Water Purification and Ocean Salinity: The Colligative Properties and Engineering Naval Solutions

The National Academy of Engineering's (NAE) Grand Challenges of Engineering (NAE, 2008) are a list of 14 critical challenges that society faces and that can be addressed by engineers. The Grand Challenges highlight the many ways that engineering is helping others and also emphasize the collaborative, creative, and interdisciplinary work that engineers do. Framing engineering as an altruistic or communal career is believed to increase interest for more girls, underrepresented racial/ethnic minority, and first-generation college students. In a grant funded by the Office of Naval Research (ONR), our interdisciplinary team has developed multiple hands-on lab and classroom activities for the high school classroom that use the Grand Challenges and Naval contexts as a motivation for the lab. In "Water Purification and Ocean Salinity: The Colligative Properties and Engineering Naval Solutions", students explore the relationship of boiling point and salinity in the context of purifying water. This quote (http://www.rickcampbellauthor.com/styled/index.html#topic9) provides an intriguing context for the lab: "Submarines have to distill water they need for drinking, cooking, and bathing from seawater and have a limited supply, so the crew takes "submarine showers" - turn the water on and wet down, turn the water off and soap up. Turn the water on and rinse off. Repeat until done. You can't ever leave the water running anywhere. Not even while brushing your teeth." Students are tasked with helping the Navy adapt their water purification systems for different regions of the ocean where salinity varies. The lesson has both a scientific and engineering "briefs". The scientific brief describes variation of ocean salinity and the "halocline" of salinity and water temperature in the deep ocean. The engineering brief shows how a "still" can be used to produce fresh water from a salt water source. Students are then asked to determine how the local salinity of the ocean would affect the efficiency of a still in producing fresh water. Grade level: Grade 8-12 Learning objectives: > Students will calculate the expected change in boiling point based on a specified salinity and with the provided equation. They will compare this result to their observed data. > Students will connect this real-world example to the concept of intensive or colligative properties, including osmotic pressure and boiling point elevation. > Students will discuss how colligative properties have real world consequences for providing access to clean water through engineered solutions. > Students will identify strengths and weaknesses of their boiling water still and describe improvements to the design. Materials/procedure: Students use a hot plate and a "still" comprising an Erlenmeyer flask with a rubber stopped that has a thermometer and hose inserted. "Ocean" water of different salinities are provided. Water vapor travels up the hose and is collected in a small beaker. Students monitor the boiling point and the amount of water produced over a specific time period. Students can also use test strips to ensure the water produced is less salty. Evaluation: Each has been evaluated through student surveys and through implementation by teacher. The lab has been refined through teacher feedback.