Conductivity of an electrolytic solution depends on ______. (Two or more than two options may be correct.) (i)

NCERT Exemplar · Q46

Q. Conductivity of an electrolytic solution depends on ____________. (Two or more than two options may be correct.)

(i) nature of electrolyte.

(ii) concentration of electrolyte.

(iii) power of AC source.

(iv) distance between the electrodes.

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Concept understanding — Molar Conductivity

From Resistance to Conductance: Flipping the Idea

You already know resistance ( $R$ ) — it tells you how much a material opposes the flow of current. A high resistance means the wire fights the current; a low resistance means it lets current through easily.

Now flip that thought. Instead of asking "how much does it resist?", ask "how easily does it let current flow?" That's exactly what conductance measures.

Note

Conductance ( $G$ ) is the reciprocal of resistance:

$G = \frac{1}{R}$

Unit: siemens (S) — named after Werner von Siemens. 1 S = 1 A/V (ampere per volt).

If a wire has $R = 10\ \Omega$ , its conductance is $G = 0.1\ \text{S}$ . If $R = 0.5\ \Omega$ , $G = 2\ \text{S}$ — it conducts twice as well.

Ohm's Law in Conductance Form

You know $V = IR$ . Rearranging:

I = \frac{V}{R} = G V

So current = conductance × voltage. A high-conductance material draws a large current for the same voltage — it's a "good conductor."

Now, Conductivity: The Material's Intrinsic Property

Resistance depends on two things: the material itself (its "resistivity" $\rho$ ) and the geometry (length $L$ , cross-sectional area $A$ ):

R = \rho \frac{L}{A}

Conductance also depends on geometry. A thicker wire (larger $A$ ) or a shorter wire (smaller $L$ ) has higher conductance. To isolate the material's inherent ability to conduct, we define conductivity ( $\sigma$ ):

\sigma = \frac{1}{\rho}

Conductivity is the reciprocal of resistivity. It tells you how well the material itself conducts, independent of shape and size.

Unit: siemens per metre (S/m).
High $\sigma$ → good conductor (copper: $\approx 5.8 \times 10^7\ \text{S/m}$ ).
Low $\sigma$ → poor conductor / insulator (glass: $\approx 10^{-12}\ \text{S/m}$ ).

Watch out

Don't confuse conductance (property of a specific object, depends on geometry) with conductivity (property of the material, independent of geometry). A short thick copper wire has high conductance; a long thin copper wire has lower conductance — but both have the same conductivity.

The Big Picture in One Table

Quantity	Symbol	Definition	Depends on	Unit
Resistance	$R$	$V/I$	Material + geometry	$\Omega$
Resistivity	$\rho$	$R A / L$	Material only	$\Omega \cdot \text{m}$
Conductance	$G$	$1/R$	Material + geometry	S
Conductivity	$\sigma$	$1/\rho$	Material only	S/m

Intuitive Analogy

Think of a water pipe:

Resistance = how hard it is to push water through (narrow, long pipe).
Conductance = how easily water flows (wide, short pipe). …

Why this formula?

Conductance and Conductivity: Why the Formulas Hold

Let's build this from first principles — understanding the why before the what.

1. The Core Idea: How Easily Does Current Flow?

Think of a conductor (like a copper wire). When you apply a voltage across it, electrons drift through the material. Two questions arise:

How much current flows for a given voltage? → This is conductance ( $G$ ).
How well does the material itself allow current? → This is conductivity ( $\sigma$ ).

The key distinction: Conductance depends on the size and shape of the object. Conductivity is an intrinsic property of the material.

2. Ohm's Law in Terms of Conductance

You know Ohm's law:

V = IR

But we can rewrite it as:

I = \frac{V}{R}

Define conductance $G$ as the reciprocal of resistance:

G = \frac{1}{R}

So:

I = G V

Why this makes sense:

A larger $G$ means more current for the same voltage — the conductor "conducts" better.
$G$ has units of siemens (S) = $\text{A/V}$ .

3. From Resistance to Conductivity: The Geometry Factor

Resistance of a uniform conductor depends on:

Length $L$ (longer → more resistance)
Cross-sectional area $A$ (thicker → less resistance)
Material property $\rho$ (resistivity)

The formula:

R = \rho \frac{L}{A}

Now, conductivity $\sigma$ is the reciprocal of resistivity:

\sigma = \frac{1}{\rho}

So:

R = \frac{1}{\sigma} \cdot \frac{L}{A}

Why this form?

If you double the length, electrons have to travel twice as far, colliding more → resistance doubles.
If you double the area, there's twice as many "lanes" for electrons → resistance halves.

4. The Key Formula: Conductance in Terms of Conductivity

Since $G = 1/R$ , we get:

G = \sigma \frac{A}{L}

This is the central relationship. Let's see why it holds:

$\sigma$ tells you how well the material conducts (intrinsic).
$A/L$ tells you how the geometry amplifies or reduces that.

Intuition:

A fat, short wire ( $A$ large, $L$ small) has high conductance.
A thin, long wire ( $A$ small, $L$ large) has low conductance.
A material with high $\sigma$ (like copper) gives higher $G$ than one with low $\sigma$ (like iron), for the same shape.

5. Microscopic Derivation (Why $\sigma$ Exists)

At the microscopic level, conductivity arises from electron motion:

\sigma = n e \mu

Where:

$n$ = number of free electrons per unit volume
$e$ = electron charge …

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Conductivity depends on the nature and concentration of the electrolyte, but not on the power of the AC source or the distance between electrodes — the correct options are (i) and (ii).

