Problem 35 - Chemistry: An Atoms-Focused Approach

Question

A solution of table salt is a good conductor of electricity, but a solution containing an equal molar concentration of table sugar is not. Why?

Step-by-Step Solution

Answer

Answer: Table salt conducts electricity when dissolved in water because it dissociates into free mobile ions (Na+ and Cl-), which are necessary for conducting electricity. In contrast, table sugar does not conduct electricity as its molecules do not dissociate and no free mobile ions are formed in the solution.

Step 1: Understand the nature of table salt and table sugar.

Table salt is an ionic compound (Sodium Chloride, NaCl) and table sugar is a molecular compound (Sucrose, C12H22O11). When dissolved in water, both substances interact with water molecules differently.

Step 2: Explore dissociation in table salt.

When table salt (NaCl) is dissolved in water, the ionic bond between sodium (Na+) and chlorine (Cl-) breaks, and the ions are surrounded by water molecules. This process is known as dissociation and can be represented by the following equation: NaCl(s) → Na+(aq) + Cl−(aq) The presence of free mobile ions (Na+ and Cl-) in the solution enables it to conduct electricity, as electricity is the flow of charged particles.

Step 3: Explore the properties of table sugar in a solution.

Unlike table salt, table sugar (sucrose) is a covalent compound and does not dissociate into ions when dissolved in water. Instead, sugar molecules disperse evenly throughout the water, maintaining their molecular structure. This results in a solution without free mobile ions.

Step 4: Explain why table sugar solution does not conduct electricity.

The table sugar solution does not contain any free mobile ions, which are necessary for conducting electricity. Since the sugar molecules remain intact when dissolved in water, the solution remains a non-electrolyte and does not conduct electricity. In conclusion, a solution of table salt conducts electricity because it dissociates and forms free mobile ions in the solution. On the other hand, a solution of table sugar does not conduct electricity because sugar molecules do not dissociate and no free mobile ions are formed.

Key Concepts

Ionic Compounds

Ionic compounds are made up of positively and negatively charged ions held together by strong electrostatic forces. These forces create what is known as an ionic bond. A classic example of an ionic compound is table salt, or sodium chloride (NaCl).

In the solid state, the ions in an ionic compound are fixed in place within a crystal lattice structure. However, when dissolved in water, these ionic bonds can break, allowing the individual ions to move freely.

• This movement of ions is key to many properties of ionic compounds.
• For instance, in solutions, free ions enable the material to conduct electricity.
• It’s the dissociation into these ions that distinguishes ionic compounds from covalent compounds in solutions.

Covalent Compounds

Covalent compounds form when elements share pairs of electrons, resulting in molecules. A well-known covalent compound is table sugar, scientifically known as sucrose (\[C_{12}H_{22}O_{11}\]).

In these compounds, there are no free ions present because the atoms are bound by shared electrons, making them stable molecules. When a covalent compound such as sugar dissolves in water, it disperses evenly but does not break apart into ions:

• The molecules remain intact and retain their structure.
• Consequently, there are no free-flowing charged particles in the solution.
• This lack of dissociation means that covalent compounds typically do not conduct electricity when dissolved.

Dissociation

Dissociation is the process by which ionic compounds separate into ions when dissolved in a solvent like water. This process is crucial for the electrical conductivity of ionic solutions.

Taking sodium chloride (\[NaCl\]) as an example, dissociation occurs as follows:
\[NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)\]

• The strong ionic bond between \[Na^+\] and \[Cl^-\] ions is overcome by the interaction with water molecules.
• These ions are then free to move independently and distribute uniformly throughout the solution.
• The ease of dissociation contributes to ionic compounds being good electrolytes.

Without dissociation, ionic compounds would not be able to conduct electricity in solution, as movement of free ions is critical in carrying charge through the liquid.

Electrical Conductivity

Electrical conductivity is the ability of a substance to allow the flow of electric current. In solutions, it largely depends on the presence of free ions.

Solutions of ionic compounds, due to dissociation, have many free-swimming ions:

• These charged particles can carry and transfer electrical current through the liquid.
• This is why solutions like saltwater are good conductors of electricity.

Conversely, solutions of covalent compounds generally lack this property:

• As no ions form in the solution, there are no charged particles to facilitate conductivity.
• Sugar solutions, despite their sweetness, cannot conduct electricity, making them non-electrolytes.

Understanding electrical conductivity in solutions highlights the importance of ion presence and movement, which is essential for many practical applications, from batteries to salt lamps.