10 Salinity and water density

Experiment: Changes in water density due to salinity

Compare the density of fresh water and saline water. Make colored samples of each and see what happens when they are combined.

Time: 30 minutes

Level: Intermediate

Materials

Container for mixing, ½ or 1 liter
tap water, any temperature
food coloring
scale or teaspoon measures
table salt
4 identical clear containers, either glass jars (such as mason jars or food jars) OR small clear plastic film canisters
2 thin pieces of smooth waterproof sheet to fit over the tops of the jars (such as laminated paper, waxy milk carton, or a thin plastic material)
stirring sticks or spoons
a tray or container to work in in case the water spills

Specifics and purchasing links are on this spreadsheet Materials list for CCSAE experiments

Science Notes

Measurement and units:

Salinity is often quantified in percent (parts per hundred) or ppt (parts per thousand). The average salinity of seawater is 35 ppt or 3.5%. This can be thought of as 3.5 grams of salt per 100 grams of water, or 35 grams of salt per liter of water. Note that 1 milliliter of water = 1 gram of water, so 1 liter = 1000 g water.

Unit conversions:

1 ml = 1 cm3
1000 ml = 1 liter
3 teaspoons = 1 tablespoon
2 tablespoons = 1 ounce = 35 grams
1 tablespoon of salt weighs 17 g

Reference and more information: https://manoa.hawaii.edu/exploringourfluidearth/physical/density-effects/measuring-salinity

Procedure

Step 1

Make a liter of salt water ~35 ppt by adding 35 g salt or 2 tablespoons salt, to 1 liter of water.

Optional: Taste a drop of the water - this is in the range of seawater!

Add food coloring and mix it in.

Step 2

Pour the salt water into 2 of the jars or canisters, to the very top.

Mark the containers, or otherwise note that this color is the salty water.

Pour plain water into the other 2 containers, to the very top.

Mark the containers, or otherwise note that this color is the fresh water.

Step 3

Cover one of each container with a piece of thin waterproof material.

Carefully invert each jar, place it over the jar of the opposite color and align them.

Step 4

Be ready to record images or videos of this step!

Quickly, but carefully, pull out each sheet while keeping the top jar on top of the bottom one.

Observe what happens to the fluids.

Step 5

For the set of jars that doesn’t mix, try some things:

shake it up/down
turn it sideways and create layers
tip it back to original orientation to get layers of each color plus a mix in the middle

Extension:

See a demonstration of ocean currents created by waters of different salinity on this video What causes currents in the ocean? https://www.youtube.com/watch?v=uMH2-2nnJGs

Draw your own conclusion (open this box to see our suggestions)

Suggestions:

Salt water is more dense and sinks

Sea water is heavier than fresh water

Identify the physical, chemical or biological mechanism that is happening (open this box after you have an idea)

The mechanism is density due to salinity

Salty water is more dense, and sinks when mixed with fresh water.

Relate this mechanism to climate change

Find the arrow (→) in the framework that represents this mechanism (starts at the cause phenomenon and goes to the effect phenomenon). (Use your printed copy of the framework, or open the one here.)

Pathway for Changes to Atlantic Ocean Circulation.pdf

Open this box to check your connection

Then go to the Explainer pages to learn more about the Cause and Effect phenomena

Connection 10

Cause: Regional changes in water salinity

What is happening: water density drives deep ocean currents; water that is less salty doesn’t sink

Effect: Changes to Atlantic Ocean circulation

We’d love to hear how you used this tool in your community, how the students responded, and your suggestions to improve the experiments and other resources. Contact us at ccsae@mit.edu.