Class 10 Science Chapter 2 Acids, Bases and Salts

Key Concept: pH Scale

c) The solution is basic.

[Solution Description] The pH scale measures the acidity or alkalinity of a solution. A pH less than 7 indicates an acidic solution, a pH of 7 is neutral, and a pH greater than 7 indicates a basic or alkaline solution. Since the given solution has a pH of 10, it is basic or alkaline.

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Key Concept: Neutralization Concept

c) Salt and water
[Solution Description] The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) forms salt (sodium chloride) and water. The balanced chemical equation for this reaction is:$HCl + NaOH \rightarrow NaCl + H_2O$
Therefore, the products are salt and water.

Notes:

• Definition: Neutralization is a chemical reaction between an acid and a base to form water and salt.
• General Equation:
  Acid + Base → Salt + Water
• Example:
  HCl (acid) + NaOH (base) → NaCl (salt) + H₂O (water)
• Reaction: The hydrogen ions (H⁺) from the acid combine with the hydroxide ions (OH⁻) from the base to form water.
• Types of Neutralization:
  • Strong acid + Strong base
  • Weak acid + Strong base
  • Strong acid + Weak base
• Applications:
  • Antacid tablets neutralize stomach acidity.
  • Soil treatment using lime (base) to neutralize excess soil acidity.

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Key Concept: Indicator Function

b) To indicate the endpoint by color change
[Solution Description] Phenolphthalein serves as an indicator during a neutralization reaction by changing color at the endpoint, which helps identify when the reactants have been fully neutralized.

Notes:

• Definition: Indicators are substances that change color to indicate the presence of an acid or base.
• Types of Indicators:
  • Natural Indicators: Extracted from plants, e.g., litmus, turmeric, and red cabbage juice.
  • Synthetic Indicators: Prepared artificially, e.g., phenolphthalein, methyl orange.
• Litmus Test:
  • Blue litmus turns red in acidic solutions.
  • Red litmus turns blue in basic solutions.
• pH Paper: A universal indicator that shows a range of pH values.
• pH Scale: Ranges from 0 to 14; 7 is neutral. Values below 7 indicate acidity, above 7 indicate alkalinity.
• Importance: Helps to determine whether a substance is acidic, basic, or neutral.

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Key Concept: Reaction Equation Completion

c) $\text{ZnCl}_2 + \text{H}_2\text{O}$
[Solution Description] Zinc oxide (ZnO) reacts with hydrochloric acid HCl to form zinc chloride $(\text{ZnCl}_2)$ and water $(\text{H}_2\text{O})$.

The balanced chemical equation is:$\text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}$

Notes:

• Acid + Base → Salt + Water
  • Example: HCl + NaOH → NaCl + H₂O
• Acid + Metal → Salt + Hydrogen Gas
  • Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑
• Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
  • Example: HCl + Na₂CO₃ → NaCl + H₂O + CO₂↑
• Acid + Metal Oxide → Salt + Water
  • Example: 2HCl + CuO → CuCl₂ + H₂O
• Base + Non-metal Oxide → Salt + Water
  • Example: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

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Key Concept: Tooth Decay pH

c) 5.5
[Solution Description] Tooth enamel begins to demineralize at a pH below 5.5, making teeth more susceptible to decay.

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Key Concept: Salt pH Analysis, Neutralization Reaction Analysis

c) Sodium sulfate is a neutral salt with a pH of exactly 7.
[Solution Description] Sulfuric acid $(\text{H}_2\text{SO}_4)$ is a strong acid, and sodium hydroxide (NaOH) is a strong base. When they react, they form sodium sulfate $(\text{Na}_2\text{SO}_4)$, a neutral salt because both the acid and base are strong. The balanced equation for the reaction is:
$\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$
Since both components are strong, the resulting solution will have a pH of 7.

Notes:

Salt pH Analysis:

• Salts are formed by the neutralization of acids and bases.
• The pH of a salt depends on the strength of the acid and base used.
  • Neutral salts: Formed from strong acid and strong base (e.g., NaCl, pH = 7).
  • Acidic salts: Formed from strong acid and weak base (e.g., NH4Cl, pH < 7).
  • Basic salts: Formed from weak acid and strong base (e.g., Na2CO3, pH > 7).

