50 Important Questions For Chemistry Practical | Viva Questions | Class 12 CBSE 2025 |Sourabh Raina

50 Important Viva Questions for Class 12 Practical

Salt Analysis and Qualitative Analysis

• Qualitative Analysis: This method determines the positive and negative ions present in a compound, helping to identify its constitution.

• Definition of Radical: A radical is defined as an atom or group of atoms that carries a charge and behaves as a single unit in chemical reactions.

• Aqua Regia: A mixture of concentrated HCl and HNO3 in a 3:1 ratio, used to dissolve noble metals like gold and platinum.

Acidic and Basic Radicals

• Acidic vs. Basic Radicals: Salts consist of positively charged (basic radicals derived from bases) and negatively charged (acidic radicals derived from acids) parts.

• Examples of Radicals: Sodium ion (Na⁺) is an example of a basic radical, while sulfate ion (SO₄²⁻) is an acidic radical derived from sulfuric acid.

Chemical Reactions and Tests

• Carbonate Ion Reaction: Heating carbonate ions (CO₃²⁻), such as in magnesium carbonate (MgCO₃), produces CO₂ gas.

• 24-DNP Test: The full form is 2,4-Dinitrophenylhydrazine; it detects aldehydes and ketones by forming bright yellow/orange/red solids upon reaction.

Color Indicators in Chemistry

• Color of Iron Salts: Ferrous salts are light green due to Fe²⁺ ions, while ferric salts appear brown due to Fe³⁺ ions.

• Nessler's Reagent: K₂HgI₄ used for detecting ammonia; forms brown precipitate if ammonia or ammonium ions are present.

Detection Methods

• Tollens' Reagent: Ammoniacal silver nitrate solution used for detecting aldehydes by producing a silver mirror effect.

• Sodium Nitroprusside Formula: Na₂[Fe(CN)₅(NO)] used for detecting sulfur/sulfide ions, resulting in purple/violet coloration when present.

Flame Tests

• Flame Test Chemistry: Valence electrons get excited by flame heat; they emit visible radiation when returning to ground state, showing different colors based on the element tested.

• Use of Concentrated Acid for Paste Preparation: Concentrated acid helps prepare salt paste effectively for flame tests instead of using glass rods which can impart unwanted colors due to sodium silicate content.

Gas Detection with Lime Water

• Lime Water Test for CO₂ Detection: Lime water turns milky when CO₂ is bubbled through it due to calcium carbonate formation from the reaction with carbon dioxide.

• Other Gases Affecting Lime Water: SO₂ also turns lime water milky by forming calcium sulfite precipitate alongside calcium carbonate.

What is the Color of Nickel and Manganese Salts?

Color Characteristics of Salts

• Nickel salts are described as bright green, while manganese salts exhibit a light pink color.

Importance of Preliminary Tests in Qualitative Analysis

• Preliminary tests help simplify the analysis process by providing essential information about the salt's characteristics, such as color and flame test results.

• These tests do not confirm specific ions but offer crucial clues regarding the presence of certain cations and anions.

Why Does Concentrated HNO3 Turn Yellow?

Chemical Reactions Involved

• Concentrated nitric acid (HNO3) turns yellow due to the production of nitrogen dioxide (NO2), which occurs when nitric acid thermally decomposes over time.

• The dissolution of NO2 in solution contributes to the yellow coloration observed in concentrated HNO3.

How Do Ionic Salts Ionize When Dissolved?

Mechanism of Ionization

• Ionic salts ionize into positive and negative ions when dissolved in water due to their polar nature, allowing them to separate effectively.

• Water molecules attract these ions because of their polarity, leading to dissociation from solid structures into individual charged particles.

Distinguishing Between CO2 and SO2

Testing Methods

• To differentiate between carbon dioxide (CO2) and sulfur dioxide (SO2), passing both gases through acidified potassium dichromate solution will convert SO2 into a green color while CO2 shows no effect.

Why is Dilute H2SO4 Preferred for Acid Radical Testing?

Comparison with Other Acids

• Dilute sulfuric acid (H2SO4) provides more accurate results compared to dilute hydrochloric acid (HClO2), making it preferable for testing acid radicals.

