Given below are two statements labelled as Assertion (A) and Reason (R). Select the most appropriate answer fr

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Q. Given below are two statements labelled as Assertion (A) and Reason (R). Select the most appropriate answer from the options given below : Assertion (A) : Conductivity decreases with decrease in concentration of electrolyte. Reason (R) : Number of ions per unit volume that carry the current in a solution decreases on dilution. (A) Both (A) and (R) are true and (R) is the correct explanation of (A). (B) Both (A) and (R) are true, but (R) is not the correct explanation of (A). (C) (A) is true, but (R) is false. (D) (A) is false, but (R) is true.

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Concept understanding — Conductivity of Electrolytes

Conductivity of Electrolytes – From Intuition to Precision

Think of a copper wire. You know it conducts electricity because electrons flow through it. Now imagine dipping two metal plates into a beaker of salt water and connecting them to a battery. The bulb glows. The salt water is conducting — but not with electrons. Something else is carrying the charge.

That something is ions. When an electrolyte (like NaCl, HCl, or NaOH) dissolves in water, it splits into positive and negative ions. These ions are free to move. When you apply a voltage, positive ions drift toward the negative electrode, negative ions toward the positive electrode. That directed motion of charged particles is an electric current. That is the core idea: electrolytic conductivity is the ability of a solution to carry current via the movement of its ions.

Note

The more ions present, and the faster they can move, the higher the conductivity. But it is not that simple — concentration changes both the number of ions and how they interact with each other.

The Precise Definition

Conductivity (symbol $\kappa$ , units S m $^{-1}$ or S cm $^{-1}$ ) is the reciprocal of resistivity. For a solution placed between two parallel electrodes of area $A$ and separation $l$ , the resistance $R$ is:

R = \frac{1}{\kappa} \cdot \frac{l}{A}

So $\kappa$ is the conductance of a 1 m × 1 m × 1 m cube of the solution. It depends on:

Number of ions per unit volume (concentration)
Charge on each ion
How fast the ions move (their mobility)

But here is the catch: as you dilute a solution, $\kappa$ does not simply drop proportionally. Why? Because dilution changes both the number of ions and the degree of dissociation (for weak electrolytes). To compare the conducting power of different electrolytes fairly, we need a quantity that normalises for concentration.

Molar Conductivity – The Fair Comparison

Molar conductivity ( $\Lambda_m$ , units S m $^2$ mol $^{-1}$ ) is defined as:

\Lambda_m = \frac{\kappa}{c}

where $c$ is the concentration in mol m $^{-3}$ . It tells you: if I had exactly one mole of electrolyte dissolved in a solution, what would the conductivity of that entire solution be?

\Lambda_m = \frac{\kappa}{c}

For a strong electrolyte like KCl, $\Lambda_m$ decreases slowly as concentration increases. Why? Because at higher concentrations, ions are closer together — they feel each other's electric fields, slow each other down (ion-ion interactions). At infinite dilution ( $c \to 0$ ), ions are so far apart they move independently, and $\Lambda_m$ reaches a maximum value called $\Lambda_m^\infty$ (limiting molar conductivity).

For a weak electrolyte like acetic acid, $\Lambda_m$ rises sharply on dilution. That is because dilution increases the degree of dissociation — more ions are formed from the same number of molecules. At infinite dilution, all molecules are dissociated, and $\Lambda_m^\infty$ is the sum of the individual ion contributions (Kohlrausch's law). …

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Conductivity depends on the number of charge carriers per unit volume; dilution reduces ion concentration, so conductivity falls. Both statements are true, and the reason correctly explains the assertion.

Understanding Conductivity vs. Molar Conductivity

The key to this question lies in distinguishing between conductivity ( $\kappa$ ) and molar conductivity ( $\Lambda_m$ ). These are related but behave oppositely with dilution, and confusion between them is the most common trap.

Conductivity measures how well a solution conducts electricity per unit volume. It depends on two factors:

\kappa = \sum_i n_i z_i e \mu_i

where $n_i$ is the number density (ions per unit volume), $z_i$ is the charge, and $\mu_i$ is the mobility. In simpler terms, conductivity is proportional to the total number of ions available in a given volume to carry current.

When you dilute an electrolyte solution, you're adding more solvent while keeping the total number of ions roughly constant (ignoring dissociation changes for strong electrolytes). The ions spread out over a larger volume, so the number of ions per unit volume decreases.

Step-by-Step Analysis

Examine the Assertion (A): "Conductivity decreases with decrease in concentration of electrolyte."

As concentration falls (dilution increases), fewer ions occupy each unit volume of solution. Since conductivity is directly proportional to ion density, $\kappa$ must decrease. This statement is true.
Examine the Reason (R): "Number of ions per unit volume that carry the current in a solution decreases on dilution."

