Activity 3: Salinity


  • Learn about salinity, a water quality characteristic.
  • Use a refractometer to measure the salinity of mystery samples and identify the type of water body it could have come from.
  • Change the aquaculture project as needed.

Learning Targets

  • Understand what salinity is and how it affects aquatic life.
  • Categorize bodies of water based on salinity.
  • Understand what can change salinity.
  • Practice measuring salinity.


60-80 minutes.


Included in this kit:

  • 7 sets – Salinity Cards
  • 7 sets – Measuring Salinity Cards
  • 7 Refractometers (1 per 3-4 students)
  • 7 Books of lens paper
  • 10 Pipettes
  • 10 Plastic beakers
  • 1 Bottle table salt
  • 1 Scoopula
  • 1 Scale
  • 10 Tare/weighing boats
  • 1 Water pitcher
  • Distilled water (to calibrate refractometers)

Not included but needed for this activity:

  • Tap water
  • Evaluations (choose one or more of the following):
    • Exit Ticket (1 for each youth)
    • KLEWS Chart (1 for each youth)
    • Journals (1 for each youth)

full set-up of items for the salinity activity


  • Salinity: Concentration of salt in water.
  • Estuary: a region where freshwater and saltwater mix; includes the tidal portion of a river.
  • Brackish: water that is saltier than freshwater, but less salty than saltwater; found in estuaries.
  • Briny: water that is saltier than saltwater, for example, the Red Sea.
  • Distilled Water: water that has been boiled (or evaporated) and then condensed into a new container; any substances that have a higher boiling point than water (e.g. salts) will be left behind in the original container; distilled removes (most) salts, chemicals, and bacteria. [Note: there are no health benefits to drinking distilled water instead of potable (tap) water.]
  • Deionized Water: water that has had all ions (including salts) removed; deionization removes salts (and other ions). [Note: there are no health benefits to drinking deionized water instead of potable (tap) water.]

Background Information for Facilitators

Note:  All links are provided purely for educational purposes. No responsibility is assumed for any content on the linked site(s).

Salinity videos:

  • Salinity Impacts on Marine Life (YouTube), Earth Rocks!/City College of San Francisco (length: 3:28)
    • Summary: Kinds of ions dissolved in the ocean, how ocean salinity can change, what an estuary is, never drink seawater
  • Salty Seas (YouTube), Earth Rocks!/City College of San Francisco (length: 14:57)
  • Long, but useful. Salinity and units covered before 1:30. Changes to salinity covered after 9:18. We do encourage watching the whole video if possible, as it includes information that is useful for later activities (e.g. algae blooms)
  • Summary: Definition of salinity, salinity units, (the rest is a little chemistry-heavy, don’t worry if it’s been a while), calcium carbonate shells and ionic bonds and how it dissolves in water, kinds of salt in water, the residence time of ions, mixing time of the ocean, how ion composition can change, nutrients and plankton, how salt ions get into the ocean, how salinity changes, precipitation (or the opposite of a solid being dissolved in water), table salt = NaCl = sodium chloride, how salinity affects plants and animals, salt concentrations in freshwater (it’s not 0)

Optional Salinity and Density videos:

  • Making waves: The power of concentration gradients (YouTube), TED-Ed (length: 5:19)
    • This video deals with both salinity and temperature, and how they affect the density
    • Summary: Thermohaline circulation, concentration gradient, equilibrium, how temperature affects density, how salinity affects density, how density differences can lead to a current


No endorsement is implied nor is discrimination intended against similar products or services.

Mystery Solutions Salt Calculations Table

Beaker size 250 mL
Solution #1 0 psu Put in beaker: 0 g of salt
then fill to: 250 mL line with tap water
Solution #2 3 psu Put in beaker: 0.75 g of salt
then fill to: 250 mL line with tap water
Solution #3 10 psu Put in beaker: 2.5 g of salt
then fill to: 250 mL line with tap water
Solution #4 33 psu Put in beaker: 8.25 g of salt
then fill to: 250 mL line with tap water
Solution #5 35.5 psu Put in beaker: 8.875 g of salt
then fill to: 250 mL line with tap water
Note: be sure that your mass of salt does not include the mass of the beaker

  1. put empty beaker on scale
  2. zero scale
  3. add salt to beaker as needed
A note on units:
psu practical salinity units psu = (grams salt) / (grams solution)
ppt parts per thousand density of water = 1 g/mL
1 psu = 1 ppt 1 g water = 1 mL water
  • Place weigh boat on scale, and press “tare” or “zero” button
  • Use scoopula to add salt to scale until you have reached the desired amount
  • Put salt into beaker
  • Fill a beaker with water to the 250 mL mark
  • Stir beaker until all salt is dissolved in the water solution
  • Repeat for each mystery sample


Before you begin, review the discussion norms that youth agreed to during the Introduction to Aquaculture activity. These norms are intended to help youth have productive, respectful conversations. Reminding youth of their norms helps youth to remember to follow their norms.

