Question

Which pure substance will not conduct electricity? (a) Molten \(\mathrm{NaCl}\) (b) Molten KOH

Short answer

Neither molten \(\mathrm{NaCl}\) nor molten KOH fits the description of a pure substance that will not conduct electricity as both contain free-moving ions that allow them to conduct electricity.

Step-by-step solution

Understanding Electrical Conductivity in Substances

Electrical conductivity in substances is usually made possible by the presence of free-moving charged particles. In solids, these charged particles are the electrons in the metal lattice. In liquids, like molten salts, these charged particles are ions. A pure substance will not conduct electricity if it does not contain free-moving charged particles.

Analyzing Option (a) - Molten \(\mathrm{NaCl}\)

Molten \(\mathrm{NaCl}\) is a liquid form of sodium chloride. In its molten state, \(\mathrm{NaCl}\) dissociates into sodium (\(\mathrm{Na}^{+}\)) and chloride (\(\mathrm{Cl}^{-}\)) ions. These ions are free to move and can conduct electricity.

Analyzing Option (b) - Molten KOH

Molten KOH, or potassium hydroxide, is similar to molten \(\mathrm{NaCl}\) in that it also dissociates into potassium (\(\mathrm{K}^{+}\)) and hydroxide (\(\mathrm{OH}^{-}\)) ions. These ions are also free to move and can conduct electricity.

Determining the Correct Answer

Since the question asks for a pure substance that will not conduct electricity, and both molten \(\mathrm{NaCl}\) and molten KOH can conduct electricity due to the presence of free-moving ions, neither option (a) nor option (b) represents a pure substance that will not conduct electricity.

Key concepts

Molten Salts and Conductivity

When we talk about salts such as sodium chloride (aCl)}), we typically imagine them as white crystalline solids. Yet, when these salts are heated to a certain point, they turn into a molten (liquid) state. In this state, their conductivity properties change significantly. Unlike the solid form, where ions are locked in place, molten salts allow ions to move freely. This movement is crucial for electrical conductivity because it's the flow of charged particles that carries electric current.

In simplest terms, the mobility of ions in molten salts such as molten aCl)} and molten KOH enables the material to conduct electricity. This is because, when dissolved or molten, the salt dissociates into positively charged sodium ions (a^+})), and negatively charged chloride ions (Cl^-})), both of which are eager participants in carrying an electric current. During this process, one end of the battery attracts positively charged ions (cations), while the other end attracts negatively charged ions (anions), allowing for the circulation of charge that we identify as an electric current.

Advantages and Uses of Molten Salt Conductivity

Due to their high conductivity, molten salts have several industrial applications. They are used in molten salt batteries, electrolysis for producing metals, and even as a heat transfer fluid in some types of nuclear reactors.

Ionic Dissociation in Molten Substances

Ionic dissociation is a process that refers to the breaking apart of ionic compounds into individual ions. This phenomenon is particularly evident in molten substances. What exactly leads to ionic dissociation? It's the heat. As the temperature of an ionic solid increases, the energy given to the substance starts to overcome the forces holding the ions together in a crystal lattice form.

Once you provide enough heat, these ionic solids eventually melt, breaking the rigid structure, and allowing ions to roam freely in the liquid. It is this 'freedom' that makes molten ionic compounds good conductors of electricity. As the ions are no longer fixed in place, they can move and carry charge throughout the substance when an electric field is applied. This forms the basis for the electric conductivity we observe in molten salts.

Understanding through Examples

To better comprehend ionic dissociation, let’s look at examples like molten sodium chloride or molten potassium hydroxide. Upon melting, aCl)} dissociates into a^+}} and Cl^-}}, while KOH separates into K^+}} and OH^-}}. These free ions act as charge carriers and facilitate the movement of electricity. It's a dance of positive and negative charges hopping from one place to another, which makes the study of molten salts and their conductivity a fascinating subject in the field of chemistry and materials science.

Conductivity of Pure Substances

Moving on to the concept of conductivity in pure substances, it is essential to clarify what 'pure' means in this context. We often refer to a material as pure if it contains no mixtures or other chemical compounds — just a single type of molecule or element. When it comes to electrical conductivity, pure substances fall into one of two categories: conductors or insulators.

For a pure substance to conduct electricity, it must have mobile charged particles. In metals, these are electrons that can move freely throughout the metallic lattice. However, many pure substances, especially covalent compounds like oils or distilled water, lack these free-moving charged carriers and thus do not conduct electricity.

Why Don't Some Pure Substances Conduct Electricity?

The absence of free-moving charged particles, like ions or electrons, is the main reason certain pure substances cannot carry an electric current. A perfect example is solid crystalline aCl}}, which, despite being made of ions, doesn't conduct electricity because the ions are held too tightly in the lattice to move freely. Hence, pure aCl}} in solid form is an insulator. On the other hand, when substances are not purely composed, like molten salts or aqueous solutions, the presence of ions makes them good conductors because of the ionic dissociation that allows charge movement.