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Pollutants—Sink, Suspend, Float
Students explore the impacts of marine pollutants on marine food webs in terms of how their properties (size and density) affect their position in the water column. They first collect and classify “garbage” during a field trip into types of pollution. They then predict and test the buoyancy of different types of pollutants, including plastic micro-beads from manufactured liquid products, to compare the density of each one to that of water. Finally, they use a variety of media to support their participation in a class discussion about the potential negative impacts of plastic pollution on marine food webs and how the impact might be minimized.
Essential Question(s)
How do the physical properties of different synthetic materials determine their impacts on living organisms?
How does the density of particular pollutants determine their buoyancy in waterways and the ocean?
How does the size of particular pollutants determine their impacts on marine life?
PDF Investigation
Time Required
Field Trip + 1 week of 1-hour sessions
Physical Science
Investigation Type
Field Trip, Classroom
Grade Level
Middle School
NGSS Performance Expectations
Materials Needed

Various types of pollutants such as lead sinkers, oil, aluminum foil, plastics # 1-7, orange, dye in tablet and/or liquid form to represent pharmaceuticals), nylon or polypropylene thread, soap with microbeads (to represent micro-pollutants), cigarette butts, glitter

  • Glass beakers or containers: enough for all groups of students
  • Group data sheet and individual project journal
  • Fine-meshed paper of metal coffee filters
  • Optional: Geology Sieves
  • Small containers with caps to collect water samples
  • Stop watch
  • Salt
  • Table comparing the densities of materials
  • Pencils
  • Paper

NGSS Performance Expectation

Students who demonstrate understanding can:

Gather and make sense of information to describe that synthetic materials come from natural resources and impact society. (MS-PS1-3)

Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment. (MS-ESS3-3)


Knowledge - Students will know that:
  • Density is a property of matter that determines whether something will float, sink, or suspend in water.
  • Production and disposal of pollutants, including plastic waste, in the ocean has significantly altered the biosphere, with the potential to impact some species negatively. Water can vary in density due to salt content and temperature.
  • Different types of plastics and other pollutants have different densities, which affects where marine organisms encounter and interact with them in the water column.
Skills - Students will be able to:
  • Gather and make sense of information about impacts of synthetic materials
  • Make predictions about the properties of matter and infer impacts on marine organisms
  • Design a method for monitoring and/or minimizing the human impact of plastic pollution.
Local and Cultural Connections
Waterway and ocean pollution is an issue in Alaskan communities, both from local sources as well as for marine debris that arrives on Alaska beaches on currents and during storms. Students can participate in “Litter-Free” events and community clean-ups and develop strategies that will minimize their own contribution to plastic pollution and influence other people in their community to minimize their contribution.
Teacher Preparation

Prepare for the field trip (Select site, obtain signed permission slips, arrange busses and chaperones) and gather water sampling containers.

Obtain materials and equipment for the lab.

You can do the lab with tap water. If, however, you would like students to compare the buoyancy of the pollutants to that of salt water, mix 11 Tablespoons of salt (kosher salt works best) into a gallon of water to approximate the salinity of the ocean. Use distilled water, if possible. The salt water solution may be a bit more dense than sea water, on average, if you use tap water because most tap water has some minerals. Make up the solution several hours before it will be needed so all of the salt dissolves and it will be at room temperature.

Prepare pollutant samples. To filter out the micro-beads from soap using a metal coffee filter, add a small amount of product to the filter and hold under running water to rinse out the soapy part. Be careful to avoid the soap foaming up and overflowing; the microbeads will be carried out with the bubbles. You can use your fingers to “stir” the product and make sure all of the soapy water is rinsed out. Then let the product dry and use a paintbrush to wipe out the plastic onto a piece of paper so it can be poured into whatever container is desired.

To use a coffee maker and paper coffee filtering system, it’s also important to add only a small amount of product to the filter. Run a tank of water through the coffee maker and let the filter dry.

The geology sieves can be used to sieve through a sample of sediment to see if there are any micro-plastics present. Start with the biggest mesh sieve to sift out the large rocks and pieces of plastic, if present, and then sieve using progressively smaller-meshed sieves.

Prepare science notebooks or project journals. Make copies of the Pollutant Data Sheet and the Sink, Suspend, or Float? Data Sheet.

