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Brent Cullimore

Tiny Bubbles was a hit song by Don Ho (before even my time). Tiny Density Gradients just won’t make the charts.

But that’s what happens when you take a pure fluid substance to high enough pressure or temperature: the distinction between liquid and vapor disappears. Instead, you instead get a dense, spongy state called a “supercritical fluid.” It happens easily with carbon dioxide (aka CO2 in the chemical world or r744 in the refrigerant world) because of its relatively low critical temperature (31°C, 87.8°F).