What is black ice

We give the name black ice to the phenomenon of invisible ice on a road. In practice, anything applying a pressure to solid ice will cause a similar depression of the freezing temperature to that of the skater, so a car or heavy vehicle travelling over ice will also cause a momentary melting of the ice beneath its wheels. This water-on-ice causes the car to skid - often uncontrollably - and leads to many deaths every year. Such ice is particularly dangerous whereas an ice skater wants the ice to be slippery, a driver does not. 

We move from the qualitative argument that r(meit) decreases as p increases, and next look for a quantitative measure of the changes in melt temperature with pressure. We will employ the Clapeyron equation  

In fact, it does not matter whether AH relates to the direction of change of solid - liquid or of liquid - solid, provided that AV relates to the same direction of change. 

The molar change in volume A /m has units of nrVmor1. Values typically lie in the range 10-5-10-6m3 mol-1. 

The minus sign of A /m reflects the way water expands on freezing. This expansion explains why a car radiator cracks in cold weather (if it contains no de-icer ) the water freezes and, in expanding, exerts a huge a pressure on the metal. 

---