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Chapter 11: Problem 30

Table sugar \(\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)\) or urea \(\left[\left(\mathrm{NH}_{2}\right)_{2} \mathrm{CO}\right]\) can be used by road crews to melt ice on roads, but solutions of \(\mathrm{CaCl}_{2}\) are generally used instead. Assuming equal costs per pound of substance, why is \(\mathrm{CaCl}_{2}\) used instead of table sugar or urea?

Short Answer

Expert verified
Calcium chloride (CaCl2) is used instead of table sugar (C12H22O11) or urea [(NH2)2CO] in melting ice on roads because it is an ionic compound that dissociates into more particles in solution, resulting in a greater freezing point depression. Additionally, its hygroscopic nature allows it to form a brine more readily, making it more effective and cost-efficient for melting ice.

Step by step solution

01

Discuss the ionic nature of the substances

The first thing to note is that CaCl2 is an ionic compound, meaning it consists of oppositely charged ions. C12H22O11 (table sugar) and urea [(NH2)2CO] are covalent compounds where atoms share electrons. Ionic compounds can dissociate into ions when dissolved in water, increasing the number of particles that can interact with the ice, thus improving the melting process. On the other hand, covalent compounds do not dissociate into ions when dissolved.
02

Discuss the freezing point depression property

Freezing point depression is a colligative property that depends on the number of solute particles dissolved in the solution. The more solute particles, the lower the freezing point of the solution. In the case of CaCl2, it dissociates into three ions in water, whereas table sugar and urea do not dissociate when dissolved. This means that a given weight of CaCl2 produces more particles in solution than an equal weight of table sugar or urea, resulting in a greater freezing point depression, making it more effective at melting ice.
03

Discuss the hygroscopic nature of the substances

Calcium chloride is hygroscopic, which means it can attract and hold water molecules from its surrounding environment. This property allows CaCl2 to form a brine more readily when in contact with ice. The brine formed lowers the freezing point and helps in melting the ice. On the other hand, table sugar and urea do not have similar hygroscopic properties, making them less effective in melting ice.
04

Discuss the cost-effectiveness

Although the problem assumes equal costs per pound of substance, it is essential to consider the amount needed to achieve the desired effect. Since CaCl2 produces more particles in solution leading to a greater freezing point depression, less quantity of CaCl2 is needed for the same effect compared to table sugar or urea. As a result, using CaCl2 would be more cost-effective. In conclusion, calcium chloride (CaCl2) is used instead of table sugar (C12H22O11) or urea [(NH2)2CO] in melting ice on roads because it has superior properties such as the ionic nature, freezing point depression, and hygroscopic nature, making it more effective and cost-efficient for melting ice.

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