CUET UG Chemistry Booster Test – 2 Conductance of Electrolytic Solutions

Electrical resistance is the opposition offered by a conductor to the flow of electric current. According to Ohm's law, resistance is defined as the ratio of potential difference to current, R = V/I. The SI unit of resistance is ohm (Ω). To express ohm in SI base units, we first express volt in base units. One volt is equal to one joule per coulomb. Since joule is kg m² s⁻² and coulomb is A·s, volt becomes kg m² s⁻³ A⁻¹. Dividing volt by ampere gives the SI base unit of resistance as kg m² s⁻³ A⁻². NCERT uses these derived units while discussing conductance, conductivity and resistivity. Understanding SI base units helps in dimensional analysis and verification of formulae in electrochemistry and electrical measurements. Therefore, the correct SI base unit representation of electrical resistance is kg m² s⁻³ A⁻².

Conductance is the ability of a substance to allow the flow of electric current. According to NCERT, the resistance of a conductor or solution column is directly proportional to its length and inversely proportional to its cross-sectional area. Therefore, R = ρl/A Taking the reciprocal, G = 1/R = A/(ρl) Since conductivity (κ) is the reciprocal of resistivity (ρ), κ = 1/ρ Substituting in the conductance equation, G = κA/l This relationship shows that conductance increases with increase in conductivity and cross-sectional area, while it decreases with increase in length of the solution column. In electrochemistry, this equation is important for determining conductivity of electrolytic solutions using conductivity cells. NCERT explains that conductivity is a characteristic property of a material, whereas conductance depends on the dimensions of the conductor or solution column. Thus, G = κA/l is the correct expression.

Resistivity is a characteristic property of a material that indicates how strongly it opposes the flow of electric current. According to NCERT, resistivity is defined as the resistance offered by a conductor having unit length and unit cross-sectional area. The mathematical relation is: ρ = RA/l If l = 1 m and A = 1 m², ρ = R × 1/1 ρ = R Thus, the numerical value of resistance becomes equal to the resistivity of the material. Resistivity depends only on the nature of the material and temperature, not on the dimensions of the conductor. Materials with high resistivity are poor conductors, whereas materials with low resistivity are good conductors. This concept is fundamental in electrochemistry and electrical conductivity measurements. Therefore, when the conductor has unit length and unit cross-sectional area, its resistance is numerically equal to its resistivity.

NCERT introduces several important electrical quantities in electrochemistry. Conductance is the reciprocal of resistance and is measured in siemens (S). Resistivity represents the resistance of a conductor having unit length and unit cross-sectional area and is expressed in Ω m. Conductivity is the reciprocal of resistivity and is measured in S m⁻¹. The cell constant is defined as l/A, where l is the distance between electrodes and A is the electrode area. Since length is measured in metres and area in square metres, the unit of cell constant becomes m⁻¹. Understanding these units is important because many numerical problems involve conversion among conductance, conductivity, resistivity and cell constant. Correct association of quantities with their units is frequently tested in board examinations and CUET-type examinations. Hence the correct matching is 1-c, 2-a, 3-d and 4-b.

Conductivity is a measure of the ability of a material or solution to conduct electricity. According to NCERT, conductivity is represented by the symbol κ and is mathematically equal to the reciprocal of resistivity: κ = 1/ρ The SI unit of conductivity is siemens per metre (S m⁻¹). Conductivity depends on the concentration and nature of ions present in solution and is widely used in electrochemistry for studying electrolytes. NCERT further notes that the term "specific conductance" was used earlier, but IUPAC recommends the term "conductivity." Therefore, statements 1, 2 and 4 are correct. Conductivity provides valuable information regarding ion mobility and the conducting power of electrolytic solutions. It is one of the most important concepts used in determining molar conductivity and analysing electrolyte behaviour.

Conductivity is a measure of the ability of a substance to conduct electric current. According to NCERT, conductivity is generally expressed in S m⁻¹, although S cm⁻¹ is also commonly used. To compare the given values, they must first be converted into a common unit. 1. 1 S cm⁻¹ = 100 S m⁻¹ Therefore: 1. 1 S cm⁻¹ = 100 S m⁻¹ 2. 1 S m⁻¹ = 1 S m⁻¹ 3. 0.01 S cm⁻¹ = 1 S m⁻¹ 4. 0.001 S m⁻¹ = 0.001 S m⁻¹ After conversion, the highest value is 1. This is followed by 3 and 2, which become equal after conversion, and finally 4. Conductivity comparisons frequently appear in electrochemistry because different unit systems are used in laboratory measurements. Students should always convert values into a common unit before comparison. Hence the decreasing order is 1, 3, 2, 4.

