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DENJIRO: I have here a glass of ice water. Ice floats in water, right? This is a special phenomenon that occurs precisely because the ice is in water. Let’s perform an experiment using water’s strange properties. The surface of the ice water is 0 degrees Celsius. So what’s the temperature at the bottom?
GIRL: It’s 4 degrees!
DENJIRO: Normally, warm part would rise to the top. So why is this happening? Take a look at this. It’s called a Galileo thermometer. There are a number of glass floats suspended in oil. So how does this measure temperature? Let me submerge it in water to cool it down.
GIRL: A float is rising!
DENJIRO: When a liquid’s temperature drops, its density rises. The different floats have their weight adjusted and have different buoyancy. For example, the float that says 26 degrees floats when the liquid goes below 26 degrees. By looking at which float has floated to the top, you can tell what the temperature of the liquid is, and therefore the room.
Now, what happens if you do the same thing with water? I have a test tube filled with water and another filled with alcohol instead of oil. They both have glass floats at the bottom. Now, let’s cool down the test tubes by placing them in saltwater that’s -10 degrees. The floats are rising. Let’s cool them even more.
GIRL: Huh? The float in water is sinking again.
DENJIRO: The colder a liquid gets, the higher the density becomes. However, water is an exception. Water reaches maximum density at 4 degrees. But it’s truly a mysterious liquid because when the temperature goes below that, the density goes down as well. That’s why the float that rose to the top started to sink again. Water sinks to the bottom at 4 degrees because that’s when its density is highest. But as it reaches 3 to 1 degrees, the density drops. When it becomes ice, the density is extremely low. It floats on water since it is light. Due to this property of water, the bottom of oceans and lakes don’t freeze, and that’s why the Earth doesn’t freeze over.
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