Question

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

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

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.

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.

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.

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.

Key concepts

Ionic Compounds

Ionic compounds are made up of charged particles, called ions, which are held together by electrostatic forces. These ions are usually formed when metals react with non-metals. When an ionic compound like calcium chloride (${\text{CaCl}}_2$) is dissolved in water, it separates into its constituent ions. For instance, ${\text{CaCl}}_2$ splits into one calcium ion (${\text{Ca}}^{2+}$) and two chloride ions (${\text{Cl}}^{-}$). This dissociation increases the number of particles in the solution, allowing for more interactions with the external environment.
$\text{Covalent}$ compounds, such as table sugar (${\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}$) and urea (${\text{(NH}}_2\text{)\_2CO}$), do not dissociate into ions when dissolved. This difference means ionic compounds generally have more solute particles, making them particularly useful in applications like ice melting on roads.

• Ionic compounds form by the complete transfer of electrons, unlike covalent bonds which share electrons.
• The dissociation of ions in water enhances their ability to lower the freezing point.

Hygroscopic Properties

Substances with hygroscopic properties can attract and retain moisture from their environment. Calcium chloride is a well-known hygroscopic compound. When exposed to air, it absorbs moisture and forms a liquid brine, which is a mixture of water and salts. This characteristic makes it extremely effective for deicing roads.
This brine significantly lowers the freezing point of water, making it easier to melt the ice and prevent new ice from forming. On the contrary, substances like table sugar and urea lack significant hygroscopic properties. They do not attract moisture as effectively, making them less efficient in forming the brine necessary to lower the freezing point.

• Hygroscopic substances are crucial in applications like road salting as they readily create brine solutions.
• Brine effectively lowers water's freezing point, accelerating the ice melting process.

Colligative Properties

Colligative properties depend on the number of particles in a solution and not on the identity of the solute. One such property is freezing point depression, which is pertinent when dealing with melting ice. The more solute particles present, the lower the freezing point will be. Calcium chloride is especially effective here because it dissociates into more particles compared to other solutes such as table sugar or urea.
When you dissolve ${\text{CaCl}}_2$ in water, it splits into three ions (one ${\text{Ca}}^{2+}$ and two ${\text{Cl}}^{-}$), effectively tripling the number of particles in the solution compared to a similar mass of table sugar or urea which do not produce additional ions.
This increased number of particles enhances freezing point depression, making calcium chloride highly effective for tasks like deicing roads.

• Colligative properties are key in understanding how solutes affect solutions, particularly in industry.
• Freezing point depression is directly related to the number of particles dissolved, not their chemical nature.