Students pour 250 milliliters of
water and 250 milliliters of ethyl alcohol into a graduated cylinder. The graduated cylinder shows a
total volume of 480 milliliters. Why does the cylinder contain less
than 500 milliliters of liquid?
In this question, we are told that
students combine 250 milliliters of water with 250 milliliters of ethyl alcohol. If we add these two volumes
together, we get a total volume of 500 milliliters. So we might expect that when we
pour the water and ethyl alcohol into the same graduated cylinder that the liquids
would fill the cylinder to the 500 milliliters mark. But we are told in the question
that the final reading of the graduated cylinder is 480 milliliters. This is a smaller volume than we
expected. To answer this question, we must
explain why the volume of the two liquids together is less than 500 milliliters.
In all forms of matter, there are
spaces between the atoms or molecules that make it up. For example, in liquid water there
are empty areas between the water molecules. The space between molecules is
called intermolecular space. So what happens to the ethyl
alcohol molecules when we add ethyl alcohol to water? The ethyl alcohol molecules fit
into the empty areas between the water molecules. This means that some of the volume
of the cylinder that we thought would have been filled is not because alcohol
molecules fill in some of the empty space in the water.
So why does the cylinder contain
less than 500 milliliters of liquid? The answer is “Alcohol molecules
fill the space in between water molecules.”