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What if I don't have any baby food jars?
If you don't have any baby food jars and you don't
have a baby who will empty some jars for you, we suggest that you buy a
jar of strained peaches or applesauce baby food. You can empty the jars
over vanilla ice cream for a tasty dessert or mix them in a blender with
milk and ice to make a smoothie. If you don't want to buy baby food, we
have also done this experiment using two identical small, clear, drinking
glasses. You just have to make sure that the mouths of the glasses match
up perfectly, without a leak.
Why
does the water mix so quickly when the glass of hot water is on the bottom?
If you've already made a
Salt Volcano
or a
Glitter Globe
, you probably know
that some liquids float on top of other liquids. Oil floats on water. Alcohol
floats on oil. That's because these liquids have different densities. Whenever
you put together two liquids that don't mix, the liquid that is less dense
will float on top of the denser liquid. A drop of oil weighs less than a
drop of water the same size. The oil is less dense than the water, so it
rises to the top.
When you
heat up water, the water molecules start moving around faster and faster.
They bounce off each other and move farther apart. Because there's more
space between the molecules, a volume of hot water has fewer molecules in
it and weighs a little bit less than the same volume of cold water. So hot
water is less dense than cold water. When you put the two together with
the hot water on the bottom, the hot water rises to the top, mixing with
the cold water along the way and creating purple water.
Why doesn't the water mix when the hot water is on top?
When the cold water is on the bottom, the hot water doesn't have to rise--it's
already on top. The cold blue water stays on the bottom and the hot red
water stays on top.
How can I experiment further?
What do you think would happen if you tried this experiment with a jar of
salt water on top and a jar of water without salt on the bottom? Try it
and see. Use food coloring to color the salt water a different color than
the plain water so that you can see what happens.
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Fill one of the jars with very
hot tap water. Add a drop of red food coloring. What happens to the drop?
Watch for a minute, then put the red jar into the baking pan
.
Fill the other jar with cold water. Add a drop
of blue food coloring. What happens to that drop?
Cut about
2 inches off one short side of the index card. You should end up with a
square about 3 inches on a side.
Slowly add more water to the
blue jar until you can see a bulge of water over the rim of the jar.
This part is very tricky. You
may want to practice it a few times over the sink with a jar of plain water.
Pick up the blue jar and turn it straight upside-down. You don't need to
put your hand on the card. The water will hold the card in place. (Just
flip the jar over. Don't hesitate. If the jar is tilted but not turned over
completely, the water will gush out and make a mess.)
Lay the square card carefully onto the top of the
blue jar. Tap the card gently with your finger. (Don't poke it. You want
the card to be flat and form a seal with the water and the jar.)
Put the upside-down blue jar
right on top of the red jar.
Have someone hold onto both jars
while you very slowly and carefully pull the card out.
What happens? What color is the
water in the top jar? What color is the water in the bottom jar?
Empty both jars. Rinse them.
Repeat steps 1 through 6--but put the jar with the blue-colored cold water
in the baking pan and put the card on top of the jar with the red-colored
hot water. Turn the red jar upside-down and put it on top of the blue jar.
Slowly pull out the index card. What
happens? What color is the water in the top jar? What color is the water
in the bottom jar?
