|MadSci Network: Chemistry|
One liter of water has a greater mass, because water has a greater value for its density. The density of water is approximately 1.0 gram (g) for every milliliter (mL) or 1.0 g/mL. This means that every mL (there are about 30 mL in an ounce) has a mass of 1.0 g. One liter of water would weigh about 1000 g.
The alcohol has a density of less than 1.0 g/mL, about 0.79 g/mL. A liter of alcohol would then weigh about 790 g. The reasons for this difference in density could be several, but the easiest way to understand the differences is that in water the molecules are packed together well and held together pretty tightly. In the alcohol, the molecules are not held together as tightly, so they aren't packed together very tightly. They have more empty spaces or larger empty spaces between them. So, there is not as much matter packed into a mL in the alcohol. There is more matter packed into each mL of water.
Dan Berger adds:
Another possibility is this:
Water is mostly (by both mass and volume) oxygen atoms, while ethanol has, in addition, two carbon atoms (and four more hydrogen atoms). A molecule of ethanol will take up a volume roughly similar to that of three water molecules; meanwhile, the mass of the ethanol molecule is only about 85% of the mass of three waters. Therefore, even if the binding between ethanol molecules were as strong as that between water molecules, we'd expect ethanol to have a lower density.
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