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!
WHAT YOU'LL NEED
4 clear, glass bottles with narrow mouths
pitcher or measuring cup for pouring water
cold and hot water
2 waxy playing cards
towel for clean up (or you can use a tray or plate)
LET'S GET STARTED!
Put 4 drops of blue food coloring into two of the bottles, and 6 drops of yellow food coloring into the other two bottles.
Pour cold water into the two bottles with the blue coloring; fill to the very top of the bottle rims!
Pour hot water (the hottest from the sink will work fine) into the two bottles with yellow coloring and fill to the very top of the bottle rims.
Next, place a playing card over the top of one of the YELLOW bottles, and keep your fingers on the card while you flip the bottle over. The liquid will stay in the bottle.
Place the bottle over a BLUE bottle, and move your fingers out of the way so that the card is the only thing between the two bottles.
Stop! Predict what will happen when you remove the card!
Now, carefully slide the playing card away, keeping the two bottles touching as it's removed (a few drops will spill - that's okay!)
Observe what happens!
Now place the other playing card over the remaining BLUE bottle and flip it over onto the remaining yellow, hot bottle.
Stop! Predict what will happen to the waters when they touch this time.
Remove the card as before. What happened this time?
Check out this video to see how our experiment went in the Yellow Scope lab:
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!