick together.
When water cools, what happens is really unusual as solids go! Instead of the molecules being drawn closer together, the polar nature of the water molecule actually pushes the molecules apart a bit into quite a loose lattice. This is why water expands as it freezes into ice - we know that because we don't fill an ice cube tray to the top or it all overflows and fuses together. We also know that ice is less dense than water because ice cubes float in our drinks - this is due to the open lattice form of ice. Similarly, icebergs float. But this is very different from the typical solids described earlier. So teachers need to be careful about using water as an example ... because it is so different
The atoms in a solid are so attracted to each other that they vibrate and don't move past each other. The molecules of a liquid areattracted to each other, but move more freely and past one another
The important point is that the molecules themselves are NOT different. Water is H2O whether it is in solid, liquid or gaseous form (states). It is the ARRANGEMENT and DEGREE of MOVEMENT that is different. What follows is a simplified explanation that would seem to be appropriate to the asker of the question. Yes, there is a lot more to it!
In a typical solid, the molecules are quite close together and there are forces between the molecules (INTERmolecular, not intramolecular as one poster said below, inter means between, intra means inside) that keep them arranged into fairly rigid lattice shapes. Each molecule can vibrate, it is not still, but it doesn't travel. When heat energy is applied, the molecules vibrate more and eventually some will have enough energy to overcome those intermolecular forces and break free of the lattice. This is melting and the solid will turn into a liquid. Typically it happens around the edges of the solid first as these are the first molecules to be bumped out of the lattice arrangement. Molecules in liquids move a bit more freely (so liquids can flow) but still don't travel long distances and don't rush around madly (as sometimes portrayed - that's more like a gas). Even though a change of state has occurred, the substance is chemically the same as before. This is a physical change only.
It is also important to realise that water is NOT TYPICAL of most solids and liquids. That is due to its chemical nature as a polar molecule - the Oxygen end of the molecule is very attractive to the electrons it is 'sharing' with the H atoms (you can think of the oxygen as 'greedy' but it has no thoughts or feelings!), so the electrons spend more time at that end, giving it a slightly negative charge. The H atoms don't get to spend much time with their electrons so the H end is slightly positive. This sets up additional attractive forces between different water molecules - the negative end of one molecule is somewhat attracted to the positive ends of others. Called Hydrogen bonding, do not confuse these cohesive forces with the bonds that bind the 2H to the O to make a water molecule (which are intramolecular). Water as a liquid thus tends to 'stick together' - one reason why plants can pull water all the way from roots to leaves in the very narrow tubes of the xylem vessels. As water is lost from the leaf by evaporation (transpiration) so more water follows along behind it as the water molecules stick together.
When water cools, what happens is really unusual as solids go! Instead of the molecules being drawn closer together, the polar nature of the water molecule actually pushes the molecules apart a bit into quite a loose lattice. This is why water expands as it freezes into ice - we know that because we don't fill an ice cube tray to the top or it all overflows and fuses together. We also know that ice is less dense than water because ice cubes float in our drinks - this is due to the open lattice form of ice. Similarly, icebergs float. But this is very different from the typical solids described earlier. So teachers need to be careful about using water as an example ... because it is so different
