How can water be 0°C or colder without being frozen? Assuming there are no impurities.
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There are two freezing temperatures in reality, the first is the temperature at which water will freeze (under atmospheric conditions) in the presence of a seed crystal or nucleation site for a crystal to form. The second is the temperature at which something called crystal homogenous nucleation is reached, which for water under the conditions we’re talking about is around -48C.
It truly depends on the pressure at that point. There lots of graphs that show what is known as a phase diagram. It shows the phases of a substance at pressures vs Temps. So when a liquid is not frozen, given pure water, then it must be warmer than 0⁰/32⁰ or be pressure regulated. There is also this thing called a triple point. Look it up, very interesting.
Mmhh.. I’ll try to answer like you’re five.. all of this is only valid for unsalted pure water.
When temperature reaches 0°C the water wants to freeze (well, not that water has desires but it becomes energetically more favorable to crystallize)
Now imagine water is on a staircase with only two steps, the higher water is the higher energy water has. When water is liquid it is thus on the higher step but now it freezes and water wants to go down. But to go down, water needs a little bit of energy, you know, to move her legs and find the motivation to move. Because when it’s 0°C, water is like you in the morning, it doesn’t want to move at all.
Usually some impurities can create enough Brownian motion (particles moving) to bring this energy. But in the case of pure water if nothing bring this little bit of energy then water will stay liquid. It’s an “overcooled system” and this state is called metastable because a liiiittle bit of energy such as poking the bottle would cause it to instantly freeze. There’s a lot of YouTube videos about that where people carefully remove the bottle from the freezer and just tap the bottle to make it freeze instantly.
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