SCIENCE – Learn how salt affects the density of water with this fun, quick and easy experiment with items found around the house.
Learning Objective:
Expanding on the normal sink and float experiment, students will learn about how salt changes the density of water.
Education Standards
Next Generation Science Standards
- K-PS3-1 Make observations to determine the effect of sunlight on Earth’s surface. Examples of Earth’s surface could include sand, soil, rocks, and water. Assessment of temperature is limited to relative measures such as warmer/cooler.
- SEP.3 Planning and Carrying Out Investigations
- SEP.3.E Test two different models of the same proposed object, tool, or process to determine which better meets criteria for success.
- SEP.3.D Make predictions about what would happen if a variable changes.
- SEP.1.B Ask and/or identify questions that can be answered by an investigation.
- SEP.3.F Make predictions based on prior experiences.
What you need
- Two tall glasses (big enough to fit an egg)
- 2 eggs (either raw or boiled)
- Warm water
- Salt
Experiment:
Step 1 –
- Start by filling one glass about 2/3 of the way full with water.
- Ask the kids what will happen if you carefully drop an egg into the glass of water.
- Give it a try!
Step 2 –
- In the other glass, fill to the same height with water.
- Now stir in 3 tablespoons of salt.
- Mix well to dissolve the salt!
- Ask the kids what they think will happen this time and demonstrate!
Learn:
Density refers to how heavy something is relative to the space it takes up. You can learn more about density and buoyancy from our Lego boat lesson.
HOW DOES SALT CHANGE THE DENSITY OF WATER?
Adding salt to water makes the water denser. As the salt dissolves in the water, it adds mass (more weight to the water). This makes the water denser and allows more objects to float on the surface that would sink in fresh water.
About 3.5 percent of the weight of seawater comes from the dissolved salts. The heat of the sun also forces evaporation at the ocean’s surface, which puts water vapor into the atmosphere but leaves minerals and salts behind, keeping the ocean salty.
Salt in the ocean comes from rocks on land. The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water).
As the rain erodes the rock, acids in the rainwater break down the rock. This process creates ions, or electrically charged atomic particles. These ions are carried away in runoff to streams and rivers and, ultimately, to the ocean.
Two of the most common ions in seawater are chloride and sodium. Together, they make up over 90 percent of all dissolved ions in the ocean. Sodium and Chloride are ‘salty.’ In fact, sodium chloride is the chemical name for salt!
Questions for Further Thought:
- Are you more likely to float in a pool or in the ocean?
- What makes ocean water salty?
- Can you name some of the biggest mammals on Earth that can float in the ocean?
Extensions
Grades 3-5, 6-8, 9-12: Learn about the water cycle and how you can desalinate water.
T he water cycle is the changes of state and movement of water throughout the Earth system: the atmosphere, biosphere, lithosphere, and hydrosphere. Evaporation is caused by the sun’s rays changing liquid water from Earth’s surface into gas. As these gaseous vapors rise and circulate in the atmosphere, they cool and change back into a liquid state. This is known as condensation. When water vapor condenses, tiny droplets form clouds, which return the water to Earth as precipitation.
Learn how to make a homemade solar still to mimic the natural process of desalination or separating pure water from a saltwater mixture in Solar Still, Part I: Salt Water from PBS Learning Media. Check out Part II where they conduct the same experiment using juice.
Additional Resources:
Try this chapter on density for middle school chemistry from the American Chemical Society.
Try this water density lesson for grads 1 – 8 from PBS Learning Media.
Check out some ocean science activities to do from Little Bins for Little Hands.
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