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Sci Tech
Ice tray and rock salt
PHOTO: M.J. PRABU
Why does the ice tray not stick to the freezer of the fridge when it is kept on rock salt?
J. Paulson Nayagam
Coimbatore, Tamil Nadu
The temperature of the freezer box is around -18 degree Celsius. At this temperature ordinary water would freeze and solidify.
The ice tray usually sticks to the freezer box because the layer of water between the tray and the bottom of the freezer box gets converted to solid ice which binds the two to stick to each other.
On the other hand, if this layer can be prevented from freezing the tray would not stick. This is what happens when the tray is placed on rock salt.
Rock salt is a mineral called halite and chemically sodium chloride — the same as common salt. It has acquired this name because it is found as a mineral and is mined as rock like deposits in places where, in the geological past, there existed salty sea water which evaporated. The deposits subsequently got buried.
Rock salt 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. In fact it is used to achieve as low a temperature as - 20.3 degree 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. Thereby the two do not stick to each other.
H. K. Sahu
Scientific Officer
IGCAR, Kalpakkam, Tamil Nadu
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