Masses are additive but what about volumes? At first it seems reasonable but mixing equal volumes of water and ethyl alcohol will result in less than the total volume of the individual components.
• Intermolecular forces
• Polar molecules
• Hydrogen bonding
50 mL, Ethyl alcohol, C2H5OH, 95% anhydrous
50 mL, distilled or deionized water
100 mL buret
100 mL beaker
Green food coloring
Alternately; two 50 mL graduated cylinders and one 100 mL graduated cylinder
This activity requires the use of hazardous substances and has the potential for hazardous reactions. Carefully review the Safety Precautions section and relevant Materials Safety Data Sheets.
- Add about 60 mL of distilled or deionized water to the 100 mL beaker and stir in three drops of green food color.
- With the stopcock closed, pour the colored water into the buret up to the 50 mL mark.
- Discard the remaining water and add about 60 mL of ethyl alcohol to the 100 mL beaker.
- Whit the buret held at approximately a 45o angle; slowly and carefully pour enough ethyl alcohol into the buret to bring the combined volume up to the top graduation mark. If this is done carefully the water and alcohol will hardly mix. The more dense water will remain at the bottom and the less dense alcohol will float on top. If there is a small amount of mixing at the boundary between the liquids the demonstration will not be effected. The green coloring in the water will help to reveal when this step is done properly.
- Have students observe the total combined volume of the two liquids, before mixing, brings the level up to the top mark.
- Sealing the top of the buret with your finger (or stopper) invert the buret slowly allowing the liquids to mix. Invert the buret several times to allow the two liquids to thoroughly mix and the heat of solution to dissipate. The air bubble will assist in the mixing process.
- Observe that the final volume of the liquid has fallen several milliliters below the original level.
If no buret is available the demonstration could be done using two 50 mL graduated cylinders and one 100 mL graduated cylinder.
- Measure out, very carefully, 50 mL of distilled or deionized water in one 50 mL graduated cylinder.
- With equal care, measure 50 mL of ethyl alcohol into the other 50 mL graduated cylinder.
- Pour both liquids into the 100 mL graduated cylinder and stir with a stirring rod until all bubbles have come to the top.
- Observe that the final volume in the cylinder is less than 100 mL.
When 50 mL of water is added to 50 mL of water or when 50 mL of alcohol is added to 50 mL of alcohol, the final volume will always be 100 mL. In this demonstration, when the alcohol is added to the water, intermolecular forces cause the two different molecules to pack closer together. This is believed to be due to strong hydrogen bonding.
Hydrogen bonding is an especially strong example of dipole-dipole interaction. A dipole-dipole interaction is the attraction of the positive end of one polar molecule for the negative end of another polar molecule. In hydrogen boding, a hydrogen atom serves as a bridge between two highly electronegative atoms (nitrogen, oxygen, or fluorine).
Ethyl alcohol is a dangerous fire risk; it is flammable. The addition of denaturant (usually methyl alcohol) makes ethyl alcohol poisonous by ingestion. Wear chemical splash goggles, chemical-resistant gloves, and chemical-resistant apron.
Consult a reliable reference manual for appropriate disposal methods.