Learning Experiences


(30 mins. discussion + field trip)

Before the field trip:

  1. Lead a short class discussion: Where does pollution come from? How does garbage (pollution) affect people, animals, and the ocean? What are some characteristics of debris that make them more or less detrimental to living things in the ecosystem?

  2. Brainstorm what might be common pollutants in the waters around your area. Discuss where to look for them (you may need to steer the conversation toward the idea of looking in the water for micro-pollutants or in the sediment for buried debris).

Field trip:

  1. Students go to a local waterway or beach, observe and collect samples of common pollutants (including really small pollutant bits of plastic and styrofoam).

  2. Students collect marine or stream debris, record where they were found, and bring them back to class to use in analysis and experiments. If the beaches/river banks are fairly debris-free, you can collect trash along the roadway to use. Some students may want to collect a sample of sediment to sift later to look for microplastics-a three-gallon bucket will be sufficient. Other students might want to use a plankton net to collect micro-debris in the water.

After the field trip: Students write a journal entry about a pollutant found on the clean-up day. Where did this pollutant originate? What journey has it experienced? What life did it live before ending up in our waste pile? How might have ended up in the ocean?


(90 min)

  1. Brainstorm with the class: Which types of pollutants float? Which ones sink? Which ones become suspended in the ocean water column? (See link for ocean water column zones.)

Show students these graphics and discuss the different types of pollutants shown and their effects in relation to where they end up in the water column (floating, suspended, or sunk to the bottom):

Oil spill impacts

The ocean water column

  1. Lead a class discussion (or have students work in small groups) to define the types of information to research about the different types of pollutants to determine their potential effects on a marine food web. This might include how the position of the pollutant in water (floating, sinking, suspended) determines which organisms it will affect, how its position affects how it will move in the system (i.e., up or down in the water column, with surface or deep currents), and the ways in which the particular pollutant interacts with different types of organisms.


  1. To explore the possible impacts of these pollutants, students will work in groups to design and test a method for determining which pollutants will float or sink in water
  • by comparing the density of water to the pollutants. They will have access to measuring tools, graduated cylinders, scales, and other science lab equipment that might be useful.
  • by figuring out a way to calculate the densities of the pollutants (using the displacement method to find the volume of a plastic bottle will be tough, but it’s possible!)

[It might be important to note that ocean water varies in its salinity and temperature, and both of these factors affect its density. Lead will not float in any liquid water, but are the plastics close enough to the density of water to be buoyant in some and sinkable in another.]

  1. After students have determined their plan and described its constraints, they will make predictions about which items will sink, float, or become suspended.

  2. Students test their methods. They use teacher-prepared samples of pollutants, recording each type of pollutant to be tested on their data sheet. (The teacher-prepared samples will include more types of pollutants than will be found in the field by students such as, lead, pharmaceuticals, oil, etc.) Students add their predictions on data sheet.

  3. For each pollutant, students will fill their glass container with water and add a sample of the pollutant and then record their observations. They repeat the test for each pollutant three times.

  4. After all macro-samples are used, mix the micro-pollutants in the water to suspend these pollutants. (Micro-plastics are defined as bits of plastics that are 5 mm or less in size.)

  5. Students should be able to observe the micro-pollutants in the water. (They may be hard to see.) They then decant the water through the coffee filter to collect and confirm the presence of the micro-pollutants.

EXTENSION: Students can compare the density of the pollutants to that of tap or distilled water and also to that of salt water.


(60 min.)

Following the lab activity, students write and draw a reflection of what they have observed/learned in their journal or the teacher can lead a class debriefing:

  • What design issues did you have when you were writing your protocol? Do you have any suggestions for changes to the approach you used? Why?
  • What is the problem with pollutants? What specific characteristics do specific pollutants have that make them especially difficult to deal with effectively? What challenges does a plastic bottle in the ocean pose that a glass bottle does not?


(60 min.)

  1. Show students a video about animals eating plastic. Show as many of the other resources (see below) as you have time for or assign them to students to explore on their own.

  2. Lead a class discussion about the following:

  • How might pollutants in the environment kill or harm animals? (e.g., plastics filling up stomachs and causing starvations, toxins, cancer-causing chemicals) This article demonstrates that micro-plastics are now in animals in the deepest part of the ocean.
  • What questions come up when students look at a poster showing the distribution of debris in the Gulf of Alaska with respect to ocean currents?
  • What questions come up about watching a short video about the Pacific Garbage Patch (Gyre)?
  • The statement on this website that plastic bags are the best choice for the environment
  • Should the use of long-lived manufactured plastics like micro-beads be regulated?
  • How can the plastic pollution be monitored and minimized? What can you do? What could our community do?
  1. Students write in their Project or Science Journal to explain what difference it makes in a marine ecosystem whether pollutants float, are suspended, or sink in the ocean.