This is a direct statement of fact. Dilution means spreading the same number of ions over more volume, reducing $n_i$ (the number density). This statement is also true.
Check if R explains A: Does the decrease in ion density (R) cause the decrease in conductivity (A)?

Yes, absolutely. Conductivity is fundamentally determined by how many charge carriers are present per unit volume. When dilution reduces this number, conductivity must fall. The reason is the direct physical cause of the assertion. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 15 on this concept.

CBSE 2026Set ANNUAL1 mark
$Q. Conductivity of electrolytic solutions ______ with increase of temperature (fill in the blank).$
›Reveal solutionSolution

Raising the temperature increases ionic mobility (lower viscosity, faster ion movement) and, for weak electrolytes, increases the degree of dissociation, so conductivity goes up.

…

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CBSE 2026Set ANNUAL1 mark
$Q. Write True or False: Conductivity of a solution increases with dilution.$
›Reveal solutionSolution

False - conductivity decreases on dilution (molar conductivity increases).

Conductivity (specific conductance, kappa) is the conductance of ions present in 1 cm3 of solution. On dilution the total number of ions per unit volume decreases, so kappa decreases.

…

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CBSE 2025Set ANNUAL1 mark
$Q. Why does the conductivity of a solution decrease with dilution?$
›Reveal solutionSolution

Conductivity (κ) is conductance per unit volume; diluting spreads the same ions over more volume, so ion density falls.

Conductivity ( $\kappa$ ) is a measure of the conducting power per unit volume of the solution, i.e. it depends on the number of ions present per unit volume.

On dilution, the total volume of the solution increases while the total number of ions (for a fixed amount of electrolyte) essentially stays the same (or increases only slightly for weak electrolytes due to greater dissociation) — so the concentration of ions per unit volume decreases. Since $\kappa$ is directly proportional to ion concentration, $\kappa$ decreases with d …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. A weak electrolyte:$
(a) Does not dissociate into ions.
(b) Dissociate completely into ions.
(c) Dissociate into ions but not completely.
(d) None of these.
›Reveal solutionSolution
A weak electrolyte partially ionizes in solution (degree of dissociation $\alpha \ll 1$ ), unlike a strong electrolyte which ionizes almost completely.

Electrolytes are substances whose aqueous solutions conduct electricity because they furnish ions. They are classified by how completely they ionize:

Strong electrolytes (e.g. NaCl, HCl, KOH) dissociate almost completely (~100%) into ions in solution. …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. Which of the following statements is true regarding the conductivity of electrolytic solutions ?$
(a) It is independent of the size of the ions
(b) It is independent of the viscosity of the solution
(c) It decreases with temperature
(d) It depends on the solvation of ions present in the solution
›Reveal solutionSolution

Ionic conductivity depends on ion size (and its solvated/hydrated size), charge, and how easily ions move through the solvent — all linked to solvation.

Conductivity of an electrolytic solution is governed by the mobility of the ions, which in turn depends on: the size of the bare ion, the extent of solvation (a heavily solvated ion behaves as a larger, slower-moving species), the viscosity of the solvent (higher viscosity hinders ion movement), and temperature (conductivity of electrolytic solutions INCREASES with temperature, because increased thermal motion decreases the extent of ion solvation and sol …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Which of the following will have the highest conductivity at 298 K ?$
(i) 0.01 M HCl solution
(ii) 0.1 M HCl solution
(iii) 0.01 M CH₃COOH solution
(iv) 0.1 M CH₃COOH solution
›Reveal solutionSolution
Conductivity (unlike molar conductivity) increases with both the concentration of ions AND their degree of dissociation; a more concentrated strong electrolyte therefore conducts more than a dilute one or a weak electrolyte.

Conductivity ( $\kappa$ ) depends on the number of current-carrying ions per unit volume of solution — so, unlike molar conductivity, it generally increases with concentration for a given electrolyte, provided the electrolyte is fully (or mostly) dissociated.

HCl is a strong acid — fully dissociated into H⁺ and Cl⁻ at both 0.01 M and 0.1 M.

CH₃COOH is a weak acid — only partially dissociated, so it has far fewer ions per unit volume than HCl at the same concentration. …
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CBSE 2024Set ANNUAL1 markMCQ
$Q. Conductivity of an Electrolytic solution depends on -$
(a) Nature of electrolyte
(b) Power of AC Source
(c) Distance between the electrodes
(d) Surface area of electrodes
›Reveal solutionSolution

Conductivity is an intrinsic property of the electrolyte itself, not of the cell's geometry.

Conductivity (specific conductance, $\kappa$ ) of an electrolytic solution depends on the nature of the electrolyte — i.e. its degree of dissociation/ionisation, the number, charge and mobility of the ions it produces, and the concentration of the solution. Options (ii)-(iv) (power of AC source, distance between electrodes, surface area of electrodes) instead af …

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CBSE 2024Set D1 markMCQ
$Q. Who gave the theory of ionisation?$
(a) Faraday
(b) Arrhenius
(c) Ostwald
(d) Rutherford
›Reveal solutionSolution

The theory of ionisation (electrolytic dissociation) was given by Arrhenius.

