08 Change in salinity of sea water

Experiment: Investigate the impact of additional fresh water on the ocean

Investigate how the density of salt water changes as fresh water is added.

Use a floating carrot to visualize the changes in density.

Time: 30 minutes

Level: Beginner

Materials

salinity reader - refractometer This is optional but is an inexpensive tool that is very engaging. This refractometer on Amazon is $17.
Clear jars, glass or plastic, 500 ml or larger
mixing cup, about 2 cups or 500 ml
Small measuring cups for 10 - 50 ml quantities, or plastic syringe
Measuring spoon, teaspoon or tablespoon
table salt
water, room temperature
fresh carrot, cut into pieces about 1” or 2.5 cm

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

Science notes

This experiment highlights density and uses a piece of carrot as a "density measuring tool".

An object will float if it is less dense than the liquid it is in, and it will sink if it is more dense. Fresh water and ocean water have different densities.

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 (volume)
1000 ml = 1 liter
1 cm3 water = 1 gram water (mass)
3 teaspoons = 1 tablespoon
1 tablespoon of salt weighs 18 g ➔ 1 teaspoon salt = 6 grams

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

Procedure

Step 1 - Do a control trial

Fill a jar about half way with plain tap water at room temperature.

Drop a piece of carrot in it and observe what it does.

(If it does not sink to the bottom, check that it is not covered with air bubbles).

Step 2 - Prepare salt water at a range of "saltiness".

The salinity of water (saltiness) corresponds to the density of water and is often measured as parts per thousand (ppt). That means the amount of solid (in grams) dissolved in 1000 grams of water.

(the average salinity of sea water).

From the conversion table above you can see that 1 tsp salt = 9 grams and 1 tbsp salt = 18 g. And that 1 cubic centimeter (cm3) = 1 gram of water.

Mix these with water measured in liters to get the parts per thousand (ppt).

For example:

2 tsp salt in 500 ml water = 12 g salt / .50 l, which converts to 60 ppt.

For multiple groups of students , make water samples of some of these different values: ~30 ppt, ~36 ppt, ~42 ppt, ~48 ppt, ~54 ppt, ~60 ppt.

If you have a refractometer, use it to measure the salinity in ppt and add salt or water as needed.

Step 3 - Do trial 1

Place the piece of carrot in the salt water and observe where is stays.

If it is positively buoyant (less dense than the salt water), a small part of the carrot will be above the water as it floats on top.

If it is negatively buoyant (more dense than the salt water), it will sink to the bottom.

Step 4 - Adjust the salinity until the carrot floats

Add salt or fresh water until the carrot piece is floating in the salty water (not touching the bottom, and not showing above the water).

Start with a small amount of water (such as 20 ml), or a small amount of salt (1/2 tsp). Add it to the salt water mixture with the carrot in it and mix it until all the salt is dissolved.

Observe the level of the carrot piece.

Keep track of the total amount of salt (in grams) and water (in liters) that is added in a table such as the one shown below.

Repeat this procedure until the carrot is floating.

Using the data in the table, calculate the density at which the carrot floats.

If you are using a refractometer, compare your calculation with the measured value.

Step 5 - Think about density

The density of pure water is 1 g / ml, aka 1 g / cm3

Use a density calculator such as the example shown here to calculate the density of the salt water in your trials.

https://www.omnicalculator.com/physics/water-density

Look up “density of a carrot” in grams per cubic centimeter (g/cm³). online and compare to tap water and to the salty water in your trials. Put them in order of most to least dense to find the heaviest to the lightest.

The carrot here was used as a “density measuring tool”. What can you conclude about the density of salt water as fresh water is added to it?

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

Suggestions:

A carrot can float in salt water but not fresh water

Carrots are less dense than salt water, and more dense than fresh water

The density of salt water changes when fresh water is added to it

Melting glaciers and ice sheets are making regions of the ocean less salty

(decreasing the salinity of the ocean)

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

The mechanism is Change in salinity of sea water

The addition of fresh water to salt water makes the water less salty.

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 8

Cause: Melting glaciers and ice sheets

What is happening: fresh water is added to salt water and changes the salinity

Effect: Regional changes in water salinity

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.