Question

The following aqueous solutions were tested with a light bulb conductivity apparatus, as shown in Figure 4.1. What resultdark, dim, or bright - do you expect from each? (a) \(0.10 \mathrm{M}\) potassium chloride (b) \(0.10 \mathrm{M}\) methanol (c) \(0.10 \mathrm{M}\) acetic acid

Short answer

(a) Bright, (b) Dark, (c) Dim.

Step-by-step solution

Understand Conductivity Apparatus

A conductivity apparatus tests the ability of a solution to conduct electricity. If the solution conducts electricity well, the light bulb lights up brightly. If the solution is a poor conductor, the bulb will light dimly or not at all.

Analyze Potassium Chloride (KCl) Solution

Potassium chloride (KCl) is a strong electrolyte. When dissolved in water, it dissociates completely into potassium (K⁺) and chloride (Cl⁻) ions, which carry electrical current efficiently. Therefore, we expect the bulb to light brightly.

Analyze Methanol (CH₃OH) Solution

Methanol is a non-electrolyte when dissolved in water. It doesn't dissociate into ions, and so there are no charged particles to conduct electricity. Consequently, the bulb will remain dark.

Analyze Acetic Acid (CH₃COOH) Solution

Acetic acid is a weak electrolyte. It partially dissociates into ions in solution. This partial dissociation produces fewer ions compared to a strong electrolyte, resulting in a dim light on the bulb.

Key concepts

Strong Electrolyte

A strong electrolyte is a substance that completely dissociates into ions when dissolved in water. This means as soon as it enters a solution, it breaks apart to form free-floating positive and negative ions. These ions are the key players in conducting electricity. They act like tiny messengers, shuttling electric charges from one point to another.
For example, potassium chloride (KCl) is a strong electrolyte. When KCl is added to water, it quickly divides into potassium ions \(K^+\) and chloride ions \(Cl^-\).
This complete separation allows the solution to conduct electricity very effectively.

• The light bulb in a conductivity apparatus will light up brightly with strong electrolytes.

This is because the flow of numerous free ions facilitates a strong passage of electrical current.

Weak Electrolyte

Weak electrolytes only partially dissociate into ions in solution. When these substances dissolve, only a fraction of their molecules split into ions.
As a result, the number of ions present is significantly less compared to that in strong electrolytes. This means that weak electrolytes won't conduct electricity as efficiently.
Acetic acid (CH₃COOH) is a good example of a weak electrolyte. In water, some acetic acid molecules dissociate to form hydrogen ions (H⁺) and acetate ions (CH₃COO⁻), but many remain intact.

• This partial dissociation results in a dimmer light bulb when using a conductivity apparatus.

The fewer ions available, the less effective the electrical current is, leading to a weaker light.

Non-Electrolyte

Non-electrolytes are substances that do not generate ions when dissolved in water. When such substances enter a solution, they stay as whole molecules without breaking apart into ions.
Because there are no charged particles formed, these solutions do not conduct electricity at all.
Methanol (CH₃OH) is an example of a non-electrolyte. Despite being soluble in water, it doesn't dissociate into ions and so can't conduct an electrical current.

• This results in a dark light bulb when a conductivity apparatus tests such a solution.

Non-electrolytes are essentially invisible to electrical currents, meaning they won't provide a path for electricity to travel.

Conductivity Apparatus

A conductivity apparatus is a useful tool for testing how well solutions conduct electricity. It typically consists of a light bulb connected via wires to two electrodes that are dipped into the solution being tested.
How the light bulb behaves gives us a visual indicator of the solution's electrical conductivity:

• If the solution is a strong electrolyte, many ions move freely, and the bulb lights up brightly.
• If it's a weak electrolyte, fewer ions are present, and the bulb glows dimly.
• If it's a non-electrolyte, no ions are present to conduct electricity, leaving the bulb dark.

This setup helps students and scientists alike quickly assess the conductive properties of different solutions.