How does alcohol act as a dehydrant? I was doing histological slide preparation with rat's liver. There as a part of the microtechnique the fixed tissue had to be dehydrated using alcohol in the order 30%, 50%, 70%, 90% and 100% for 20-30 min each. 
This had to be done to get rid of the water (possibly extracellular) before it was embedded in paraffin for microtomy because paraffin is immiscible with water.

My question is how does the alcohol extract water out of the tissue?
The alcohol displaces water gradually through the process of osmosis. To prevent rupture of cells from the osmotic pressure, samples are immersed in a series of baths of increasing alcohol concentration.

After immersion in 95% or stronger alcohol, it is sometimes the practice to displace the alcohol with toluene or other non-polar solvent so that the specimen can be permanently mounted with paraffin or balsam.