Question

Why does a solution of sodium chloride in water conduct an electric current?

Short answer

A solution of sodium chloride in water conducts an electric current because when NaCl dissolves in water, it forms charged solvated ions (Na+ and Cl-) that are free to move around in solution. When a voltage is applied, these ions move towards their respective electrodes, generating an electric current. This conductivity arises due to the presence and movement of the charged particles in the solution.

Step-by-step solution

Understand the structure of sodium chloride

Sodium chloride (NaCl) is an ionic compound, which means it consists of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). In its solid state, these ions are arranged in a crystal lattice structure where each Na+ is surrounded by Cl- ions and vice versa.

Dissolving sodium chloride in water

When sodium chloride is added to water, the polar water molecules (H2O) interact with the ions present in the NaCl crystal lattice. The negatively charged oxygen atoms in water (with their partial negative charge) are attracted to the positively charged Na+ ions, while the positively charged hydrogen atoms (with their partial positive charge) are attracted to the negatively charged Cl- ions. As a result, the attractive forces between the water molecules and the Na+ and Cl- ions overcome the forces holding the ions together in the crystal lattice, causing the NaCl to dissolve.

Formation of charged ions in solution

Once the NaCl has dissolved, the sodium ions (Na+) and chloride ions (Cl-) are now free to move around in solution. The water molecules effectively separate individual Na+ and Cl- ions from each other, surrounding them with a "shell" of water molecules known as a hydration shell. These mobile charged particles are referred to as solvated ions.

Ionic conductivity of sodium chloride solution

The presence of solvated ions in the sodium chloride solution allows it to conduct an electric current. When a voltage is applied across the solution, the negative electrode (cathode) attracts the positive Na+ ions, while the positive electrode (anode) attracts the negative Cl- ions. This movement of ions towards their respective electrodes generates an electric current. Thus, a sodium chloride solution conducts electricity due to the presence and movement of these charged particles (solvated ions) in solution.

Key concepts

Electrolyte Dissolution

The process of electrolyte dissolution is critical for understanding the conductivity of solutions. Let's take sodium chloride (NaCl) as an example. In its solid form, NaCl is made up of a lattice of ions that are tightly held together by ionic bonds. Upon being introduced into water, these bonds are disrupted. The water molecules, which have a polar nature, interact with the Na+ and Cl- ions. This interaction is strong enough to pull the ions away from their lattice and into the solution.

Despite their solubility, not all substances dissolve to produce a conductive solution. Only when a substance dissociates into ions, which are charged particles, does it become an electrolyte. A strong electrolyte like NaCl completely dissociates in water, which is why a NaCl solution can conduct electricity well.

Solvation Process

Diving deeper into the solvation process helps us to understand the transformation of ions from a solid state into a dispersed state in solution. For NaCl, once the ions are separated from the lattice, individual Na+ and Cl- ions are surrounded by water molecules. The slightly negative oxygen end of the water molecule faces the Na+, and the slightly positive hydrogen ends face the Cl-. This creates a stable arrangement called a hydration shell.

The solvation process is crucial because it allows ions to move freely. The better the solvation process, the more mobile the ions in the solution, which directly impacts the solution's conductivity. This is integral for electricity to pass through, as the flow of charges is what essentially constitutes an electric current.

Electrical Conduction in Solutions

The electrical conduction in solutions can be understood in terms of the movement of ions. When an electric field is applied, the charged solvated ions are compelled to move towards their oppositely charged electrode: cations move towards the cathode, and anions move towards the anode. This flow of ions constitutes an electrical current.

The sodium chloride solution acts as an electrolyte, meaning it can support the movement of ions, which in turn allows for the flow of electric current. An important factor to keep in mind is that the mobility of each ion and the number of ions are key determinants of the solution's overall conductivity. In our example, NaCl, when dissolved in water, disassociates completely into Na+ and Cl-, ensuring a high number of charge carriers, making the solution a good conductor.

Ionic Compounds in Water

Understanding how ionic compounds behave in water is vital to grasping their role in electrical conduction. When ionic compounds like NaCl are added to water, they dissolve because the interaction between the ions and the water molecules is more favorable than the ionic bonds in the solid state. Thus, these compounds tend to disperse evenly throughout the water, creating a uniform solution.

Once these ions are in solution, they contribute to the solution's properties, such as boiling point elevation, freezing point depression, and of course, electrical conductivity. Ionic solutions are central to many natural processes, industrial applications, and scientific experiments due to these unique properties.