Density of Water

    March, 2016:  

    Kids, this is an experience that you may have noticed before, particularly if you go swimming in the summer. Fish take advantage of this principle in the winter time. 

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.


    • Hot water
    • Cold water (ice water)
    • Food coloring
    • 4 clear plastic cups/containers with no pour spout
    • 2 or more 4x6 index cards

    Try this:

    • Pour some hot tap water into two glasses.  In two other glasses put an ice cube and cold water into each.  Make sure that the cups are filled to the brim.
    • Make the two hot cups colored with one color of food coloring and the two cold cups with a different color.  Many people use red for hot and blue for cold but yellow and blue make for an interesting combination too.  Do you notice a difference between which water disperses the food coloring fastest?  What color do you expect if the colors mix?
    • Place a few 4 x 6 index cards on top of the hot water and the cold water cups.  
    • This is where things might get messy so an adult can help with this step.  One at a time, quickly flip the cups with index cards on them and place one of the hot water cups on top of one of the cups with cold water. Make sure that hot is on top of cold.
    • Likewise, quickly flip the remaining cold water cup and place it on top of the remaining cup with the hot water. Make sure that cold is on top of hot.
    • Pull out the index cards. 

    Many of you have experienced density before and you may have referred to it as “Sink or Float”.  Now you’re comparing hot water to cold water.  When you allowed the water to mix with the hot on top of the cold did anything happen?  How about when you had the cold on top of the hot?  Did you find that the cold water sank to the bottom?  You should also have noticed that the two colors mixed nicely in one case but not in the other.  If you mixed yellow and blue you should have obtained a green mixture.  You can use a color wheel to help you determine which color you get.  You should get a color in between the two colors that you mix.

    You may have noticed that the hot water dispersed the food coloring faster than the cold water did.  The reason for this is that hot things have molecules with lots of energy and motion while those that are cold have little energy or motion.  The more motion in the molecule, the faster things should mix.  If the two sets of glasses sat long enough, would they eventually look the same? Try it out!

    At the opening it was stated that you may have had this experience while swimming.  If you dive into the deep end of the pool you may have noticed that the temperature of the water changes to colder temperatures the deeper you go.  Part of the reason for this is that the cold water is more dense than the hot water.  The other reason is that the sun puts its energy into heating up the water on the top.  Fish use this principal in the winter because the oxygen that they breathe is dissolved in the cold water.  Since their pond might be frozen over, the fish have to go to the bottom of the pond where it is colder to get their oxygen.  

    This is similar to another ChemShorts article on density using sugar to increase the density:

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