Neutralization Reaction Analysis:

• Neutralization is the reaction between an acid and a base to form salt and water.
• General reaction:
  Acid + Base → Salt + Water
• Example:
  HCl + NaOH → NaCl + H2O
• Neutralization reactions are exothermic, releasing heat.

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Key Concept: Litmus Test

a) Red to blue
[Solution Description]
Basic solutions turn red litmus paper blue due to the presence of $OH^-$ ions.

Notes:

• Litmus Paper: A natural dye extracted from lichens used to test whether a substance is acidic or basic.
• Acidic Solution: Turns red when tested with blue litmus paper.
• Basic Solution: Turns blue when tested with red litmus paper.
• Neutral Solution: No color change in either red or blue litmus paper.
• Litmus Solution: A liquid form of litmus used similarly to litmus paper.
• Application: Simple and quick method for identifying acids and bases in a solution.
• Limitations: Cannot measure the strength of the acid or base.

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Key Concept: Definition Recall

b) Decreasing the concentration of ions
[Solution Description] Dilution involves adding additional solvent (usually water) to decrease the concentration of solute particles like $\text{H}_3\text{O}^+$ or $\text{OH}^-$ ions per unit volume.

Notes:

• Acids: Substances that release hydrogen ions (H⁺) in aqueous solution. Example: Hydrochloric acid (HCl).
• Bases: Substances that release hydroxide ions (OH⁻) in aqueous solution. Example: Sodium hydroxide (NaOH).
• Salts: Compounds formed when an acid reacts with a base. Example: Sodium chloride (NaCl).
• Neutralization Reaction: A reaction between an acid and a base to form salt and water. Example: HCl + NaOH → NaCl + H₂O.
• Indicators: Substances that change color to indicate whether a solution is acidic, basic, or neutral. Example: Litmus paper.

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Key Concept: Real-World Application, Reaction Equations

b) $Ca(OH)_2 + 2HNO_3 \rightarrow Ca(NO_3)_2 + 2H_2O$

[Solution Description] To neutralize acidic soil, the farmer can use lime $(Ca(OH)_2)$, which reacts with the acid present in the soil. The general reaction for neutralizing an acid with lime is:

$Ca(OH)_2 + 2HX \rightarrow CaX_2 + 2H_2O$

where HX represents the acid. The product is $CaX_2$, along with water, helping to increase pH to neutral.

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Key Concept: pH and Aquatic Life, pH and Environmental Impact

b) 2 times

[Solution Description] The change in pH represents a logarithmic scale of hydrogen ion concentration. A drop of 0.3 pH means an increase in acidity by a factor of $10^{0.3}$. Calculating this:

$10^{0.3} \approx 2$

Hence, the ocean water becomes approximately 2 times more acidic.

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Key Concept: Indicator Mechanism, Experimental Design

a) Add more base until a slight pink persists
[Solution Description] If phenolphthalein remains colorless, the solution may still be below its effective pH range of 8.2 to 10.0. The student should verify if enough base has been added to surpass the lower limit of this range. Alternatively, equipment calibration or the condition of the phenolphthalein can be checked.

Notes:

Key Concept: Reaction Balancing

b) $2\text{Fe} + 6\text{HCl} \rightarrow 2\text{FeCl}_3 + 3\text{H}_2$
[Solution Description] To balance the equation, we first look at the number of atoms on each side. The unbalanced equation is:

$\text{Fe} + \text{HCl} \rightarrow \text{FeCl}_3 + \text{H}_2$

There is 1 Fe atom on both sides, so iron is balanced. However, there are 3 Cl atoms in $\text{FeCl}_3$, so we need 3 HCl molecules to provide 3 Cl atoms. Adjusting for hydrogen, 6 H from 3 HCl will produce 3 $\text{H}_2$ molecules. The balanced equation is:

$2\text{Fe} + 6\text{HCl} \rightarrow 2\text{FeCl}_3 + 3\text{H}_2$

Notes:

• Definition: Reaction balancing ensures the number of atoms for each element is equal on both sides of a chemical equation, following the Law of Conservation of Mass.
• Steps to Balance:
  • Write the skeleton equation.
  • Count the number of atoms of each element.
  • Balance atoms of metals first, then non-metals, and finally oxygen and hydrogen.
• Example:
  Unbalanced: H₂ + O₂ → H₂O
  Balanced: 2H₂ + O₂ → 2H₂O
• Always verify by recounting atoms for all elements.
• Balancing ensures the equation is scientifically correct and reliable.

