Problem of the Week

Chapter 15: Solutions

Problem of the Week

The Big Freeze
Ice cream is a frozen dairy food made by freezing a mixture of pasteurized milk, heavy cream, sugar, corn syrup, water, and other optional ingredients. The mixture is agitated during freezing to incorporate air and insure a uniformity of consistency. Ice cream and related products such as frozen custard, ice milk, sherbet, water ice, and yogurt are generally classified as frozen deserts. The first frozen dessert is credited to Emperor Nero of Rome. This frozen dessert was made from snow, retrieved from the Apennine Mountains by slaves, mixed with wine, fruit pulp, fruit juices, and honey. Marco Polo, a 13th century explorer, brought recipes for water ices, resembling sherbets, to Europe from his adventures in China.

From their first arrival through the 18th century, the closely guarded ice cream recipes traveled the royal houses of Europe. King Charles I of England served a cold creamy dessert prepared by his French chef DeMirco at a state banquet and declared that this dessert was not for commoners. Caterer Phillip Lenzi announced the sale of various confections, including ice cream, in a New York newspaper upon his return from London in 1774. Dolly Madison, wife of U.S. President James Madison, served ice cream at her husband's Inaugural Ball in 1813. Some form of frozen dessert has been enjoyed for thousands of years.

Good ice cream is determined not only by the taste but also the texture. The texture of ice cream is dependent on the process used. Making ice cream at home requires the use of an ice cream machine. The steps of preparing homemade ice cream are similar to the commercial processing stages. The ice cream mix is prepared, aged to increase consistency, and loaded into a container equipped with blades and a dasher. This container is placed into a bucket that is loaded with ice and salt for the freezing process.

The freezing procedure has a two-fold purpose, the removal of heat from the mix and the incorporation of air into the mix. Heat is removed by conduction through the metal to the salt-water brine surrounding the freezing can. This transfer of heat depends upon the temperature of the brine, the speed of the dasher, and how well the dasher scrapes the cold mix from the surface of the freezer can. The dasher speed and surface contact are important to achieve complete removal of the frozen ice cream from the wall of the freezer can. Salt-water brine is made by adding enough salt to crushed ice to make a good freezing mixture. The ice cream mixture usually takes between 20 to 30 minutes to freeze and the freezing container is then placed into a freezer to allow the ice cream to harden.

Commercial ice cream is considered an emulsion. Define an emulsion and explain how it is a colloidal system.

When 1200.0g of sodium chloride added into 7300.0 g of ice the resulting solution has a density of 1.089 g/mL. Determine the molarity, molality, mass percent of NaCl, and mole fraction of NaCl for this solution?

Determine the freezing point depression and the freezing point of a 20.0% salt solution. Remember that sodium chloride is an ionic compound that dissociates into 2 mol ions/mol NaCl when dissolved in water. The K_fp of water is 1.86 C°/molal.