Engage: Salinity Cards (10 minutes)

Use the salinity card sort to explore how to categorize different bodies of water.

The first part of the item set-up for the salinity activity.


  • Give a set of Salinity Cards to small groups (2-3 youth).
  • Ask youth to sort cards into categories (5-10 minutes). Youth can pick whichever categories they like but must be able to explain the characteristics of their categories.
  • Journals: have youth record card categories on page 14.
  • After all groups have had time to sort the cards into categories, ask groups to share and describe the categories they chose.
  • Go over the salinity of the water bodies on the cards:
    • Go over salinity units first, so youth can understand the units in the answers.
      • ppt = ‰ = parts per thousand = particles of salt per every 1000 particles
        • Note: we recommend using ‰ instead of ppt because ppt can also be used for parts per trillion
      • psu = practical salinity units = grams salt per kilograms liquid
        • Note: Tuva uses “PSU”, which is the same thing
      • 1 ppt = 1 psu
    • Journals: have youth write salinity units on page 13.

Explore (time)

Definition of Salinity

As a group, discuss and agree on a definition of ‘salinity’.

  • KLEWS: brainstorm in the K column. Following discussion, write the definition in the S column.
  • Journals: brainstorm on page 14. Following discussion, record definition on page 13.

If needed, use the following questions to prompt discussion:

  • What does the word ‘salinity’ make you think of? (Usually, someone thinks of salt.)
  • What do you know about salt in water?
  • Should you drink saltwater? (Answer: NO. It never helps.)
  • What about plants and animals living in water? How does salinity affect them?
  • (Eventually, youth should decide that ‘salinity’ has to do with the amount of salt in water.)

Explain: What Changes Salinity

As a group, discuss how salinity can change.

Ask youth: “How can salinity change?”

  • KLEWS: brainstorm in the K column.
  • Journals: brainstorm on page 14.

Keep asking questions until youth have brought up and agreed on (most of) the following:

  • Rain (adding more water – salinity of added water depends on how that water flows to the water body)
  • Heat (evaporation removes water, leaving salt behind)
  • Tides (mixing of fresh and salty water)

Measuring Salinity

How to Measure Salinity

  • Use the Salinity Measurement Cards to learn how to measure salinity.The second part of the item set-up for the salinity activity.
  • Give groups of 2-4 youth a set of Salinity Measurement Cards.
  • Give youth 5 to 10 minutes to read cards, discuss, and decide which cards describe valid ways to measure salinity.
  • Bring youth back to a whole group and go over the answers to the Salinity Measurement Cards.
  • Be sure to emphasize that, while tasting is a valid way of measuring salinity, you should never eat samples in a lab situation.

Refractometers (Activity)

Use refractometers to measure the salinity of the mystery water samples that were mixed up before the youth arrived. Youth should also measure the salinity of the water samples they gathered after the Introduction lesson/activity if they collected water samples.

There should be one refractometer for every 3-4 youth. Each youth should measure the salinity of each sample. This provides multiple measurements and enables youth to compare and discuss their measurements. Read the Refractometer Care and Use Guide (PDF) for instructions on how to use the refractometer.

  • Journals: record results on page 14.

Extend: Aquaculture Project (10-15 minutes)

Use Supplemental Resource #4: Researching Species and Water Bodies (PDF) to help youth with their research.

Research the salinity range for your target species. Research the salinity range for your target water body. Determine if the salinity range for your chosen species is a good match for the measured salinity of your chosen water body. If not, choose a new species, or a new water body. Make sure that prior water quality characteristics are a good match for any changes.

  • Species salinity on page 15.
  • Waterbody salinity on page 16.

Evaluate (Assessment) (5 minutes)

Approximately 5 minutes before the end of the session, have youth finish up what they are doing, help clean up the materials, and do one of the following evaluation methods:

Have youth complete and turn in an exit ticket.

Have youth finish and turn in KLEWS chart.

After completing this activity youth should have the following in their journals:

  • Definition of ‘salinity’ (page 13)
  • Units for salinity (page 13)
  • Salinity measurements for mystery samples (page 14)
  • Aquaculture project: (pages 15 and 16)
    • Optimal salinity ranges for the species they are looking into
    • Salinity data for the water bodies they are looking into
    • Comparison of species and waterbody: is it a match for salinity?
    • Change Aquaculture Project as needed.