Focus on Science H4.1: Winter Roads and Ice Cream Making
Ice cream could be made in the summer only through extraordinary efforts in
the days prior to refrigerators. You have employed the methods used to make
ice cream. When an ice and water mixture has salt added to it, it lowers the
temperature at which ice melts. As you observed, the temperature of the ice and
salt mixture falls below 0°C. This places the water and salt bath at a temperature
lower than the chilled ice cream mixture. As long as the water and ice are below
0°C, heat energy will be removed from the as yet unfrozen ice cream allowing it
to freeze. Mixing (cranking) ensured a rapid cooling of all of the fluid in the can
and prevented ice crystals from forming apart from the ice cream.
On winter days where the temperature hovers just below freezing, adding salt to
the surface of the ice will lower its melting temperature below the air
temperature, thus allowing ice normally at 0°C to melt on a day that the
temperature is below freezing. On extremely cold days salt will no longer help
since the air temperature falls below the melting temperature of ice and salt.
