Students ask so many questions. As teachers, we don’t always have the answers. What is the temperature in space? How many plants are carnivorous? Why is the ocean salty? The first time a student asked me this question, I honestly had no idea how to answer it. However, the great thing about being a science teacher is that we can INVESTIGATE things we don’t know! A Google search lead me to some interesting information, and an idea sparked for a lab activity.
Why is the Ocean Salty?
Grade Level: 3rd-6th | Duration: 45-60 minutes | Concepts: Physical Science– Mixtures & Solutions, Earth Science– Weathering and Erosion, The Water Cycle
Materials: Sand, rock salt (ice cream salt), table salt, clay, gravel, small plastic cups with holes in the bottom, plastic food container with lid (2+), water, beaker, graduated cylinder, eye dropper or pipette, hand lens, paper towel, aluminum foil, hot plate or some other means to quickly evaporate water.
Play-dough can be substituted for clay, but I highly recommend clay because you can seal it in a bag and reuse it every year. It also looks much more like actual rocks.
Objective: The purpose of this lab is to guide students through several concepts that account for the salt found in oceans. First, students must understand that salt dissolves into water to make a solution. They must also understand the erosive nature of water, and its ability to break down and carry things to other places. Last, they must know the fundamentals of the water cycle.
The Lab
Before the investigation begins, we talk about the water on Earth…How much of the Earth’s surface is water (70+%), how much is fresh water (3%), and how much is salt water (97%). Then I ask students why the ocean is salty, and have them record a hypothesis. We discuss their ideas, and then I introduce the “test” we will be doing to prove or disprove their predictions.
The first step in this process is to make some observations of rock salt and a nice, broken rock that shows mineral grains. After the students have had a chance to write down their findings, we discuss the properties of the salt (especially solubility!) and compare them with the rock.
The goal should be to help students make the connection that rocks contain many types of minerals, like salt, and those minerals can weather, dissolve and wash away. A rock sample with visible grains, nice pocks and holes is VERY helpful in making this point.
Next we move on to the real investigation. Each student group gets a lab setup, which consists of a plastic container with sand, gravel and water at the bottom, as well as half of the lid. This represents the ocean and the land.
Students then place their “clay rocks” in the ocean and on the land. Using the holey cup, the students then make it rain. After the rain, they also gently rock the the container to create waves. Students should observe the salt is dissolving from the “rocks”.
Each group will then take a water sample from their ocean. Using an aluminum foil cup (or I have just let them put it in a pot these days), place them on the hot plate and watch the magic happen.
Once the water evaporates, there should be salt left behind.
*Teacher note: Make sure to add salt to the sand or the water in advance when you are prepping your materials so there will be plenty left behind.
So…why IS the ocean salty? Over time, water and rain wear down rocks on land and in bodies of water. These rocks contain minerals, like salt, that are washed into moving bodies of water like streams and rivers. The salt is eventually dumped into the ocean.
Water evaporates from the ocean, and the salt is left behind. Since the water in the ocean doesn’t flow back IN to fresh water sources, the salt stays and continues to build up.
After the lab, some students may ask higher level questions like, “Will we eventually run out of salt on land?”, which can lead to a discussion of the rock cycle.
Be sure to check out the lab sheets I made to go with this here!
Do you have a unique way to teach solutions or weathering and erosion? Tell me about it in the comments.
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Sockmonkey Science 
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