Electronic conductance in metals occurs due to the movement of free electrons. NCERT explains that metallic conductors allow current to pass because electrons are mobile within the crystal lattice. The magnitude of electronic conductance depends on the structure of the metal, the number of available electrons and temperature. The viscosity of a solvent affects ionic conductance because ions move through a liquid medium. When viscosity increases, ionic mobility decreases. However, metallic conductors do not require a solvent for conduction. Current passes through them via free electrons. Therefore, viscosity has no role in determining the electronic conductance of metals. This distinction between metallic conduction and electrolytic conduction is important. Metals conduct electronically, whereas electrolytic solutions conduct through ions. Hence, the viscosity of the surrounding solvent is not a factor affecting electronic conductance through metals.

When direct current passes through an electrolytic solution, ions move toward oppositely charged electrodes. Positive ions migrate toward the cathode, while negative ions migrate toward the anode. At the electrodes, oxidation and reduction reactions take place. NCERT explains that these electrode processes consume reactants and generate products, leading to changes in the composition of the solution. This phenomenon is known as electrolysis. The conductivity of electrolytic solutions arises because ions carry charge through the solution. The composition change is not caused by polymerization, substitution or ordinary neutralization reactions. Instead, it results from electrochemical reactions occurring at the electrode surfaces. Such reactions are fundamental to electrochemistry and are responsible for many industrial applications such as electroplating and extraction of metals. Therefore, electrochemical reactions are responsible for the observed change in composition.

NCERT discusses the development of conducting polymers as an important advancement in materials chemistry. Unlike ordinary polymers, conducting polymers possess conjugated π-electron systems that allow electrons to move through the polymer chain. This gives them electrical conductivity. Important examples include polyaniline, polypyrrole and polythiophene. Research on such polymers led to significant developments in the field of organic electronics. These materials are used in batteries, sensors, antistatic coatings and electronic devices. Ordinary polymers such as polyethylene, polypropylene, PVC and Teflon are generally electrical insulators because they do not possess extended conjugated structures. Conducting polymers bridge the gap between metals and conventional plastics. Therefore, the group containing polyaniline, polypyrrole and polythiophene correctly represents conducting polymers.

Materials are classified as conductors, semiconductors and insulators depending on their electrical conductivity. According to NCERT, substances such as glass, Teflon, rubber and many plastics possess extremely low conductivity. They contain very few mobile charge carriers and therefore do not permit the easy flow of electric current. Because of their poor conducting ability, these materials are used as electrical insulators. Teflon is widely used in wire coatings, while glass is commonly used in electrical equipment requiring insulation. Their conductivity is far lower than that of semiconductors and conductors. These materials are neither superconductors nor conducting polymers. Their primary characteristic is resistance to electrical conduction. Therefore, they are classified as insulators having very low conductivity.

Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators. According to NCERT, materials such as silicon and gallium arsenide are important semiconductors widely used in electronic devices. Their conductivity can be controlled by temperature, doping and external electric fields, making them essential in transistors, integrated circuits and solar cells. Semiconductors occupy a unique position between highly conducting metals and poorly conducting insulators. Their technological importance has made them the foundation of modern electronics. Statement 1 is correct because semiconductors possess intermediate conductivity. Statement 2 is correct because silicon and gallium arsenide are standard examples. Statement 3 is also correct because these materials form the basis of electronic industries. Statement 4 is incorrect because infinite conductivity is associated with superconductors under special conditions and not with semiconductors. Therefore, only statements 1, 2 and 3 are correct.

Materials are classified according to their ability to conduct electricity. Conductors such as copper have very high conductivity because of the presence of free electrons. Insulators such as Teflon have extremely low conductivity because charge carriers are absent or very limited. Between these two extremes lie semiconductors. Doped silicon is one of the most widely used semiconducting materials. The addition of small amounts of impurities significantly modifies its electrical conductivity, making it suitable for electronic applications. According to NCERT, semiconductors possess conductivity intermediate between conductors and insulators. Their conductivity can be controlled and manipulated, which makes them extremely useful in modern electronics. Therefore, doped silicon is the material specifically identified as having conductivity between conductors and insulators.

The passage states that electrolytes furnish their own ions when dissolved in water. These ions become charge carriers responsible for conducting electricity through the solution. Conductivity in electrolytic solutions depends directly on the number and mobility of ions present. Pure water undergoes only a very small degree of self-ionization and therefore contains extremely low concentrations of H⁺ and OH⁻ ions. Consequently, its conductivity is very low. When an electrolyte such as sodium chloride or potassium chloride dissolves in water, it dissociates into ions, substantially increasing the number of charge carriers available. The increased ionic concentration enhances conductivity significantly. Therefore, electrolytic solutions conduct electricity much better than pure water. This principle forms the basis of electrolytic conductance discussed in NCERT electrochemistry.

