Worksheet Grade 6 Science

The Marlingford Brine Well: Modeling Salt Dissolved in Water

A grade 6 worksheet where students model water and salt particles, then use percent to plan membrane treatment for the salty new well at Marlingford.

Grade: Grade 6
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: Calculator, Colored pencils

Standards

MS-PS1-1 NGSS

Develop models to describe the atomic composition of simple molecules and extended structures.

How this material aligns: Items 1 through 3 build the particle counts for a water molecule and for table salt as a repeating structure, and item 10 has students use that model to explain how salt spreads through the water.

All materials for MS-PS1-1 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Students find a percent of a quantity in items 5, 7, and 8 while planning how much dissolved solids the Marlingford membrane unit must remove.

All materials for 6.RP.A.3 →

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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 model the composition of a water molecule and of table salt as a repeating structure.
Students will calculate the percent of dissolved solids a treatment unit must remove to meet a stated goal.
Students will explain why dissolved salt spreads evenly through water and stays behind when water evaporates.

Vocabulary

Key words: dissolved solids, brackish, membrane, reject water, recovery

Teacher overview

The town of Marlingford drilled a new well and found brackish water, which is water that is saltier than a lake but far less salty than the ocean. This worksheet pairs a particle model of water and table salt with the percent arithmetic an operator would use to size a membrane unit for that well. The misconception to address head on is that boiling or filtering with paper removes salt. Students routinely predict that a coffee filter will catch dissolved salt the way it catches sand, so item 11 makes them reason about what is left in the pot when half the water boils away.

The Marlingford Brine Well: Modeling Salt Dissolved in Water — Grade 6 Science

Name: ____________________________ Date: ______________

Student Worksheet

The town of Marlingford drilled a new well last year. The water from that well tastes salty. A lab measured the total dissolved solids in the well water at 2,500 milligrams per liter. The Marlingford water board set a goal of 500 milligrams per liter for the water it delivers to homes. The town plans to push the well water through a membrane unit. That unit removes 96 percent of the dissolved solids. For every 40 gallons the unit takes in, 30 gallons go on to town and 10 gallons leave as reject water. Use these numbers to answer the questions.

A water molecule is made of 2 hydrogen atoms and 1 oxygen atom. How many atoms are in one water molecule?
How many atoms are in 8 water molecules?
One repeating unit of table salt holds 1 sodium particle and 1 chlorine particle. How many particles are in 12 repeating units?
The well water holds 2,500 milligrams per liter of dissolved solids. The goal is 500 milligrams per liter. How many milligrams per liter must be removed to reach the goal?
Starting from 2,500 milligrams per liter and aiming for 500 milligrams per liter, what percent of the dissolved solids must be removed?
In water, 1 milligram per liter is the same as 1 part per million. Write 2,500 milligrams per liter as parts per million.
The membrane unit removes 96 percent of the dissolved solids. Starting from 2,500 milligrams per liter, what level is left in the treated water?
The unit takes in 40 gallons and sends 30 gallons on to town. What percent of the water is recovered for use?
The other 10 gallons leave as reject water, and they carry away almost all of the salt that was removed. Is the reject water saltier or less salty than the well water? Explain your answer.
Use your models from items 1 and 3. Explain why salt spreads evenly through a glass of well water instead of staying in a lump at the bottom.
Suppose you boil a pot of the well water until half the water is gone. Explain what happens to the amount of dissolved solids in the water that is left in the pot.

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: 3 atoms.
Add the atoms in the model: 2 hydrogen atoms plus 1 oxygen atom gives 3 atoms.
Answer: 24 atoms.
Each molecule holds 3 atoms, so 8 molecules hold 8 x 3 = 24 atoms.
Answer: 24 particles.
Each repeating unit holds 2 particles, so 12 units hold 12 x 2 = 24 particles.
Answer: 2,000 milligrams per liter.
Subtract the goal from the starting level: 2,500 - 500 = 2,000 milligrams per liter.
Answer: 80 percent.
Divide the amount removed by the starting amount, so 2,000 divided by 2,500 gives 0.8. Multiply by 100 to get 80 percent.
Answer: 2,500 parts per million.
In water the two units match one to one, so the number does not change.
Answer: 100 milligrams per liter.
Removing 96 percent leaves 4 percent behind. Multiply the starting level by 0.04, so 2,500 x 0.04 = 100. Students may also subtract, since 96 percent of 2,500 is 2,400 and 2,500 - 2,400 also gives 100.
Answer: 75 percent.
Divide the water sent to town by the water taken in, so 30 divided by 40 gives 0.75. Multiply by 100 to get 75 percent.
Answer: Saltier than the well water.
Nearly all the salt from 40 gallons ends up packed into just 10 gallons, so the salt is more crowded there. Accept any answer that says the reject water is saltier and explains that the same salt is now in less water.
Answer: Water particles surround the salt particles and carry them through the liquid.
Look for the idea that moving water particles pull the repeating salt structure apart into separate particles, and that those particles then spread through the whole glass. A response that only says "it dissolves" without describing the particles earns partial credit.
Answer: The dissolved solids stay in the pot, so the water left behind is about twice as salty. Boiling sends water into the air as vapor and leaves the salt behind. The same amount of salt now sits in half as much water. Accept any answer stating that the salt stays and the remaining water gets saltier.

Differentiation

Support

Give students punch-out paper circles in two colors so they can build the water molecule in item 1 and the repeating salt structure in item 3 by hand before counting. For items 5 and 7, supply a double number line running from 0 to 2,500 milligrams per liter across from 0 to 100 percent so the Marlingford percentages have a visual anchor.

Extension

Ask students to design a second membrane stage for Marlingford that treats the 10 gallons of reject water and recovers half of it, then calculate the total recovery for the two stages together and explain what happens to the salt that is still removed.

ELL supports

Pre-teach "dissolved," "brackish," "membrane," and "reject water" with a simple three-panel drawing showing water entering the unit, clean water leaving one side, and salty water leaving the other. Provide sentence frames for items 9 through 11, such as "The reject water is ___ because ___," and "When the water boils away, the salt ___."

Extensions

Have students weigh a measured spoonful of salt, stir it into a known volume of warm water to model the Marlingford well water, let a shallow dish of that water evaporate on a windowsill for several days, and then compare what is left in the dish with the salt they started with. A short written claim about where the salt went ties the model back to item 11.