In this lesson students will explore the role of a gasoline additive, fuel line antifreeze (generally methanol or 2‑propanol), in reducing the potential of water to block fuel lines in freezing weather. Students will prepare test tube models of water-contaminated fuel tanks and explore the effect of adding different types of fuel line antifreeze. This lesson can be used to bolster concepts about miscibility, density, intermolecular forces, phase changes (freezing), and colligative properties (freezing point depression).
This lesson will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
- Scientific and Engineering Practices:
- Analyzing and Interpreting Data
AP Chemistry Curriculum Framework
This demonstration supports the following learning objectives:
- Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.
- 2.1 Students can predict properties of substances based on their chemical formulas, and provide explanations of their properties based on particle views.
- 2.11 The student is able to explain the trends in properties and/or predict properties of samples consisting of particles with no permanent dipole on the basis of London dispersion forces.
- 2.13 The student is able to describe the relationships between the structural features of polar molecules and the forces of attraction between the particles.
- 2.15 The student is able to explain observations regarding the solubility of ionic solids and molecules in water and other solvents on the basis of particle views that include intermolecular interactions and entropic effects.
By the end of this lesson, students should be able to
- understand that water and gasoline are immiscible
- understand that water and lower alcohols are miscible
- recognize that water could freeze in a fuel line freeze causing operational problems for a car
- explain why water collects at the bottom of the fuel tank
- explain how fuel line antifreeze additives prevent freezing of water
This lesson supports students’ understanding of
- intermolecular forces
- polar vs. nonpolar molecules
- miscibility of liquids
- phase changes
- freezing point depression
Teacher Preparation: 30 minutes
- Engage: 5-10 minutes
- Explore: 40 minutes
- Explain: 15 minutes
- Elaborate: 15 minutes
- Evaluate: 10 minutes
- Nonpolar solvent, such as hexane, mineral oil, mineral spirits/paint thinner
- Water (distilled or tap water is acceptable)
- Alcohol, such as methanol, ethanol, or 2-propanol
- Blue and yellow food coloring (to color water and alcohol)
- Test tubes (one for each alcohol to be tested, plus one extra)
- Rubber stoppers to fit test tubes
- Test tube rack
- Beral pipets (one for each alcohol to be tested, plus one extra)
- 10 mL graduated cylinder
- 400mL or 600mL beaker
- Crushed ice
- Sodium chloride (table salt)
- The nonpolar solvents suggested above are generally flammable, as are the alcohols. Use these materials with adequate ventilation and keep away from sparks or flame. Avoid skin contact.
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- This resource could be used as a post-AP Chemistry exam activity.
- For the fuel model, low molecular weight hydrocarbons are preferred due to their lower viscosity. Hexanes work well. Alternatives include camp fuel (“Coleman fuel”), lighter fluid (“Zippo fluid”), charcoal lighter fluid, mineral spirits, or mineral oil. The amount of water added to the test tube may be adjusted depending on how accurately you wish to model the actual proportion of water in an automobile tank. A smaller amount of water (one or two drops in 5-10 mL of “fuel”) is more representative of the actual proportion of water that might be expected in a fuel tank in winter; however, larger amounts of water (5‑10 drops) will be easier for the student to observe.
Engage: To engage student interest, have them consider an automobile. Ask them to identify all of the different liquid materials that are used in a car. Students may need some prompting. Their list may include the following: coolant/antifreeze (radiator), motor oil, brake fluid, power steering fluid, automatic transmission fluid, windshield washer fluid, and gasoline. Note that each fluid performs a specific function in the vehicle and that it would not be appropriate to substitute one for another. For example, no one would put windshield washer fluid into the crankcase of an engine in place of motor oil, since the properties of oil and washer fluid are vastly different. Remind the students that the unique properties of each fluid (such as boiling point, solubility, lubricity) result directly from the chemical structure of the fluid components.
Explore: Introduce the activity as an exploration of a fuel tank additive (fuel line antifreeze) and its effect on the freezing point of water that may contaminate gasoline in the fuel tank. Give students the handout and help them through the activity.
Explain: After completing the activity, students will understand that water and gasoline are immiscible. All students should be able to explain the observation that water rests on the bottom of the test tube due to its higher density than gasoline (or the hydrocarbon model used in the activity). Depending on the students’ level and the unit in which this activity is introduced, students might be able to explain the immiscibility in terms of the balance between the strong water-water intermolecular forces (hydrogen bonding), weak gasoline-gasoline IMFs (London dispersion forces), and the weak forces between water and gasoline. Students should recognize that alcohol is capable of hydrogen bonding and therefore dissolves in the water phase. Students may be familiar with freezing point depression. A comparison to the radiator antifreeze is appropriate at this point.
Elaborate: Students might consider the significance of using gasoline that has been blended with ethanol. Modern gasoline may contain 10% (E10), 15% (E15), or up to 85% (E85) ethanol. Students can calculate how much ethanol is in a fuel tank filled with 60 L of E10 fuel. Is the addition of another 500 mL of alcohol likely to make a significant difference?
- Evaluate: Students’ understanding of the concepts in this activity may be evaluated by their answers to the post-lab questions.
Download the Teacher's Guide to view the rest of this lesson.