Changes in Density

You mean it’s not always 1.0 g/ml? Water’s density can change?

Yes! In fact, every substance can undergo a change in density. When a substance is heated, the atoms and molecules vibrate, which increases the substance’s volume without changing its mass. As a result, its density goes down. When a substance is cooled, the opposite happens, and the density goes up. (Ice crystals are a notable exception this rule.)

Today in class, students took part in a lab that tested this effect. They observed how hot water floated because of this change in density. All you needed was some hot water, a pipette, and a bit of food coloring. They repeated the test with cold water and saw the opposite effect. Then they tested salt water. Because salt dissolves in water, filling the spaces between the water molecules, it makes the water heavier without affecting its volume. The result? Salt water has a higher density. Because of these factors, it is actually easier to float a boat in cold, salty water than it is in warm, fresh water!

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Thanks for the help with the above video, Phoebe. And remember everyone, tonight’s homework is to finish the Post-Lab Conclusions.

September 16 – Changes in Density (pg110)

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