Science: Salt water density experiment

Wednesday 11th April 2018

This easy to set up salt water density science experiment takes the classic sink and float science experiment to a whole new level. What will happen to the objects in salt water? Will a stone float? There are so many questions to ask and predictions to make with this science activity. Our supplies included, four recycled glass jars, salt,sugar, plain water and baking soda.   

We tried using measuring jars filled to about 1 2/3 cups with room temperature water (try warm too, the salt took awhile to dissolve). In one measuring jar, we poured in about 1/3 of a cup of table salt. We mixed and mixed and it was pretty salt heavy.

We tested each item by placing a stone gently in the fresh water jar to see what happens and then into the salt water…and then the soda water…and then the sugar jar.   

Penelope, Charlie, Sophie N, Milo, Harry, Gus, Ilaria, Sophie P, June, Ava, Wolfe and Mosab all participated. June said, "The big rocks go to the bottom they don't stay on top." Penelope tested both the small and large stones. Ilaria wanted to try and fit the stones in the jars. Milo attempted to float the wooden pieces and Charlie loved watching the baking soda react with the water.   

Explaining saltwater density can be a challenge, but we were able to visually demonstrate and show that two things of similar size can weigh different amounts and that is what causes things to sink or float. Things that have a higher density may sink and things with a lower density may float. We enjoyed observing the children think out aloud, and as a group as well as make comparisons between the different jars. Through the children observing what doesn’t work is also great for reasoning skills and developing critical thinking skills.  

The salt in the water makes it more dense allowing more types of objects to float when they might not in fresh water. Fresh water is less dense so many things sink. As the salt dissolves in the water, it adds mass (more weight to the water). This makes the water more dense and thus allows more objects to float on the surface.  

The baking soda had a unique reaction. When baking soda dissolves in water some of it reacts to form carbon dioxide gas. If you look carefully you will see tiny bubbles rising from the bottom of the cup. We must have added just the right amount of baking soda to the water because when we put the jewels in the cup they hovered right in the middle!  

This is a great opening activity that will allow the children to begin to explore ocean life on planet earth.