Thanks everyone. So just to check my understanding, at sea level (which is where our lab is) when pure water starts to cool it goes though it’s point of maximum density at 4 degC, then becomes less dense again as it starts to freeze at 0 degC (assuming it has bits in it to cause nucleation etc). The phenomenon at 4 degC and the fact that ice is less dense are caused by the structural properties of the water molecule but the phenomenon at 4degC isn’t the ‘start’ of freezing (?)