text: when pure ethanol is gradually added to pure water total volume first decreases then increases why  ?   this is a very complex subject.  it is true that there are  hdydrogen bonding, but i will try to make it easier for you to picture it.  think of  a jar of beans (water), then you pour in rice (ethanol). at 1st the rice will go in between the beans. so the total volume goes down. after a while the rice will fill by itself over the beans, so the total volume goes up.  this is not excatly correct, but it gives you a model for picturing it in your mind.  in the real case, the beans and rice are always in motion and not sitting still.   i hope this helps you. to understand it in  deeper level, you will need to read more on the subjects of bonding, solvent, co-slovent, azeotropi solution, etc.  enjoy and have fun with chemistry