Hot and Cold Water Cups

Hot and Cold Water Cups


In this week’s science lesson, we’re
going to be learning about density. If something is more dense, that
means it is packed together and heavier than something that is less dense. And
it’s easy for us to see that solid things, like this cart right here, solid things are hard
and heavy. They are more dense than air. Can I push my hand through this cart and
just punch right through it? No, it’s solid. Can I push my hand through air
and punch right through it? Yes, it’s a gas, and it is not dense.
Well, did you know that cold water is more dense than hot water? I have
two containers of water right here. This one is very warm. It would be a good
temperature for taking a bath or a shower. And we’re going to fill up two of these
cups with warm water. But first, I’m going to add a little bit of food coloring.
This demonstration works without food coloring, but it’s more difficult to see.
It’s a lot easier to tell what’s going on if we color the water. So I’m going to
put three drops of yellow into this cup, and three drops of yellow into this cup
over here. And then we are going to add our warm water. And because our warm
water is less dense, it tends to want to rise. We’re going to fill the water all
the way up to the very top of the cup, and then we’re gonna do the same over here. Just like hot air balloons tend to rise, because the air inside them is less
dense than the air around them, the same thing happens with water. Warm water
rises, and cold water sinks. I have here a container of ice-cold water, and we’re
going to color the cold water blue. Two drops of food coloring in this cup,
two drops of food coloring in this cup. And now we’ll take out our ice-cold water,
and fill up these two cups with cold water. And now comes the fun part. We are
going to layer these cups in an opposite way. We’re going to have cold on top of warm here,
and warm on top of cold over here. And then the experiment, or the
question that we are asking, is what happens when we remove
the barrier between these cups and let the water mix? Will they mix
together and make green? Will they stay layered? Or will they trade places? So as
I’m setting this up, I want you to be making predictions about what you
think will happen when we remove the lids. So here is our experiment. We have our
cups set up with warm on top on one side, and cold on top on the other side, and
you have enough information to make a prediction because you know that warm water wants to rise, and cold water wants to sink. So first, we’re going to remove the barrier
right here between these two cups. Watch closely. I recommend having two people to do this,
because it is tricky to do with just one person trying to balance the cups and
remove it at the same time. But you can see that the cups stayed the same. We
have yellow on top of blue because the warm water wants to be up, and it’s
already up. The blue water wants to be down, and it’s already down. Now we’re going
to try the same thing over here with these cups. And they are mixing together. You can see
that within just a few seconds, the cups have turned green. And they’ve turned
green because that warm water that was on the bottom wanted to rise. It wanted
to go up. And that cold water that was on the top wants to sink, wanted to go down.
We have a dramatic difference in color here, but we also have a dramatic
difference in temperature. I can feel through the plastic that this is hot
water. I can feel through the plastic that this is cold water. But if I
place my hands here and here, the water temperature feels the same, not
hot, not cold, just lukewarm, because this water has mixed together to make warm.
One neat thing that you can do, to take this a level deeper, is to experiment
with changing the temperature of the water. I told you that this water
was ice-cold, and that our warm water was about a comfortable temperature for
taking a bath, but what would happen if you had a smaller difference in
temperature? What if the cold water was 40 degrees Fahrenheit, and the warm water
was 45 degrees Fahrenheit, and they were relatively close in temperature? Would
they mix to turn green sooner? Or would it take longer? What is the relationship
between the temperature difference and how long it takes for the water to mix?
Because these cups will eventually turn green, just like these ones over here did.
How long it takes depends on the temperature, and you can find out exactly
what that relationship is by doing some careful experimenting and then recording
your data. If you would like to check out our Science Fair Guide, go into the
description and there’s a link where we walk you through how to make this into a
full science investigation, or a science fair project. I hope you enjoyed this
demonstration and, as always, come back next week and join us for another one.
And a special thank you to our Navigator Patron Noelani Nomiyama.
If you would like to support the creation of these videos, check us out at
www.patreon.com/ScienceMom We’ll see you next week. And the other question that I
always get asked by kids is, how are you going to get the water out of
the cups? You can turn them sideways, and just like cracking an egg, spill the water
out. Or, and I think this is even a little bit more fun, you can offset the cups so
that they’re not lined up, and then… the water from the top cup will all fall
out, and then you can just pour out the water from the bottom cup.

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