Modifications and accommodations:

  • Pair students to facilitate scaffolding
  • Scribe or type student reflections for students with writing disabilities
  • Give instructions orally and in written form.
  • Check for understanding

Review student work in their Project Journal and on their Data Sheets to assess:

  • Students were able to classify pollutants.
  • Students were able to state a hypothesis about which pollutants would float, sink, or become suspended.
  • Students were able to design a protocol to gather data and draw conclusions.
  • Students reflected on their conclusions.
  • Students demonstrated critical thinking about why it mattered if pollutants floated, were suspended, or sunk in the ocean in terms of impacts on the biosphere.
List of products with micro-beads  
Polluted Water Popsicles Art Project  
Remote Hawaiian Island called “the dirtiest beach in the world”  
Plastic Change(micro-plastics)  
Ocean plastic smells like food to marine animals  
The Ocean Food Chain  
Plastic Trash Enters the Marine Food Chain  
Poster illustrating how long different types of marine debris last in the environment  
A citizen science effort to collect data about marine debris  
Plastic Density Chart  
Density Chart of Common Substances  
Related lessons
Plastics in the Ocean Column  
Additional K-12 curriculum related to microplastics  
Alaskan CoastWatch Curriculum  
Pollutant Data Sheet  
Float Sink or Swim Data Collection Sheet  
Teacher Background

When garbage ends up in waterways and eventually, the ocean, through littering or landfill runoff, it becomes marine debris. Garbage is also illegally dumped into the ocean. Organic matter decomposes relatively quickly and glass eventually breaks down into small particles that is similar to sand, but glass, metal, and plastics remain in the marine environment for much longer periods of time – from 2-3 weeks for a paper towel to 50 years for a tin can or foam plastic cup to 450 years for a plastic bottle. Plastics don’t ever really disappear, at least, not in our lifetimes – they just weather and fragment when exposed to sunlight, pile up on coastlines, gather into giant garbage patches in the ocean, or break down into tiny pieces.

Microplastics, pieces of plastic 5 mm or smaller, are turning up everywhere – in the stomachs and tissues of seabirds at the end of the Aleutian Islands to the depths of the ocean. In addition to being the remnants of large pieces of plastic marine debris, they are also washed down drains in the form of micro-beads, manufactured for specific purposes, including for use in personal care products (such as scrubs, bath products, facial cleaners, toothpastes). They are also used in other consumer uses including cleaning products and printer toners and in industrial products such as abrasive media (e.g., plastic blasting), industry (e.g., oil and gas exploration, textile printing, and automotive molding), other plastic products (anti-slip, anti-blocking applications) and medical applications.

The rate of plastic degradation in the marine environment depends upon a number of factors, such as chemical composition, size, molecular weight, additives, environmental conditions, temperature, wave action, exposure to sunlight, and location.

The size of plastic pieces and particles is an important physical characteristic that determine which marine organisms encounter them, ingest them, or become entangled with them. Larger pieces of plastic marine debris can be mistaken for food by seabirds who dive into the ocean after their fish and zooplankton prey. Once ingested, the plastic is too large to pass through the digestive system which reduces the storage and digestion of food ever after, Seabirds can quite literally starve to death with stomachs full of plastic. Microplastics are ingested by smaller animals, including filter feeders like clams and mussels. Marine mammals and sea turtles can become entangled, especially with “ghost nets” that continue to fish after they’ve been lost at sea.

Density is a second important physical characteristic of different plastics and other pollutants because it affects the potential for marine life to encounter them and ingest them. Certain types of plastic are more, less, or very close to the density of water. (The density of pure water is 1.0 g./ml. , which means that 1 gram of water mass occupies 1 cubic centimeter.)