Tests for Nitrate Detection

Ring Test Methodology

• The ring test for nitrates involves mixing a salt solution with freshly prepared reagents that form a dark brown ring at the junction where two solutions meet, indicating nitrate presence.

Flame Test Observations

Elemental Behavior in Flame Tests

• Sodium imparts a yellow color to flames during testing, whereas magnesium does not produce any visible color change due to its emission spectrum falling outside the visible range.

Differentiating Primary and Secondary Amines

Benzene Sulfonyl Chloride Usage

• Benzene sulfonyl chloride is utilized to distinguish between primary and secondary amines based on their reactivity patterns during chemical tests.

Detection of Phenol Group

Ferric Chloride Test

• The detection of phenolic compounds can be achieved using ferric chloride, resulting in violet or blue coloration upon reaction with neutral ferric chloride solution.

Understanding Titration Techniques

Definition and Process Overview

• Titration is defined as a technique used to determine the concentration of an unknown solution by adding it from a burette until reaching an endpoint indicated by a color change from an indicator added to the analyte solution.

Endpoint Significance in Titration

Defining Endpoint

• The endpoint in titration refers to the stage at which the reaction has just completed, marked by a distinct change facilitated by an indicator.

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What is an Indicator and Its Role in Reactions?

Understanding Indicators

• An indicator changes color to show the completion of a reaction, with examples including methyl orange and phenol.

Standard Solutions Explained

• A standard solution contains a precise concentration of an element or substance, allowing for accurate measurements in experiments.

Normal Solutions Defined

• A normal solution contains 1 gram equivalent mass of solute per liter. The equivalent weight is calculated based on the molecular weight divided by the n-factor, which varies depending on whether it’s an acid or base.

Key Concepts in Acid-Base Chemistry

Basisity and Equivalent Weight

• For H₂SO₄, its basisity is 2 as it dissociates into 2 H⁺ ions. The equivalent weight is calculated as half its molar mass (98 g/mol), resulting in 49 g.

Reduction Half-Reaction for KMnO₄

• In acidic medium, KMnO₄ acts as an oxidizing agent and reduces to Mn²⁺. Balancing involves adding water and hydrogen ions to maintain charge balance.

Titration Techniques and End Points

Self-Indicating Nature of KMnO₄

• KMnO₄ does not require an external indicator during titration because it changes color itself at the endpoint from colorless to permanent light pink.

Understanding Molar Solutions

Definition of Molar Solution

• A molar solution consists of one mole of solute dissolved in one liter of solvent.

Double Salts and Their Formulas

Formula for Mohr Salt

• Mohr salt's formula is FeSO₄·(NH₄)₂SO₄·6H₂O, indicating it has six water molecules associated with its crystal structure.

Rinsing Procedures in Titration

Rinsing Titration Flasks

• Titration flasks should not be rinsed with other liquids as residues can affect concentration; instead, they should be rinsed with the same solution used for titration.

Concentration Measurements: Molarity vs. Molality

Differences Between Molarity and Molality

• Molarity measures moles of solute per liter of solution (temperature-dependent), while molality measures moles per kilogram of solvent (temperature-independent).

Normality: A Key Concept in Solutions

Defining Normality

• Normality refers to the number of gram equivalents of solute present in one liter of solution, crucial for understanding reactive capacities in solutions.

Primary vs. Secondary Standards

Characteristics of Primary Standards

• Primary standards are pure substances that can create standard solutions directly through dissolution without significant error due to moisture absorption or impurities.

Secondary Standards Explained

• Secondary standards cannot be prepared directly due to their hygroscopic nature; they require careful handling to ensure accuracy when creating standard solutions.

Oxidation and Color Properties of KMnO4

Oxidation Process of Ferrous Sulfate

• The oxidation by air in ferrous sulfate (FeSO4) involves the conversion of Fe²⁺ ions to ferric ions (Fe³⁺), resulting in the formation of ferric sulfate.

Color Characteristics of KMnO4 Solution

• The color of potassium permanganate (KMnO4) solution is purple, which is a significant characteristic used in various chemical applications.

• The purple color arises from the presence of manganese in its +7 oxidation state (MnO₄⁻), which absorbs specific wavelengths of light.

Chemical Reactions Involving KMnO4

• When discussing KMnO4, if it were to be reduced to Mn²⁺, it would involve a change in oxidation state, indicating a potential reaction pathway.