Electrolytes and Electrical Conductivity in Aqueous Solutions

Electrical Conductivity in Pure vs. Impure Water

• Pure (de-ionized) water
  • Contains virtually no dissolved ions.
  • When an electric current is applied, no observable conduction occurs because no charged particles are present to carry the current.
• Tap water
  • Contains trace solutes such as fluoride and cleaning additives.
  • Still a poor conductor; ion concentration remains too low for significant current flow.

Role of Dissolved Species

• Key requirement for conductivity: the solution must contain mobile charged particles (ions).
• Dissolution ≠ conductivity
  • Example: table sugar (sucrose) dissolves readily, but the molecules stay neutral; therefore, the solution does not conduct electricity.
  • Dissolving alone is insufficient—ionic dissociation is needed.

Solid vs. Dissolved Ionic Compounds

• Solid NaCl (sodium chloride)
  • Crystal lattice locks \text{Na}^+ and \text{Cl}^- in fixed positions.
  • Charged particles are present but immobile, so the solid does not conduct.
• NaCl in water (salt water)
  • Lattice breaks; ions disperse and become mobile: \text{NaCl}{(s)} \rightarrow \text{Na}^+{(aq)} + \text{Cl}^-_{(aq)}.
  • Result: high electrical conductivity.

Electrolytes

• Definition: A substance that, when dissolved in water, yields a solution capable of conducting electricity due to the presence of mobile ions.
• Classification
  • Strong electrolytes
  • Dissociate (or ionize) completely.
  • Provide a large concentration of ions → high conductivity.
  • Most soluble ionic compounds fall into this category (e.g., NaCl, KBr, \text{CaCl}_2).
  • Weak electrolytes
  • Dissociate partially; equilibrium favors the un-ionized form.
  • Produce fewer ions → moderate/low conductivity.
  • Example given: ammonia (NH₃), which reacts with water to form a small amount of \text{NH}_4^+ and \text{OH}^-.
• Non-electrolytes
  • Dissolve without forming ions → no conductivity.
  • Example: sugar solutions.

Acid Behavior (preview)

• Instructor hints that acids show varied electrolyte strength:
  • Some acids ionize completely (strong acids) → behave as strong electrolytes.
  • Others ionize partially (weak acids) → behave as weak electrolytes.
  • Detailed discussion postponed, but the link is emphasized: extent of ionization ↔ conductivity.

Biological & Practical Implications

• The body relies on ions (electrolytes) to conduct bio-electric signals (nerve impulses, muscle contractions).
• Commercial sports drinks emphasize “getting electrolytes” to replenish ionic balance for optimal physiological electrical activity.

Key Takeaways

• Mobility + charge are both mandatory for electrical conduction in solutions.
• Solids with fixed ions do not conduct; dissolution that yields ions does.
• Classify substances in water as strong electrolytes, weak electrolytes, or non-electrolytes based on the degree of ion production and resultant conductivity.