In homeopathic remedies the original substance is often diluted to a point where statistically there should be not a single molecule left of the original substance. The most common explanation by homeopaths, on how it still works although there is no substance left, is that water has a memory. That the "essence" or "imprint" of a molecule can stay in water and continue to have some effect even after the molecule itself is removed.
Is this at all possible from a physical point of view?
Can water molecules (in the liquid phase) form structures that are stable over long periods of time?
Water forms strong intermolecular forces between its molecules. This is called hydrogen bonding and is a form of permanent dipole/permanent dipole interaction. Oxygen is more electronegative (its nucleus attracts a bigger share of the electron cloud of the covalent bond) than hydrogen. This causes water to form a permanent dipole where the oxygen has a small negative charge and the hydrogens have small positive charges.
This causes electrostatic attraction between water molecules and they can form structures for a small amount of time. Other molecules in the water can affect these short lived structures and water does retain some 'memory' of these molecules.
This is how some proponents of homeopathy claim it works. This memory somehow has an opposite effect to the toxin or other chemical that was diluted, although there has not been any mechanism proposed for this.
However, the duration of the water memory has been scientifically tested and shown to be very short (less than one billionth of a second). This means that the memory has gone by the time the patient even takes the dose.
Even if water did have a long term memory, it would not prove homeopathy. There would also need to be evidence that this water memory had the medical effects that have been claimed.
This was demonstrated by Nobelist Luc Montagnier virologist (discoverer of HIV) in his study and his results were confirmed by his colleges and other scientists. This is well demonstrated and explained in the Water Memory documentary.
Basically in one of their recent experiment (2009) they've recorded EMF reading of water which was containing diluted DNA and exported into the audio file (wav), then sent from France to University of Sannio in Italy (famous for the quality of lab specialised in molecular biology) where tube of purified water was "listening" to the recording for about an hour and "memorizing" it.
The test was successful. The water it-self started to emit reconstituted DNA signals.
So according to Prof Montagnier, the highly diluted DNA in water has retained the memory of the original DNA traces and it returns them under the form of electromagnetic signals.