Question Video: Explaining Why the Volume of an Alcohol and Water Solution Is Less Than Expected | Nagwa Question Video: Explaining Why the Volume of an Alcohol and Water Solution Is Less Than Expected | Nagwa

Question Video: Explaining Why the Volume of an Alcohol and Water Solution Is Less Than Expected Science • First Year of Preparatory School

Students pour 250 mL of water and 250 mL of ethyl alcohol into a graduated cylinder. The graduated cylinder shows a total volume of 480 mL. Why does the cylinder contain less than 500 mL of liquid?

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Video Transcript

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.”

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