The Volume of Water and Alcohol

Fill a graduated cylinder with 1000 mL of water. Measure out a separate 18 g of water. Add the 18 g of water to the graduated cylinder. The volume inside the graduated cylinder will rise to 1018 mL. Then, fill a second graduated cylinder with 1000 mL of ethanol (C2H5OH). Measure out a separate 18 g of water. Add the 18 g of water to the second graduated cylinder. The volume inside the second graduated cylinder will rise to 1014 mL. What is the volume of 18 g of water?

We have just followed the procedures for measuring the volume of 18 g of water by displacement… first by displacing a volume of water and then by displacing a volume of ethanol. When displacing water, 18 g of water has a volume of 18 mL. But when displacing ethanol, 18 g of water only has a volume of 14 mL. How is this possible?

Remember, displacement is only an indirect measurement of an object’s volume. All indirect measurements are based on certain assumptions. Usually that is not a problem, but sometimes it is. Consider two boxes of balls. The green balls occupy 1 m3 of volume. The blue balls occupy another 1 m3 of volume. If you mix the green and blue balls together, will the balls occupy 2 m3 of volume? Will they completely fill both boxes?

The answer is no. Because of their shape, there is quite a bit of empty space between the green balls. And because the blue balls are quite a bit smaller than the green balls, some of the blue balls are going to fit into those empty spaces. You could imagine that if we had a box of thousands and thousands of greens balls, pouring one small box of blue balls into it might not increase the volume of the box needed to hold them at all. This is kind of what happens when you pour a “box” of water molecules into a “box” of ethanol molecules.

An ethanol molecule (C2H5OH) is about three times as large as a water molecule (H2O). An ethanol molecule is about 4 × 10-8 m across, while a water molecule is only 1.7 × 10-8 m across. But molecules are not the same as green and blue balls, and the space between molecules is not the same as the space between green and blue balls either. Much of the space between molecules is created by the motion of the molecules. This is not empty space that is just sitting there for a smaller molecule to move into. The reason why water molecules fit so well between ethanol molecules is because water molecules are smaller than ethanol molecules… and because water molecules and ethanol molecules are strongly attracted to each other. This attraction pulls the water molecules and the ethanol molecules close together, decreasing the volume.

If you would like to see this phenomenon yourself, all you need is two graduated cylinders, water, and a bottle of rubbing alcohol from your local pharmacy. The rubbing alcohol sold in most pharmacies is a mixture of isopropyl alcohol (C3H8O) and water. You can find a weaker mixture that is 70% alcohol and a stronger mixture that is 91% alcohol. This experiment will work best with 91% isopropyl alcohol.

Fill the first graduated cylinder with 50 mL of 91% isopropyl alcohol (rubbing alcohol). Fill the second graduated cylinder with 50 mL of water. Add the 91% isopropyl alcohol in the first graduated cylinder to the water in the second graduated cylinder. Observe what happens.

The first thing you should notice is that the water and alcohol mixture shimmers. Then the graduated cylinder containing the water and alcohol mixture should get really warm. When we first did this experiment with inexpensive plastic graduated cylinders, the plastic actually melted from the heat. Finally, after the shimmering has stopped (it may take a few minutes) and you measure the volume of the water and alcohol mixture in the second cylinder, you should see that the volume is less than 100 mL (≈95 mL).

An isopropyl alcohol molecule (C3H8O) is even larger than an ethanol molecule. And like an ethanol molecule, an isopropyl alcohol molecule is strongly attracted to water molecules. Both the ethanol and isopropyl alcohol molecules have slight negative (−) charges on their oxygen atoms and slight positive charges (+) on the attached hydrogen atoms. (You will learn why oxygen atoms often have slight negative (−) charges later in this unit.) This causes the ethanol and isopropyl alcohol molecules to behave like small magnets.