Why Does Water Freeze from the Top Down?
We drink water, weِ swim inِ it, weِ wash withِ it, andِ we cool thingsِ downِ with it.
Because water isِ so common, manyِ of usِ fail toِ notice justِ howِ strange itِ isِ compared toِ otherِ substances.
When weِ plop anِ ice cube orِ two intoِ our drink duringِ the warmer months, weِ watch itِ float aboveِ the liquid inِ our glass withoutِ a secondِ thought.
Water freezes fromِ the top down—which allows ice toِ float—because ofِ a strange quirk inِ howِ water’s density behaves atِ falling temperatures.
For mostِ compounds, falling temperatures causeِ the compound’s volume toِ decrease whileِ itsِ density increases—with theِ atoms andِ molecules becomingِ more tightly packed together.
For example, a pocket ofِ warm air rises andِ expands becauseِ it isِ less dense thanِ the cooler air aroundِ it.
At 4 °C water isِ still inِ its liquid form.
If water insteadِ froze fromِ the bottom ofِ a lake orِ river toِ theِ top, thereِ would beِ profound ecological consequences.
Shallow lakes wouldِ freeze solid; unlessِ theِ plants, animals, andِ other organisms living thereِ hadِ some sort ofِ adaptation thatِ would keepِ their tissues fromِ freezing, theyِ would die.