Magic Water Barrier

Amaze your kids with this magic water experiment! Teach them about how the density of water changes with different temperatures in this colorful and mystifying project.

This trick relies on the difference in density between hot and cold water. When water is heated up, its water molecules move more quickly, expanding the space between individual molecules. This increases the water's overall volume and decreases its density.

  1. Ages: 9 - 16

  2. <30 minutes

  3. Messy


Materials you'll need

Step-by-step tutorial

  • Step 1

    Place both cups on the tray. Carefully fill one of the cups with hot water and color it with 2 drops of red food dye. Fill the other jar with cold (refrigerated) water. Add 2 drops of blue food dye. Make sure the water is filled to the top so it is almost spilling over.

    Observe: Before stirring, does one color seem to mix into the water more quickly?

    Photo reference of how to complete step 1

  • Step 2

    Take the plastic sheet and press it firmly over the top of the blue cup. This should create a vacuum so that when the cup is turned upside-down, the water does not spill out.

    Photo reference of how to complete step 2

  • Step 3

    This step is tricky! In a tray, smoothly lift the blue cup with the plastic sheet and turn it upside-down. Align it over the red cup and set it down so that it sits perfectly centered.

    Photo reference of how to complete step 3

  • Tip

    It took a couple of times to perfect this step so practice a few times if necessary. Cardstock also works as a plastic sheet alternative, but the plastic sheet was easier to work with. Be careful when handling hot water.

  • Step 4

    Ask a friend to carefully remove the plastic sheet from between the cups as you keep them aligned and steady. Right away you should see the colors begin to mix until eventually the water becomes a purple color.

    Photo reference of how to complete step 4

  • Step 5

    Empty both cups, rinse, and then repeat steps 1 and 2. Now place the card over the red cup, turn it upside-down and place it on top of the blue cup. Remove the plastic sheet. What happened different with the hot water on top? It seems like there is a magic water barrier!

    Photo reference of how to complete step 5

  • Learn moremagnifying icon graphic

    What's going on?

    Since hot water is less dense than cold water, it floats on top of cold water but only if it starts out on top. If the denser cold water is placed on top of the less-dense hot water, the cold water immediately starts to sink down, and the waters begin to mix. When hot water starts on top, it will continue to float as long as it remains much less dense than the cold water below. As the hot water cools off, and the cold water warms up, the two batches of water begin to have similar densities, and eventually mix together.

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