Lesson plan Grade 9 English Language Arts
Explaining Reverse Osmosis to Marrow Bay's Council: Grade 9 Lesson
A grade 9 informative writing lesson where students draft a memo explaining reverse osmosis chemistry to a fictional coastal council weighing a desalination plant.
Objectives
- Students will explain, in writing, how reverse osmosis reverses the natural direction of osmosis.
- Students will organize a technical explanation into a clear sequence a nonexpert reader can follow.
- Students will cite at least one real fact about a working desalination plant to support the memo's explanation.
Vocabulary
- Reverse osmosis
- Reverse osmosis cleans water by pushing it through a membrane with holes so tiny that salt and most other dissolved things cannot fit through. It can even turn seawater into fresh water.
- Brine
- The extra-salty water left over after desalination pulls fresh water out of seawater. Plants must dispose of it carefully so it does not harm sea life.
- 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: osmosis, membrane
Teacher overview
The fictional coastal town of Marrow Bay is considering a desalination plant to reduce its dependence on water piped in from inland reservoirs, and the town council has asked a student intern to write a one-page memo explaining, in plain language, how the process actually works. This lesson has students draft that memo, organizing the chemistry of reverse osmosis into a sequence a nonexpert reader can follow. A common misconception this lesson addresses is that reverse osmosis is simply "extra-strong filtering"; students often skip past the role of pressure and membrane pores entirely, so the procedure asks them to explain the process in terms of what moves, what is blocked, and why, rather than describing it as a generic filter.
Procedure
Hook (5 min): Ask students: "If you put a bag of salt water and a bag of fresh water on either side of a barrier with tiny holes, which way do you think water would move, and why?" Take two or three guesses without correcting them yet.
Direct instruction (10 min): Explain ordinary osmosis: water moves from the side with fewer dissolved particles toward the side with more, evening out the concentration, with no pump needed. Introduce reverse osmosis as running that process backward: engineers apply pressure to the salty side, forcing water molecules through a membrane's tiny pores while the larger, water-surrounded salt ions are left behind. Share that a real plant in Carlsbad, California produces about 50 million gallons of drinking water a day this way, supplying roughly 10 percent of San Diego County's water.
Guided practice (15 min): In pairs, students use the memo planning organizer to sequence four ideas for their memo: (1) what ordinary osmosis does, (2) what pressure changes in reverse osmosis, (3) what the membrane blocks and lets through, (4) one real-world fact showing the process works at full scale. Circulate and check that each pair's order moves logically from the simple process to the applied one.
Independent practice (15 min): Each student drafts a memo of at least three paragraphs to the Marrow Bay town council, explaining reverse osmosis using their planning organizer's sequence, written for a reader who does not already know the term "osmosis."
Closure / exit ticket (5 min): Students answer in writing: "In one sentence, what is the single biggest difference between ordinary osmosis and reverse osmosis?" Collect and scan for the correct idea: that pressure is applied to reverse the natural direction of water movement.
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: In ordinary osmosis, water moves on its own from less-salty to more-salty water; in reverse osmosis, engineers apply pressure to force water to move the opposite direction, away from the salt. A correct response names the applied pressure as the key difference, whether or not it uses those exact words.
Differentiation
Support (below grade level)
Provide a simple two-column diagram (Ordinary Osmosis / Reverse Osmosis) showing arrows for water movement and a label for where pressure is applied, and let students reference it while writing each paragraph of the memo. Reduce the memo length requirement to two paragraphs.
Extension (above grade level)
Ask students to add a fourth paragraph addressing one likely objection a council member might raise, such as the plant's energy cost, and to respond to that objection using only the facts provided in class.
ELL supports
Pre-teach "membrane," "pressure," "concentration," and "brine" with a labeled diagram before the memo planning begins. Allow the memo's first draft to be organized as a labeled diagram with short captions before being expanded into full paragraphs.