Pollutants that are less dense than water, so float: oil, some plastics (#2), paper, styrofoam

Pollutants that are as dense as water so go into suspension: some plastics, some pharmaceuticals

Pollutants that are denser than water so they sink: lead, some plastics (#1, #3), some chemicals, metals (aluminum cans)

For more background information on specific types of pollutants and the density of water:

Article on Microbeads
Density of Woods and Plastics
Resin Identification Codes (Wikipedia)
Industry Information for Resin Codes
Water Density Information

Microplastics are easily ingested or filtered by filter-feeders in the ocean. They can pass through the digestive system of larger organisms, so don’t cause starvation like ingestion of large pieces of plastic. In smaller organisms, however, they may cause blockages or internal abrasion and damage. In larger animals, the small particles can travel beyond the digestive systems into other tissues. Some forms of plastic leach toxic substances like phthalates that are known to be endocrine disrupters that reduce fertility or cause cancers in reproductive systems. (The numbering system for plastics used to indicate which are easily recycled is also an indication of their chemical composition and which are likely to leach phthalates or other toxic substances.) In addition, other toxic substances, like persistent organic pollutants (POPs), become concentrated on plastics.

The impact of microplastics on marine food webs is an area of active research, but approximately 60,000 - 80,000 chemicals on the market today have never been tested for safety to humans, animals or the environment, under existing law. About 80% of those are polymers and plastics which may cause cancer.

Prior Student Knowledge:

Students may already understand the concept of density, but a number of middle schoolers may still struggle with this concept and (especially) how it is related to buoyancy. They may also struggle with how to use ratios to compare the densities of two substances.

Students should have prior understanding about food web interactions and life cycles of animals which will allow them to understand the implications of pollutants on the surface of the ocean, in the water column, or on the bottom.

Possible learner preconceptions, misconceptions and instructional clarifications:

Learner Preconception/misconception: Some students may think that water (whether hot, cold, salty, fresh) has the same density all the time.

Instructional Clarification:
Discuss the states of matter and the unique nature of water, which becomes less dense when it freezes and thus floats. Remind students that salt adds mass to water when it dissolved in it, so the density increases. Middle school students can think in terms of what’s happening at the molecular level when salt molecules occupy spaces between water molecules.

Learner Preconception/misconception: Some students may also think that all plastics are the same, and that all plastics will float in all water.

Instructional Clarification: Explain the plastics numbering system in terms of indicating different chemical compositions which affects the density of the same mass of plastic.

Components of Next Generation Science Standards Addressed

Science & Engineering Practices

Obtaining, Evaluating, and Communicating Information
Gather, read, and synthesize information from multiple appropriate sources and assess the credibility, accuracy, and possible bias of each publication and methods used, and describe how they are supported or not supported by evidence. (MS-PS1-3)

Constructing Explanations and Designing Solutions
Apply scientific ideas or principles to design an object, tool, process or system. (MS-ESS3-3)

Disciplinary Core Ideas

PS1.A. Structure and Properties of Matter
Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it. (MS-PS1-3)

ESS3.C Human Impacts on Earth Systems
Human activities have significantly altered the biosphere, sometimes damaging or destroying natural habitats and causing extinctions of other species But changes to Earth’s environments can have different habitats (negative and positive) for different things. (MS-ESS3-3)

Cross-Cutting Concepts

Structure and Function
Structures can be designed to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used. (MS-PS1-3)


Influence of Science, Engineering, and Technology on Society and the Natural World
The uses of technologies and any limitations on their use are driven by individual or societal needs, desires, and values; by the findings of scientific research; and by differences in such factors as climate, natural resources, and economic conditions. Thus technology use varies from region to region and over time. (MS-PS1-3)(MS-ESS3-3)

Cause and Effect
Relationships can be classified as causal or correlational, and correlation does not necessarily imply causation. (MS-ESS3-3)

Common Core

Integrate information presented in different media or formats as well as in words to develop a coherent understanding of a topic or issue.

Trace and evaluate the argument and specific claims in a text, distinguishing claims that are supported by reasons and evidence from claims that are not.
Understand ratio concepts and use ratio reasoning to solve problems. (Understand the concept of a unit rate (a/b associated with a ratio a:b with b ? 0, and use rate language in the context of a ratio relationship) and apply it to solve real world problems)
Alaska Cultural Standards
B1. Regularly engage students in appropriate projects and experiential learning activities in the surrounding environment
Developed by Steve Morley and Carole Bookless, Juneau School District.
Last Updated on
Last Updated by
Marilyn Sigman
Alaska Sea Grant University of Alaska Fairbanks Alaska Department of Education and Early Development NOAA