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Key Concept: Indicator Limitations, Indicator Selection

b) Phenolphthalein
[Solution Description]
Phenolphthalein changes color from colorless to pink between pH 8.2 to 10, thus it does not effectively indicate small deviations around pH 7. Litmus and methyl orange both undergo visible transitions closer to neutrality, whereas phenolphthalein would not provide information for slight variations around pH 7.

Notes:

• Indicator Limitations:
  • Indicators only show the endpoint of a reaction, not the exact concentration of reactants.
  • They work best within a specific pH range and may not be reliable outside this range.
  • Some indicators are temperature-sensitive, affecting their performance.
  • Indicators may not give precise results in very dilute solutions or for weak acids/bases.
• Indicator Selection:
  • Choose indicators based on the pH range of the reaction.
  • For strong acid-strong base titrations, phenolphthalein or methyl orange is suitable.
  • For weak acid-strong base titrations, indicators like phenolphthalein work best.
  • Ensure clear color change at the endpoint.

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Key Concept: Hydronium Ion Formation, Real-world Application

b) Hydronium ion formation leads to the evolution of carbon dioxide gas.

[Solution Description]

The reaction between citric acid in lemon juice and sodium bicarbonate is a typical acid-base reaction where citric acid donates protons to the bicarbonate ions, forming water and carbon dioxide gas, which is responsible for the effervescence observed. The crucial part of this reaction is the formation of $\text{H}_3\text{O}^+$ ions when the acid dissolves in water, which then participates in forming $\text{CO}_2$ gas:

$\text{C}_6\text{H}_8\text{O}_7 + 3\text{NaHCO}_3 \rightarrow 3\text{CO}_2 + 3\text{H}_2\text{O} + \text{Na}_3\text{C}_6\text{H}_5\text{O}_7$

Here, the hydronium ions ($\text{H}_3\text{O}^+$) formed by citric acid are directly involved in the reaction process.

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Key Concept: Simple Concept

a) Neutralization
[Solution Description] The reaction between a metal carbonate and an acid typically results in the formation of salt, carbon dioxide, and water. This exemplifies a neutralization reaction, where an acid reacts with a base (metal carbonate).

Notes:

• Physical and Chemical Changes:
  • Physical change: No new substance formed (e.g., melting, freezing).
  • Chemical change: New substances formed (e.g., burning, rusting).
• Types of Matter:
  • Solids, liquids, and gases.
  • Matter can change states due to temperature and pressure.
• Properties of Matter:
  • Matter has mass, volume, and occupies space.
  • Physical properties: Color, density, boiling point.
  • Chemical properties: Reactivity, flammability.
• Conservation of Mass:
  • Mass remains constant during a physical or chemical change.
• Structure of Atom:
  • Atoms consist of protons, neutrons, and electrons.

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Key Concept: Indicator Efficiency, Complex Reaction

b) Vanilla and onion
[Solution Description]
Detection efficiency relies on the sensitivity of the indicator to pH variations. Onion and vanilla have notable reactions when transitioning between acid and basic mediums, while clove’s response is often subtler. For a broad range that includes different concentrations, using indicators like onion and vanilla together provides comprehensive coverage for detecting changes efficiently.

Notes:

• Indicator Efficiency:
  • Indicators are substances that change color to show whether a solution is acidic, basic, or neutral.
  • Common indicators: Litmus, phenolphthalein, methyl orange.
  • Strong acid-strong base reactions are easiest to detect, with clear color changes.
  • Weak acid-strong base or weak base-strong acid reactions require more sensitive indicators.
  • Efficiency depends on the pH range of the indicator and the strength of the acid or base.
• Complex Reactions:
  • Acids and bases react with salts to form new compounds.
  • In neutralization reactions, acids react with bases to form salts and water.
  • Double displacement reactions can occur, forming precipitates.
  • Some reactions may involve multiple steps or require specific conditions (e.g., temperature, concentration).

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Key Concept: Chemical Transformation, Detailed Mechanism

b) Absorption of water molecules
[Solution Description] The setting of Plaster of Paris involves the hydration of calcium sulfate hemihydrate to form calcium sulfate dihydrate. During this process, water molecules are absorbed into the crystalline structure of the gypsum, resulting in the formation of strong crystalline bonds that provide rigidity and hardness to the material.