According to NCERT, the effect of temperature on conductance depends on the type of conductor involved. In metallic conductors, increasing temperature increases lattice vibrations, which hinder the movement of electrons. As a result, electronic conductance decreases and resistance increases. In contrast, electrolytic conductance depends on the movement of ions through a solution. When temperature increases, the viscosity of the solvent decreases and ions move more freely. Increased ionic mobility results in greater conductance. Therefore, electronic conductance and ionic conductance show opposite trends with temperature. Electronic conductance decreases due to increased electron scattering, whereas ionic conductance increases because ions move more rapidly through the solution. This important distinction is emphasized in NCERT while discussing conductance mechanisms.

Ionic conductance depends on the movement of ions through a solution. According to NCERT, several factors influence ionic mobility, including the size of ions, their degree of solvation and the viscosity of the solvent. Smaller ions generally move more easily than larger solvated ions. Likewise, greater viscosity reduces ionic mobility. Statement 1 is correct because solvent viscosity affects ion movement. Statement 2 is correct because ionic size influences mobility. Statement 3 is incorrect because metallic conductors conduct through electrons rather than ions. Ionic movement is associated with electrolytic conductance, not metallic conductance. Statement 4 is not universally true because ionic movement occurs due to ions already present in solution; solvation affects mobility but is not an independent requirement for the existence of ionic movement. Therefore, only Statements 1 and 2 are correct.

In electrolytic solutions, ions are surrounded by solvent molecules, a phenomenon known as solvation. According to NCERT, the degree of solvation significantly affects ionic mobility. When an ion becomes highly solvated, many solvent molecules surround it and move together with it through the solution. As a result, the effective size of the ion increases. A larger solvated ion experiences greater resistance while moving through the solvent. Consequently, its mobility decreases. Since ionic conductance depends on the ease with which ions move, greater solvation generally lowers ionic mobility and may reduce conductivity. For example, small ions with extensive hydration often move more slowly than expected because they carry a large shell of water molecules. Thus, increased solvation leads to a larger effective ionic radius and lower mobility. Therefore, greater solvation increases the effective size of the ion and decreases its mobility.

Measurement of conductivity requires a specialized arrangement because direct current causes polarization effects at electrodes. NCERT describes the use of alternating current supplied by an oscillator. The oscillator acts as the AC source and prevents electrolysis during measurement. The Wheatstone bridge is used to determine the unknown resistance of the ionic solution accurately. The conductivity cell is a specially designed vessel containing the electrolyte whose resistance is to be measured. A detector such as a headphone or electronic sensing device is used to identify the balance condition in the bridge circuit. Thus, Wheatstone bridge corresponds to measurement of unknown resistance, oscillator corresponds to AC power source, conductivity cell corresponds to a vessel for ionic solutions and detector corresponds to the headphone or electronic device. Therefore, the correct matching is 1-c, 2-a, 3-d and 4-b.

When measuring the resistance of ionic solutions, direct current cannot be used because it causes electrolysis and electrode polarization. To overcome this problem, alternating current is employed. NCERT describes the use of an oscillator that supplies AC power in the audio-frequency range. The frequency range commonly used is 550 to 5000 cycles per second. This range minimizes polarization effects and allows accurate resistance measurements. The oscillator is connected to a Wheatstone bridge arrangement, enabling determination of the resistance of the conductivity cell. The use of AC is essential because ions continuously reverse their direction of movement, preventing significant chemical changes at the electrodes. Therefore, the specified audio-frequency range for the oscillator is 550 to 5000 cycles per second.

According to NCERT, the cell constant of a conductivity cell is determined by the geometry of the cell. It is defined as: Cell Constant = l/A where l is the distance between the electrodes and A is the area of cross-section of the electrodes. Since both quantities depend only on the construction of the conductivity cell, the cell constant remains fixed for a particular cell. The concentration of the electrolyte and temperature influence conductivity but do not determine the cell constant itself. Similarly, the frequency of the AC source is related to measurement technique and has no effect on the geometrical factor represented by the cell constant. Therefore, the cell constant depends solely on the distance between electrodes and their effective area of cross-section.

The cell constant of a conductivity cell cannot always be determined accurately from physical measurements of electrode distance and area. Therefore, NCERT recommends calibration using a standard solution whose conductivity is already known. Potassium chloride (KCl) solutions are commonly used because their conductivity values have been measured very accurately over a range of concentrations and temperatures. The resistance of the conductivity cell containing standard KCl solution is measured experimentally. Using the known conductivity and measured conductance, the cell constant can then be calculated. This procedure ensures reliable conductivity measurements for unknown solutions. Neither pure water nor weak electrolytes such as acetic acid are used as standard calibration solutions. Hence, potassium chloride solution is used for determining the cell constant of a conductivity cell.