Why Mixing Water and Ethanol Does Not Result in a 100% Volume Increase
When you mix 50 mL of water and 50 mL of ethanol, you do not get 100 mL of liquid. Instead, you get approximately 96 mL. This counterintuitive result is due to the molecular differences between water and ethanol. To better understand this, let's consider a simpler example and then delve into the underlying scientific principles.
Why Does Volume Not Always Add?
When you were in school, you were taught that when you add one volume to another, you get an increased volume. For instance, 50 mL of one substance plus 50 mL of another should equal 100 mL. However, in some cases, this is not the case. This discrepancy can be observed when combining certain liquids, such as water and ethanol.
Visualizing the Volume Discrepancy
To better illustrate this, imagine you have a 1-gallon container of petrol and a 1-gallon container of ethanol. If you pour them into a 2-gallon container, will they fit perfectly, or will there be some leftover space? The answer lies in the molecular structures of these liquids.
The Science Behind the Volume Change
The reason for the volume decrease is attributable to the differing molecular sizes of water and ethanol. Water molecules are larger than ethanol molecules. When these two liquids are mixed, ethanol molecules fit into the spaces between the water molecules, reducing the overall volume of the mixture.
Molecular Weight and Density
Let's break it down with a simple calculation to demonstrate the volume change. Starting with 50 mL of ethanol:
Mass of 50 mL ethanol: 50 mL x 0.789 g/mL 39.45 g
Mass of the solution: 39.45 g 50 g 89.45 g
Density of the solution 89.45 g / 96.59 mL 0.92664 g/mL
Volume of the solution: 89.45 g / 0.92664 g/mL 96.59 mL
As seen, the final volume is less than 100 mL, demonstrating the non-additivity of the mixture.
Intermolecular Forces and Liquid Structure
The molecular structure and intermolecular forces (such as hydrogen bonds) play a significant role in the behavior of liquid mixtures. Water, for example, has a highly structured and dense liquid state due to hydrogen bonding, which explains why ice floats on water. Adding ethanol disrupts these hydrogen bonds, reducing the overall density and structure-demanding properties.
Further, substances like gravel and sand illustrate a similar principle. If you mix equal volumes of these materials, the resulting mixture will have less volume than the sum of the individual volumes due to the packing efficiency of the particles.
Conclusion
In conclusion, the non-additivity of volumes when mixing water and ethanol is due to the molecular differences in the two substances. The structure and intermolecular forces of water and ethanol molecules cause the resulting mixture to have a volume less than the sum of the individual volumes. This concept applies not only to liquids but also to other substances and mixtures, showcasing the importance of understanding molecular behavior in chemistry and everyday life.