Svante Arrhenius (1887) proposed that electrolytes dissociate into positive and negative ions when dissolved in water, and that these free ions carry the electric current. Faraday gave the laws of electrolysis, Ostwald gave the dilution law (an application …

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CBSE 2023Set 56/2/11 markMCQ
$Q. Given below are two statements labelled as Assertion (A) and Reason (R). Select the most appropriate answer from the options given below : Assertion (A) : Conductivity decreases with decrease in concentration of electrolyte. Reason (R) : Number of ions per unit volume that carry the current in a solution decreases on dilution. (A) Both (A) and (R) are true and (R) is the correct explanation of (A). (B) Both (A) and (R) are true, but (R) is not the correct explanation of (A). (C) (A) is true, but (R) is false. (D) (A) is false, but (R) is true.$
›Reveal solutionSolution
Conductivity depends on the number of charge carriers per unit volume; dilution reduces ion concentration, so conductivity falls. Both statements are true, and the reason correctly explains the assertion.

Understanding Conductivity vs. Molar Conductivity

The key to this question lies in distinguishing between conductivity ( $\kappa$ ) and molar conductivity ( $\Lambda_m$ ). These are related but behave oppositely with dilution, and confusion between them is the most common trap.

Conductivity measures how well a solution conducts electricity per unit volume. It depends on two factors:
$\kappa = \sum_i n_i z_i e \mu_i$
where $n_i$ is the number density (ions per unit volume), $z_i$ is the charge, and $\mu_i$ is the mobility. In simpler terms, conductivity is proportional to the total number of ions available in a given volume to carry current.

When you dilute an electrolyte solution, you're adding more solvent while keeping the total number of ions roughly constant (ignoring dissociation changes for strong electrolytes). The ions spread out over a larger volume, so the number of ions per unit volume decreases.

Step-by-Step Analysis

Examine the Assertion (A): "Conductivity decreases with decrease in concentration of electrolyte."

As concentration falls (dilution increases), fewer ions occupy each unit volume of solution. Since conductivity is directly proportional to ion density, $\kappa$ must decrease. This statement is true.

Examine the Reason (R): "Number of ions per unit volume that carry the current in a solution decreases on dilution."

This is a direct statement of fact. Dilution means spreading the same number of ions over more volume, reducing $n_i$ (the number density). This statement is also true.

Check if R explains A: Does the decrease in ion density (R) cause the decrease in conductivity (A)?

Yes, absolutely. Conductivity is fundamentally determined by how many charge carriers are present per unit volume. When dilution reduces this number, conductivity must fall. The reason is the direct physical cause of the assertion. …
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CBSE 2023Set F1 markMCQ
$Q. Which of the following conducts electricity in a solution?$
(a) Electrolytes
(b) Non-electrolytes
(c) H2O molecules
(d) Copper wire
›Reveal solutionSolution

Electrolytes ionise in solution and conduct via the movement of ions.

A substance conducts electricity in solution if it provides mobile charge carriers (ions). Electrolytes (e.g. acids, bases, salts) dissociate into positive and negative ions in solution, which migrate to the electrodes and carry the current, so the solution conducts.

…

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CBSE 2021Set TERM11 markMCQ
$Q. A weak Electrolyte :$
(a) Does not dissociate into ions
(b) Dissociate into ions incompletely
(c) Dissociate into ions completely
(d) None of these
›Reveal solutionSolution

Weak electrolytes only partially ionise in solution, unlike strong electrolytes which ionise completely.

Electrolytes are classified by how completely they dissociate into ions in solution. Strong electrolytes (e.g. NaCl, HCl, NaOH) dissociate almost completely. Weak electrolytes (e.g. CH3COOH, NH4OH) dissociate only partially/incompletely, so an equilibrium exists between the undissociated molecule …

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CBSE 2021Set ANNUAL1 markMCQ
$Q. Electrolytic conduction -$
(a) is carried by movement of ions
(b) is carried by movement of electrons
(c) decreases with increase in temperature
(d) involves no change in chemical properties of the conductor
›Reveal solutionSolution

In electrolytic (ionic) conduction, current is carried by the physical migration of cations and anions through the solution/melt, unlike metallic conduction which is carried by electrons.

Electrolytic conductance occurs when an electrolyte dissociates into ions in solution (or in the molten state); on applying a potential difference, cations migrate towards the cathode and anions towards the anode, and this movement of ions constitutes the electric current. This is fundamentally different from metallic (electronic) conduction, where free electrons move through the metal lattice. Electrolytic conductance generally **incre …

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Understanding Conductivity vs. Molar Conductivity

Step-by-Step Analysis

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