Lesson plan Grade 6 Science

Port Almeda's Solar Still: Modeling State Change in Desalination

A 50-minute grade 6 lesson where students model evaporation and condensation to explain how a solar still turns seawater into fresh water in a fictional coastal town.

Grade: Grade 6
Subject: Science
Material: Lesson plan
Time: 50 min
Prep: Moderate prep
Materials: Shallow dark-colored basin or bowl, Salt water (mixed in class), Plastic wrap, Small clean cup, Tape, Small weight (a clean pebble works), Small support to raise the cup inside the basin, Sunny window or lamp, Four-panel diagram template (one per group), Index cards

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 draw a four-panel particle model showing water molecules speeding apart during evaporation and slowing back together during condensation inside a solar still, the state-change-with-thermal-energy model MS-PS1-4 requires.

All materials for MS-PS1-4 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will model how added and removed thermal energy changes water's state from liquid to vapor and back to liquid.
Students will explain why dissolved salt is left behind when seawater evaporates inside a solar still.

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: thermal energy, state change

Teacher overview

Port Almeda, a fictional coastal town facing a dry summer, is testing a solar still, a simple covered basin that uses only sunlight to turn seawater into fresh water, as one small-scale option alongside its main water supply. This lesson uses that device to model a state change students already know from everyday life, water turning to vapor and back to liquid, but applies it to a mixture with salt dissolved in it. A common misconception this lesson addresses directly is that "boiling" is required for evaporation to happen; the solar still works entirely below boiling temperature, using only the thermal energy sunlight adds to speed up water particles enough that some escape into the air as vapor.

Procedure

Hook (5 min): Ask students, "If Port Almeda's town wells run low during a drought, and the town sits right on the ocean, why can't people just drink the seawater directly?" Collect answers, most of which will mention salt, and tell students today's lesson explains a way to remove that salt using only sunlight.

Direct instruction (12 min): Introduce the solar still with a labeled diagram: sunlight adds thermal energy to seawater sitting in a shallow, dark basin; that added energy speeds up the water particles until some escape into the air above the basin as vapor, a state change called evaporation; the dissolved salt does not evaporate along with the water, since salt is not part of this state change, so it stays behind in the basin as the liquid water leaves; the rising vapor then touches a cooler clear cover placed over the basin, loses thermal energy, and slows back down into liquid droplets, a state change called condensation; those droplets run down the angled cover and drip into a clean collection cup as fresh water. Emphasize that every step in this cycle is driven by thermal energy being added (evaporation) or removed (condensation), exactly the relationship MS-PS1-4 describes.

Guided practice (15 min): In small groups, students complete a four-panel diagram template: (1) basin with seawater, (2) sun heating the water, (3) vapor rising and touching the cool cover, (4) fresh water dripping into the cup. In each panel, students draw water particles as dots, packed closely together and moving slowly in the liquid panels, spread far apart and moving quickly in the vapor panel, and label each transition arrow "thermal energy added" or "thermal energy removed." Students also add a symbol for salt that stays in the basin the whole time, never appearing in the vapor or collection-cup panels.

Independent practice (10 min): As a class, assemble one demonstration solar still: pour a small amount of salt water into the dark basin, stretch plastic wrap tightly over the top and seal the edges with tape, place the empty collection cup inside on a small support so it sits below the plastic's center, and set a small clean weight on the plastic directly above the cup so the plastic dips slightly toward it. Place the assembled still in a sunny window or under a lamp, keeping the lamp's cord and plug away from the water. Each student then writes a two-to-three sentence individual prediction of what they expect to find in the collection cup after 24 hours and why, using particle-motion reasoning from the diagram.

Closure / exit ticket (8 min): On an index card, students answer: "Explain why salt is left behind in the basin while the water in that same basin does leave it. Use the term 'state change' in your answer."

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Guided practice, four-panel diagram: A correct diagram shows tightly packed, slow particles in the basin (panel 1), particles gaining energy and beginning to spread as the sun heats the basin (panel 2), widely spaced, fast-moving particles as vapor touching the cool cover (panel 3), and tightly packed, slow particles again as the vapor condenses into collected droplets (panel 4), with salt shown in the basin in panels 1 and 2 and never in panels 3 or 4.
Exit ticket answer: Evaporation is a state change that only the water undergoes: added thermal energy speeds up the water particles enough for some to escape the liquid as vapor, leaving the dissolved salt behind, because the salt itself does not gain enough energy to change state and escape along with the water.

Differentiation

Support

Provide the four-panel diagram template with panel 1 (particles in the basin) already drawn as a worked example, so students focus their effort on completing panels 2 through 4 using the same particle-drawing convention.

Extension

Ask students to predict, using particle-motion reasoning, what would happen to the speed of evaporation in the solar still on a cloudy day compared with a sunny day, and to explain their prediction in terms of how much thermal energy the basin receives.

ELL supports

Pre-teach "evaporation," "condensation," "thermal energy," "desalination," and "state change" with the labeled solar-still diagram displayed throughout the lesson, plus simple arrows showing "energy in" and "energy out" at each stage. Allow the independent-practice prediction to be spoken aloud to a partner and recorded as a short phrase rather than written as full sentences.

Extensions

Check the demonstration solar still the next school day and have students compare the actual amount of fresh water collected with their predictions from independent practice, discussing any difference between the classroom demonstration and how solar stills perform in real coastal conditions over multiple sunny days. Students should not taste the collected water, since classroom containers are not food-safe.