Have you ever heard the phrase "heat rises"? You may have noticed that the smoke from a campfire or steam from a pot on the stove rises up into the air. Or that the air at the top bunk bed is warmer than the bottom bunk. That's because heat does indeed rise - but why?
It all has to do with density and convection currents. We'll explore these concepts in this month's 20 Minute Lab!You've probably mixed food coloring in water. Have you ever mixed yellow and blue together? You got green water! What do you think will happen if we put yellow HOT water in contact with COLD blue water? Let's experiment to find out with some common household items!
Safety Warning: Have an adult help you with the hot water. Make sure the bottle isn't so hot that you burn your hands!
LET'S GET STARTED!
WHAT'S GOING ON?
Why did the colors mix to make green only when you put the cold water over the hot water?
Well, it all has to do with density. Density is how much stuff is packed into a given space.
You might know that oil is less dense than water, so oil will sit on top of water. Water is denser and therefore heavier than oil, so gravity pulls the water to the bottom.
But guess what? Water can change density depending on its temperature!
In hot water, the molecules are bouncing around and more spread out, so it's not very dense. In cold water, the molecules are packed more tightly together, making it denser and heavier. So, when you put the hot (yellow) water on top of the cold (blue) water, nothing happened because the heavier water was already on the bottom.
But when you put the cold (blue) water over the hot (yellow) water, the more dense cold water wants to sink down and the less dense hot water wants to rise up.
The process of hot fluids (liquids and gases) rising and cold fluids sinking down is called convection currents.
We can see this happen on a small scale when you boil water on the stove or heat the air in your room. But it can happen on a global scale too - like water in the ocean and even in the air in the atmosphere - causing much of the weather we experience around the world!
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For more exciting experiments, check out our Yellow Scope science kits on the Shop tab of our website!