Notes:

Chemical Transformation: Chemical transformation refers to the process in which one or more substances are converted into different substances. This occurs through chemical reactions, where bonds between atoms are broken and new bonds are formed.

Detailed Mechanism of Chemical Reactions:

1. Reactants and Products: A chemical reaction involves reactants transforming into products.
2. Breaking of Bonds: Reactants undergo bond breaking, which requires energy.
3. Formation of New Bonds: New bonds form in the product molecules, releasing energy.
4. Energy Changes: Chemical reactions involve exothermic or endothermic energy changes.
5. Conservation of Mass: The mass of reactants equals the mass of products (Law of Conservation of Mass).
6. Reaction Conditions: Reactions may require catalysts, temperature, or pressure changes.

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Key Concept: Process Applications

a) Textile
[Solution Description]
Bleaching powder is primarily used for disinfecting drinking water and as a bleaching agent in various industries. It is widely used in the textile industry to bleach cotton and linen fabrics, where maintaining the whiteness of the fabric is critical.

Notes:

Process Applications:

1. Manufacture of Sodium Hydroxide: Sodium hydroxide is produced by electrolysis of sodium chloride (NaCl) in the Castner-Kellner cell. This process also yields chlorine gas and hydrogen gas.
2. Production of Hydrogen Gas: Hydrogen is produced by reacting metals like zinc with dilute acids (HCl or H2SO4) in a displacement reaction.
3. Making Lime: Calcium oxide (lime) is obtained by heating limestone (CaCO3) in a kiln.
4. Neutralization in Agriculture: Lime is used to neutralize acidic soils to improve crop yield.
5. Soap Manufacturing: Fatty acids react with sodium hydroxide to form soap (saponification).
6. Antacids: Bases like magnesium hydroxide neutralize excess stomach acid to treat indigestion.

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Key Concept: Real-World Application

c) It hardens quickly after mixing with water.
[Solution Description] Plaster of Paris, due to its properties of hardening quickly upon addition of water, makes it an ideal material for casting and mold-making. It allows for easy shaping before setting solidly.

Notes:

• Electricity and Magnetism:
  • Used in transformers, electric motors, and power generation.
  • Magnets in refrigerators, MRI machines, and speakers.
• Chemical Reactions:
  • Essential in industries for producing fertilizers, medicines, and cleaning agents.
  • Combustion reactions power vehicles and generate heat.
• Sound Waves:
  • Used in music systems, hearing aids, and ultrasound imaging.
  • Sonar systems in submarines to detect underwater objects.
• Optics:
  • Applications in eyeglasses, cameras, microscopes, and telescopes.
  • Fiber optics for fast data transmission.
• Motion and Force:
  • Essential in designing vehicles, airplanes, and robotics.
  • Applied in sports for optimizing athlete performance.

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Key Concept: Reaction Identification

c) Neutralization
[Solution Description] The reaction between carbon dioxide $(CO_2)$ and lithium hydroxide $(LiOH)$ forms lithium carbonate $(Li_2CO_3)$ and water, which is a typical acid-base neutralization reaction.

Notes:

• Types of Chemical Reactions:
  • Combination Reaction: Two or more substances combine to form a single product (e.g., 2H₂ + O₂ → 2H₂O).
  • Decomposition Reaction: A compound breaks down into two or more simpler substances (e.g., CaCO₃ → CaO + CO₂).
  • Displacement Reaction: One element displaces another in a compound (e.g., Zn + CuSO₄ → ZnSO₄ + Cu).
  • Double Displacement Reaction: Two compounds exchange ions to form new compounds (e.g., NaCl + AgNO₃ → NaNO₃ + AgCl).
  • Redox Reactions: Involves both oxidation and reduction (e.g., Fe + CuSO₄ → FeSO₄ + Cu).
• Indicators of Chemical Reactions:
  • Evolution of gas.
  • Formation of precipitate.
  • Change in color or temperature.
  • Emission of light.

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Key Concept: Industrial Salt Process, Salt Family Classification

b) Solvay process; Carbonate family
[Solution Description]
Sodium carbonate is produced via the Solvay process, where the main reactants include sodium chloride and limestone. The production involves bubbling carbon dioxide through a concentrated solution of sodium chloride and ammonia. