Why does the ice tray not stick to the freezer of the fridge when the ice tray is kept on rock salt?

The temperature of the freezer box is around -18 degrees Celsius. At this temperature ordinary water would freeze. The ice tray usually sticks to the freezer box because the layer of water between the tray and the bottom of the freezer gets converted to ice.

But the tray would not stick to the freezer if the water between them can be prevented from freezing. This is what happens when the tray is placed on rock salt.

Rock salt is a mineral called halite and chemically it is sodium chloride — the same as common salt. It has physical and chemical properties same as that of common salt which can be used to make an effective freezing mixture which lowers the freezing point to far below zero degrees Celsius.

When the tray is placed on rock salt, the intermediate water layer dissolves a part of the salt and remains in liquid form even after the water in the tray becomes ice.

Source: The Hindu