Today I read a story in the WP relating to the rotation of earths “solid” inner iron core. I looked it up from a different source that confirmed it is solid as opposed to liquid. How can it be so hot and not be molten? Thank you!
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The vast majority of time when someone mentions the melting or boiling points of a substance, they are actually talking about the melting and boiling points at standard pressure. Standard pressure is defined as 1 atmosphere of pressure, or about 15 pounds per square inch.
For example, when someone says that water boils at 100°C, what they are really saying is that the boiling point of water *at standard pressure* is 100°C. They are just dropping the *at standard pressure* part because it’s easier to say and the *at standard pressure* is already implied.
The reason we have to make the distinction of *at standard pressure*, is because the melting point and boiling point of substances isn’t constant, it changes depending on the pressure it is under. For example, the more pressure you put water under, the harder it is to get it to boil, meaning that if water is under high pressure, you have to raise its temperature much higher than 100°C to get it to boil. Likewise, if water is under very low pressure, it will boil well before 100°C. In fact, you can get water to boil at room temperature if you put it in a vacuum chamber. This is also why pressure cookers are a thing, if you put food and water in a pressure cooker and bring that water to a boil under very high pressures, that means the water is actually much hotter than 100°C and will cook your food faster.
I’ve been using water as an example, but this is true of all substances. The amount of pressure will change the boiling and melting points, and iron is no exception. The core of the earth is made mostly of iron, and while the core of the earth is indeed very hot, much hotter than the melting point of iron *at standard pressure*, it’s also under tremendous pressure from all the iron and rock above it. There’s so much pressure in fact that the melting point of the iron is raised so high that even the extreme temperatures of the core of the earth can’t actually melt the iron. However, if you start to leave the core of the earth and go up, the pressure starts to drop (because there’s less stuff above pushing down) and the melting point of the iron will start to decrease and it eventually gets low enough that the temperature of the core IS hot enough to melt iron. This is why the inner core is solid while the outer core is liquid.
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