Lab / experiment Grade 7 Science

Perrin Bay Solar Still: Pulling Fresh Water Out of Salt Water

A grade 7 science lab where students build a solar still that evaporates and condenses salt water, measure the fresh water collected, and calculate percent yield.

Grade: Grade 7
Subject: Science
Material: Lab / experiment
Time: 60 min
Prep: Moderate prep
Materials: Clear wide bowl (1 per group), Short cup that fits inside the bowl (1 per group), Salt water: 200 mL tap water with 2 tablespoons salt stirred in (per group), Plastic wrap, Small clean weight such as a marble, Graduated cylinder or measuring cup marked in mL, Sunny window or a desk lamp

Standards

MS-PS1-4 NGSS

Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

How this material aligns: Students observe water evaporating and then condensing on the cover, the exact adding-and-removing-thermal-energy state changes this standard asks them to model.

All materials for MS-PS1-4 →

MS-ESS2-4 NGSS

Develop a model to describe the cycling of water through Earth's systems driven by energy from the sun and the force of gravity.

How this material aligns: The still models two water-cycle stages, evaporation from the salt water and condensation into fresh droplets, driven by heat energy as described in analysis question 5.

All materials for MS-ESS2-4 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will build a solar still and collect fresh water from salt water.
Students will measure starting and collected volumes and calculate percent collected.
Students will explain how evaporation and condensation separate water from salt.

Vocabulary

Evaporation: Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.
Condensation: Condensation is when water vapor in the air cools down and turns back into tiny liquid water droplets. It is how clouds form and why a cold glass gets wet on the outside on a warm day.
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, yield

Teacher overview

Most of Earth's water is salty ocean water, and coastal towns short on fresh water sometimes remove the salt in a process called desalination. This lab lets students model the idea with a simple solar still, imagining a fictional coastal town, Perrin Bay, that wants fresh water from the sea. Students watch salt water evaporate and then condense into fresh droplets, measure how much they collect, and see why the method, though it works, is slow.

Perrin Bay Solar Still: Pulling Fresh Water Out of Salt Water — Grade 7 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (set up, before building): Each group gets a bowl, a short cup, and 200 mL of salt water already mixed. Do not drink or taste any water in this lab, even the water you collect. Keep the desk lamp's cord and base dry and away from the water, and let an adult move the lamp if it gets hot.

Step 2 (pour the salt water): Pour the 200 mL of salt water into the bowl. Do not let any splash into the short cup.

Step 3 (place the collecting cup): Set the empty short cup in the center of the bowl so it stands above the salt water. The cup must not float or tip; press it down gently so it stays in place.

Step 4 (cover the bowl): Stretch plastic wrap loosely over the top of the bowl and seal the edges so no air escapes. The wrap should not touch the cup.

Step 5 (make a low point): Place the marble on top of the plastic wrap, right above the cup, so the wrap dips down in a small cone pointing into the cup.

Step 6 (add heat): Move the still to a sunny window or under a desk lamp. Heat makes the salt water evaporate. Leave it for at least 40 minutes. Collected amounts after 40 minutes may be only a few mL, and an incandescent or halogen lamp gives more heat than an LED lamp. For a larger yield, leave the still in direct sun for several hours or overnight and measure at the next class.

Step 7 (watch and wait): Observe the underside of the plastic wrap. Water vapor cools there, condenses into droplets, and slides down to the low point, where it drips into the cup.

Step 8 (measure): Carefully remove the wrap. Pour the water collected in the cup into the graduated cylinder and record the amount in mL. Look at the bowl and note whether any salt is left behind.

Data & Analysis

Measurement	Amount (mL)
Salt water at the start
Fresh water collected in the cup

How much salt water did your group start with, and how much fresh water did you collect?
What percent of your starting salt water did you collect as fresh water? Use your two measurements and show the division.
A different group started with 250 mL of salt water and collected 20 mL of fresh water. What percent of their starting water did they collect?
You were told not to taste the water, so use observation instead: did the water in the cup look clear, and where did the salt go? Explain what your evidence suggests.
Which two stages of the water cycle does the solar still model? Name them and say where each happens in your setup.
Suppose a still collects fresh water at a rate of 8% of the salt water it starts with. How many mL of salt water must it start with to collect 40 mL of fresh water?
Your still is slow. Name one change that would make it collect fresh water faster, and explain why it would help.
Real desalination plants make millions of gallons a day. Using your results, explain one reason making large amounts of fresh water is hard and costly.

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: Accept the group's own two numbers. The starting amount should be 200 mL unless a group measured differently, and the collected amount is usually a small fraction of that.
Answer: Accept any value equal to (collected mL / starting mL) x 100 using the group's data. For example, 16 mL collected from 200 mL is 8%.
Answer: 8 percent. 20 / 250 = 0.08, which is 8%.
Answer: The collected water should look clear, and the salt stays behind in the bowl. Only water evaporated and condensed, so the salt was left in the salt water, which is the evidence that the still separated them.
Answer: Evaporation and condensation. Evaporation happens at the warm salt-water surface, and condensation happens on the cooler underside of the plastic wrap, where droplets form.
Answer: 500 mL. At 8%, the still collects 0.08 of the start, so start = 40 / 0.08 = 500 mL of salt water.
Answer: Accept any reasonable change with a matching reason, such as adding more heat or sunlight to speed evaporation, or using a wider bowl to give more surface area for water to evaporate from.
Answer: Accept reasonable answers, such as the process being very slow, needing a lot of energy or heat, or collecting only a small fraction of the starting water, so making large amounts takes big equipment and high energy costs.

Differentiation

Support

Build one still as a class demonstration first, and give these groups a data table with the 200 mL starting value already filled in. Provide the percent formula, collected divided by start times 100, printed on the sheet for questions 2 and 3.

Extension

Have students set up a second still with a larger surface area or extra heat, compare the fresh water collected to the first still, and write a short claim about which design worked better and why.

ELL supports

Pre-teach "evaporation," "condensation," and "desalination" with arrows on a diagram of the still showing water rising and dripping. Allow students to record observations as labeled drawings and to answer questions 4, 5, and 7 with short phrases.