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Reading passage Grade 7 English Language Arts

Turning Seawater Into Tap Water: Reverse Osmosis at Ellery

A grade 7 informational passage explaining how reverse osmosis removes salt from seawater, using a fictional coastal town and a real California plant as examples.

Objectives

  1. Students will explain how reverse osmosis separates salt from seawater.
  2. Students will weigh the tradeoffs of desalination using details from the text.

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.
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.

Also: membrane, seawater

Teacher overview

The fictional town of Ellery sits on the coast and runs short of fresh water in dry summers. The ocean is right there, but no one can drink salt water. This passage explains how a process called reverse osmosis removes salt from seawater, and it uses a real California plant as a working example. The questions ask students to trace how the process works and to weigh its costs against its benefits, which fits a unit on how towns find new water sources.

Turning Seawater Into Tap Water: Reverse Osmosis at Ellery — Grade 7 English Language Arts
Name: ____________________________ Date: ______________

Passage

Ellery has a problem that many coastal towns share. It sits right beside the ocean, yet it can run short of fresh water. Every summer, less rain falls, and the town's reservoir drops. People cannot drink seawater, because it is far too salty. Salt water makes people sicker, not less thirsty. So the town leaders asked a hard question. Could Ellery turn the ocean itself into drinking water?

The answer is a process called desalination. Desalination means removing the salt from seawater. The most common method today is reverse osmosis. The name sounds complex, but the idea is simple. Seawater is pushed hard against a special sheet called a membrane. The membrane has holes so tiny that only water can slip through. The salt is too large to pass. It stays behind on one side. Clean, fresh water collects on the other side.

The key word is pressure. Pumps push the seawater against the membrane with great force. Without that push, the water would not cross. The salt left behind is flushed away as a heavy, extra-salty liquid. This leftover must be handled with care so it does not harm sea life near the shore.

Ellery's leaders looked at a real plant for proof that this works. In Carlsbad, California, the largest desalination plant in the country turns seawater into drinking water every day. It produces about 50 million gallons a day. That supplies close to 10 percent of the water for its whole county. During dry years, the plant keeps making water while rivers and reservoirs shrink. That steady supply is the main reason a coastal town might build one.

Still, the choice is not easy. Reverse osmosis uses a lot of energy. All those pumps must run day and night to keep the pressure high. Cleaning seawater takes more energy than cleaning water from a river. That extra energy costs money, so desalinated water usually costs more per gallon. A desalination plant also costs a great deal to build in the first place.

So Ellery faces a tradeoff. On one side is water security. The ocean never runs dry, so the town would always have a supply, even in a drought. On the other side are the higher costs and the energy the plant would use. For a town watching its reservoir fall every summer, that steady supply may be worth the price. For a town with cheaper options, it may not. Ellery's leaders now have to weigh what a guaranteed glass of water is worth.

Questions

  1. According to the passage, why can't the people of Ellery simply drink seawater?
  2. What does the word "desalination" mean, as the passage defines it?
  3. Cite two details from the passage that explain how a membrane separates salt from water in reverse osmosis.
  4. Why is pressure important to the reverse osmosis process? Use the passage.
  5. What does the passage say happens at the Carlsbad plant during dry years, and why does that matter to a town like Ellery?
  6. The passage calls the choice a "tradeoff." Cite evidence from both sides that shows why the decision is hard.

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

Answer Key
  1. Answer: Because seawater is far too salty, and drinking salt water makes people sicker rather than less thirsty. The passage states this in the opening paragraph.
  2. Answer: Desalination means removing the salt from seawater. The passage defines it directly.
  3. Answer: Any two of: seawater is pushed against a membrane; the membrane has holes so tiny that only water slips through; the salt is too large to pass and stays behind. These details describe the separation.
  4. Answer: Pumps must push the seawater against the membrane with great force, because without that push the water would not cross to the clean side. The passage ties pressure to the water passing through.
  5. Answer: The Carlsbad plant keeps making water even while rivers and reservoirs shrink. This matters because it shows a coastal town like Ellery could have a steady supply during a drought.
  6. Answer: Answers will vary but should cite a benefit (a steady supply, since the ocean never runs dry) and a cost (high energy use, higher price per gallon, or the cost to build). Evidence from both sides shows why the choice is hard.

Differentiation

Support

Provide a two-column chart labeled "How it works" and "Costs and benefits" for students to fill in as they read, and allow short-phrase answers with a paragraph reference for questions 1 and 2.

Extension

Ask students to write a short recommendation to Ellery's leaders, taking a position on whether the town should build the plant, and supporting it with at least three details from the passage.

ELL supports

Pre-teach "desalination," "membrane," and "seawater" with a simple diagram of water passing through a screen while salt stays behind. Provide the frame "The passage says ___, so ___" for the inference questions.