[firedamage]

Why 1,1,1-trichloroethane is denser than water??It is because of the volume or the  molecules is closer to each other in trichloroethane?

[Arkcon]

It is because of the volume

This is one reason, 'tho I would describe the phenomena differently.  What do you mean, in this case, volume.  The volume of the molecule?  How do we usually describe that, quantitatively.  Molecular _ _ _ _?  The answer would be useful to your many other questions.

or the  molecules is closer to each other in trichloroethane?

That is also a reason.

[firedamage]

molecular mass?

[Arkcon]

molecular mass?

Good.  What is the molecular mass of each?  And while you're at it, don't neglect the other reason, why would the molecules be packed in closer in trichloroethane than in water?

[mreff555]

Hmmm... sorry firedamage I have to jump in too, now I'm curious.
If I remember correctly Water and 1,1,1-Trichloroethane would both be sp3 hybridized
Normally a sp3 hybridization makes for longer bond lengths because each substituent only has 1/4 of a sigma bond. What I can't remember is if those lone pairs on H2O would make a difference.