When walking around the boardwalk in downtown Burlington, you will see so many amazing boats out on the water. How can something as large as a boat float in water when small rocks sink? Boats, and submarines, have a special hidden compartment called a ballast tank to help them accomplish this amazing feat. Ballast tanks are filled with air. When the tank is full of air, the boat is less dense than the water and it will float! Density is the amount of mass in a given volume, or amount of space. Mass is dependent on the number of molecules in an object. 

If we change the amount of mass in the ballast tank, the boat will instead sink. Submarines use this trick to travel below the water’s surface. Water enters the ballast tank, which is more massive and contains more molecules than air, causing the submarine to sink. When the submarine wants to rise back to the surface it will push the water out of the ballast tanks and fill them with air. Though the mass inside the tanks have changed, their volume has not, and the submarine can float back up to the surface! Explore density more with this easy science experiment that you can do right from home.

 Vegetable Oil, Water, Salt, food coloring, clear glass

 Fill the glass ⅔ with water. Pour in vegetable oil to fill the rest of the glass. Add a few drops of food coloring and observe what happens. Finally, add salt and watch what happens with the oil and the food coloring! You can continue adding more salt and food coloring as you wish.

 In the beginning of the experiment, the oil settles above the water because it is less dense. When you add salt to the cup, the salt sinks to the bottom of the cup since it is heavier than the other two liquids. The salt carries a blob of oil with it, some of which contains the food coloring. As the salt begins to dissolve in the water, it releases the oil, which floats back up to the top.

 Can you figure out how ice floats in water?

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Standards for Density

Next Generation Science Standards