Intermolecular Forces Demonstration Relative Evaporation Rates of Volatile Liquids

Three watch glasses are placed under a document camera. A brown paper towel works just as well. A brown paper towel provides better color contrast of the liquid spots and the rate of evaporation compared to a white paper towel. A drop of water, a drop of ethanol, and a drop of acetone are placed on separate watch glasses or in separate areas of the paper towel and the rate of evaporation is observed. The drop of acetone evaporates much faster than the ethanol which evaporates faster than the drop of water. 

Setting the stage before doing the demonstration: Have students predict which liquid will evaporate fastest and the slowest.  Have students explain their choices.

• Which liquid will evaporate the slowest? The fastest?  Explain.

What factors determine how fast or slow a liquid will evaporate at room temperature?

• It will help to draw the Lewis structures of each molecule, determine if the molecule is polar or non-polar, and calculate the molecular mass of each compound.
• It will help to identify the types of intermolecular forces of attraction present between molecules.

Sample student data table

Rate of evaporation versus type and strength of IMF

 Type of IMFStrength of IMFRelative Rate of Evaporation
Acetone   
Ethanol    
Water   

 

 Strengths of IMFs Vapor Pressure (kPa) at 20°CMelting Point (°C)
Acetonemedium  28   -95
Ethanolmedium- strong   5.95- 114
Waterstrong   2.33   0.0
 

 

 Strengths of IMFs deltaHvap (kJ/mol)Boiling Point (°C)
Acetonemedium30.3+56.5
Ethanolmedium- strong39.3+78
Waterstrong40.6+100

A set of Power Point Slides to accompany this demonstration is available in the right menu.

Curriculum Notes 

This demo is probably best used when discussing intermolecular forces.  Students often believe the molecular mass of a compound is the major factor governing physical properties, such as boiling point and evaporation.

Learning Objectives

1.  Identification of intermolecular forces operating within liquid samples of water, ethanol, and acetone and the correlation of a physical property, rate of evaporation, with the type and strength of the IMF in the liquid.

Lead Time 
One day of lead time is required for this project.
Discussion 

Acetone evaporates much faster than water, even though its molecular mass is more than three times as much. Water molecules, in the liquid and solid state, are capable of hydrogen bonding, whereas a collection of acetone molecules in the liquid state do not. The relatively strong hydrogen bonding between water molecules slows the evaporation rate and increases the surface tension, as evidenced by acetone's flatter drop shape on the watch glass. Ethanol exhibits hydrogen bonding between ethanol molecules but does not evaporate as quickly as water. 

Materials 
  • document camera or overhead projector and screen (for a medium to large lecture hall)
  • three small watch glasses  or a brown paper towel
  • three dropper bottles with stoppers or lids, one containing water, one containing ethanol, one containing acetone
Procedure 

The three watch glasses are placed on the overhead projector. A drop of water, a drop of ethanol, and a drop of acetone are placed on separate watch glasses and the rate of evaporation is observed. The drop of acetone evaporates much faster than the drop of water or the ethanol. 

One day of lead time is required for this project. 

Safety Precautions 

Acetone is very flammable. Take appropriate precautions.Potential Health Effects of acetone: Inhalation - Inhalation of vapors irritates the respiratory tract. May cause coughing, dizziness, dullness, and headache. Higher concentrations can produce central nervous system depression, narcosis, and unconsciousness.Ingestion - Swallowing small amounts is not likely to produce harmful effects. Ingestion of larger amounts may produce abdominal pain, nausea and vomiting. Aspiration into lungs can produce severe lung damage and is a medical emergency. Other symptoms are expected to parallel inhalation. Skin Contact - Irritating due to defatting action on skin. Causes redness, pain, drying and cracking of the skin. Eye Contact: - Vapors are irritating to the eyes. Splashes may cause severe irritation, with stinging, tearing, redness and pain.Chronic Exposure - Prolonged or repeated skin contact may produce severe irritation or dermatitis.

Ethanol is flammable and can be toxic in quantity.

Footnotes 

Thanks to Dr. Julie Haack, Department of Chemistry & Biochemistry, University of Oregon Chemistry Department, for suggesting this demo. 

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group