Salt Water Properties, an Inquiry Investigation

An Examination of How Salt’s Impact on Water Affects Everyday Life

Mar 4, 2009
David R. Wetzel

Salt Water - Trip Advisor

A three part hands-on, minds-on investigation where students test two unknown liquids to identify them. Once identified, students make connections similar situations.

Salt water’s properties are similar to fresh water, with some noticeable differences because of the salts dissolved in the water. The viscosity (resistance to flow) and density (mass and volume) of salt water is greater than fresh water because of its salinity level. Salt water freezes at a temperature lower than 0 degrees Celsius and boils at temperature higher than 100 degrees Celsius. Actual freezing and boiling temperatures are determined by the salinity level of salt water.

Salt water is made during the movement of fresh water over land masses as it flows back to the oceans through watersheds. The minerals in rocks are eroded by flowing water. Minerals in the rocks combine with the fresh water and become part of the "salty" solution referred to as salt water that eventually enters earth’s oceans.

It is easier to float objects in salt water than in fresh water. Many humans can float in salt water when they go to the beach; however, they cannot float in a fresh water swimming pool. The reason is that saltwater is denser. Objects that cannot float in fresh water may be able to float in salt water.

Misconceptions about Salt Water

Many students picture table salt (sodium chloride) when they think about saltwater; however, there are many minerals that are classified as salts. Sea salt contains chloride, potassium, sulfate, sodium, magnesium, and others minerals.

The use of table salt during investigations causes students to internalize this misconception. To avoid this misconception only sea salt should be used in science investigations involving salt water.

Investigations into the Properties of Salt Water

Students use problem solving and science process skills during these investigations. Additionally, they incorporate the processes of the scientific method and problem-based learning skills.

Materials (per group)

One buzzer
One 9 volt battery
Two insulated wires with alligator clips 30 cm in length
Masking tape
Two craft sticks
Aluminum foil
Two beakers
One beaker “X” – distilled water
One beaker “Y” – salt water (made from sea salt or use actual salt water))
One egg
Two thermometers
Spoon
One hot plate or bunsen burner or alcohol burner
Safety goggles
Beaker tongs

Procedures

Part 1 - Conductivity

Students:

Cover the two craft sticks with aluminum foil; ensure the foil fits tightly around the sticks
Students clip one wire to each craft stick
Label each beaker, 1 and 2
Add water from beaker “X” to beaker “1”
Add water from beaker “Y” to beaker “2”
Connect one wire to both craft sticks
Connect one wire to the positive terminal of the battery
Connect the other wire to the negative terminal of the battery
Place both craft sticks in beaker “1” and record their observations
Place both craft sticks in beaker “2” and record their observations

Part 2 - Buoyancy

Students:

Place the egg in beaker “1” with spoon gently and record their observations
Place the egg in beaker “2” with spoon gently and record their observations

Part 3 - Boiling Point

Students:

Place a thermometer in both beakers “1” and “2”
Place both beakers on a hot plate
Record their observations and temperatures at which beaker “1” and beaker “2” begin to boil

Student Questions

Part 1

Did the buzzer sound when you group placed craft sticks in either of the beakers?
Why do you think the buzzer sounded when craft sticks were placed beaker “2”? (accept all answers)

Part 2

Did the egg float in either of the beakers?
Why do you think the egg floated in beaker “2”? (accept all answers)
Do you think there are any properties in beaker “2” that allow the buzzer to sound and the egg to float?

Part 3

Which beaker had the highest temperature when the water started to boil?
Why do think beaker “2” had the highest temperature?
Do you think there are any properties in beaker “2” that allow the buzzer to sound, egg to float, and have the highest boiling temperature?
Can you provide any examples of where you may have seen similar things happen?
What do you think is in beaker “1”?

Additional questions can be developed based on student responses.

Making Connections

As students make connections with the three parts of this investigation they develop a better understanding of scientific inquiry processes. They not only make the connection that beaker “2” contains salt water, they are learning to investigate like scientists. This is a common situation for scientists; they conduct a series of tests on unknown liquids to identify them and determine their properties.