Lab / experiment Grade 8 Engineering & Design
A Tabletop Solar Still for Brinehaven: Measuring Desalination Yield
A grade 8 lab where students build a solar still, measure fresh water collected over time, and compute a collection rate and percent recovery from salt water.
Objectives
- Students will build a solar still that separates fresh water from salt water and measure the fresh water collected over time.
- Students will compute a collection rate and a percent recovery, and explain why desalination needs so much energy.
Vocabulary
- Desalination
- Turning salty water, like ocean water, into fresh water people can use. It works, but it takes a lot of energy and leaves behind very salty water that must be handled carefully.
Also: solar still, evaporate, condense, yield
Teacher overview
Coastal towns short on fresh water sometimes turn to desalination, which removes salt from seawater. Brinehaven is a fictional coastal town weighing that option, and this lab lets students model it with a simple solar still. Salt water sits in a bowl, a cup waits in the center, and a plastic cover lets the sun evaporate water that then condenses and drips fresh into the cup, leaving salt behind. Students measure how much fresh water collects over time, then compute a rate and a recovery percent. The lab also shows first-hand why real desalination uses so much energy and equipment: an hour or more of steady heat turns only a few mL of water into vapor, because evaporating water takes a large amount of heat.
Procedure
Step 1 (safety, ongoing): Do not drink or taste any water in this lab. If a lamp is used as the heat source, an adult handles it, since the bulb gets hot. Wipe up spills right away.
Step 2 (mix the salt water, 8 min): Stir about 2 tablespoons of salt into about 2 cups of warm water in the bowl until the salt dissolves. This stands in for seawater.
Step 3 (build the still, 7 min): Set the empty cup upright in the center of the bowl without letting salt water spill into it. Stretch plastic wrap over the top of the bowl and seal the edges. Place the small weight on the wrap right above the cup so the wrap dips down toward the cup.
Step 4 (place in heat, 3 min): Move the still to a sunny window or under a desk lamp. Do not bump it, because drops on the underside of the wrap need to slide down to the low point above the cup.
Step 5 (collect data, 80 min in short checks): Every 20 minutes, gently lift the wrap, measure the fresh water in the center cup in mL, record it, and reseal. Take readings at 20, 40, 60, and 80 minutes. Between readings, leave the still undisturbed and have students work on analysis questions 3, 4, and 5, which do not need their own data.
Step 6 (finish, 4 min): After the 80-minute reading, note how much salt water is left in the bowl and whether salt is visible on the bowl. Empty and clean the materials. Do not taste anything.
Data & Analysis
| Time (minutes) | Fresh water collected (mL) |
|---|---|
| 20 | |
| 40 | |
| 60 | |
| 80 |
How many mL of fresh water did your still collect in total after 80 minutes?
Did the amount of collected water go up, go down, or stay flat over time?
Suppose a group collected 12 mL of fresh water in 60 minutes. What was their collection rate in mL per hour?
Suppose that group started with 480 mL of salt water and collected 12 mL of fresh water. What percent of the starting water did they recover? Round to the nearest tenth.
Tasting is not allowed, so predict instead: is the collected water salty or fresh, and why did the salt stay behind in the bowl?
A real desalination plant must make millions of gallons a day. Using how slowly your still worked, explain one reason desalination needs so much energy and equipment.
If you moved the still to a brighter, warmer spot, would you expect the collection rate to go up or down? Explain using your data.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: Varies by group, in mL.
This equals the group's 80-minute reading from the data table. Most stills collect only a few mL, which is a fair and expected result for a slow process.Answer: Go up.
The total collected should rise at each check, because more water keeps evaporating and condensing as time passes.Answer: 12 mL per hour.
The group collected 12 mL in 60 minutes, and 60 minutes is 1 hour, so the rate is 12 mL per hour.Answer: 2.5%.
12 mL recovered divided by 480 mL started = 0.025, which is 2.5 percent recovered.Answer: Fresh.
The collected water is fresh. When water evaporates, only water vapor rises, and salt does not evaporate, so the salt is left behind in the bowl while fresh water condenses in the cup.Answer: Evaporating water, or forcing it through membranes, takes a lot of energy.
The still collected only a few mL from over an hour of steady heat, because turning water into vapor takes a large amount of heat. A plant making millions of gallons a day must either supply that heat on a huge scale or run high-pressure pumps that force seawater through membranes, which is why desalination is energy-heavy. Accept any answer tying the small yield to the large energy input.Answer: Go up.
A brighter, warmer spot adds more heat, which speeds evaporation, so more fresh water should collect in the same time and the rate should rise. Accept answers supported by the group's own data.
Differentiation
Support
Pre-build one demonstration still so students see the target setup before building their own. Give a data table that needs only two readings (40 and 80 minutes) and provide the rate formula, rate = mL divided by hours, already written.
Extension
Have students run two stills at once that differ in one variable, such as one in sun and one under a lamp, and write a short claim comparing the two collection rates using their data.
ELL supports
Provide labeled picture steps for building the still (bowl, cup, wrap, weight, heat) and pre-teach evaporate and condense with arrows showing water rising and dripping. Let students record readings as numbers in the table rather than in sentences.