Why this question is about the intrinsic property

Conductivity ( $\kappa$ ) of an electrolytic solution is a measure of how easily ions carry charge through the solution. It is an intrinsic property — like density or refractive index — meaning it depends only on the solution itself, not on the geometry of the measuring cell or the electrical source used.

The key distinction: conductivity is not the same as conductance ( $G$ ). Conductance depends on cell geometry (electrode area and separation), while conductivity is conductance normalized for that geometry. So anything that changes the solution’s ability to conduct — the number of ions, their charge, their mobility — affects conductivity. Anything external to the solution does not.

Let’s examine each option.

Nature of electrolyte (i)

Different electrolytes produce different ions. For example, $\ce{HCl}$ gives $\ce{H+}$ and $\ce{Cl-}$ ; $\ce{CH3COOH}$ gives far fewer ions because it is weak. Ions also differ in size and charge — $\ce{Mg^{2+}}$ carries twice the charge of $\ce{Na+}$ but moves more slowly. These factors directly determine how much current a solution can carry at a given voltage.

Conclusion: Nature of electrolyte does affect conductivity.
Concentration of electrolyte (ii)

More ions per unit volume means more charge carriers. For strong electrolytes, conductivity increases nearly linearly with concentration (until ion-pairing at very high concentrations). For weak electrolytes, conductivity increases more slowly because dissociation fraction changes. Either way, concentration is a primary factor.

Conclusion: Concentration does affect conductivity.
Power of AC source (iii)

Conductivity is a property of the solution, not of the measuring instrument. The AC source provides the alternating voltage used to measure conductivity (to avoid electrolysis), but its power rating — whether it can supply 1 W or 100 W — does not change how well the solution itself conducts. The measured conductance may change if the voltage is so high that it heats the solution, but that is a secondary effect, not a dependence of conductivity itself. In standard electrochemistry, conductivity is defined at a given temperature, independent of source power.

Conclusion: Power of AC source does not affect conductivity.
Distance between the electrodes (iv) …

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Method: Factor Analysis of Molar Conductivity

This method identifies the factors that influence conductivity (κ) of an electrolytic solution by recalling the definition and the experimental setup.

Step 1: Recall the definition of conductivity

Conductivity (κ) is the conductance of a solution placed between two electrodes 1 cm apart and 1 cm² in area.

It depends on:

Number of ions per unit volume → affected by concentration and nature of electrolyte.
Charge and mobility of ions → determined by the nature of electrolyte.

Step 2: Identify factors from the options

Option	Factor	Does it affect conductivity?	Reason
(i)	Nature of electrolyte	✓ Yes	Different ions have different charges, sizes, and mobilities.
(ii)	Concentration of electrolyte	✓ Yes	More ions per unit volume increase conductivity (up to a limit).
(iii)	Power of AC source	✗ No	Conductivity is a material property — independent of source power.

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Here are the common mistakes students make on this question about Molar Conductivity, along with clear explanations of how to avoid each.

Mistake 1: Confusing Conductivity with Resistance

The Error: Students often think conductivity depends on the distance between electrodes (Option D) because they remember the formula for resistance:

R = \rho \frac{l}{A}

They see $l$ (distance) and assume conductivity ( $\kappa = 1/\rho$ ) must also depend on it.

Why it’s wrong: Conductivity ( $\kappa$ ) is an intrinsic property of the solution — it depends only on the nature of ions and their concentration. The distance between electrodes affects the measured resistance (an extrinsic property), not the conductivity itself. The instrument (conductivity cell) already accounts for the cell constant ( $G^* = l/A$ ) to extract $\kappa$ from the measured conductance.

How to avoid: Remember the hierarchy:

Conductance ( $G$ ) = $1/R$ → depends on geometry ( $l$ , $A$ ).
Conductivity ( $\kappa$ ) = $G \times (l/A)$ → independent of geometry.

✓ Correct: Conductivity depends on nature (i) and concentration (ii) of electrolyte.

✗ Incorrect: It does not depend on distance between electrodes (iv).

Mistake 2: Thinking the AC Source Power Matters

The Error: Students pick Option (iii) — “power of AC source” — because they know conductivity measurements use AC to avoid electrolysis. They mistakenly think a higher-power AC source changes the reading.

Why it’s wrong: The power of the AC source does not affect the intrinsic conductivity of the solution. Conductivity is a material property; it is independent of the voltage or current used to measure it (within the linear Ohm’s law regime). The AC source is only used to prevent polarization and electrolysis — its power rating is irrelevant to the value of $\kappa$ .

How to avoid: Distinguish between:

Measurement technique (AC vs DC) — this avoids side reactions.
Measured property ( $\kappa$ ) — this is fixed for a given solution at a given temperature and concentration.

✓ Correct: Power of AC source does not affect conductivity.

✗ Incorrect: Picking (iii) shows confusion between measurement method and measured property.

Mistake 3: Forgetting that “Nature of Electrolyte” Includes Ion Size and Charge

The Error: Students correctly select “nature of electrolyte” (i) but then fail to explain why — they think it’s just about strong vs weak electrolytes.

Why it matters: The “nature” includes:

Ionic size (smaller ions move faster → higher conductivity)
Charge on ions (higher charge → more conductance per mole)
Degree of dissociation (strong vs weak)
Solvation effects (hydration shell size affects mobility)

How to avoid: When you see “nature of electrolyte,” think of all factors that affect ionic mobility and degree of ionization — not just strength.

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Q.Conductivity of an electrolytic solution depends on ____________. (Two or more than two options may be correct.)

Why this question is about the intrinsic property

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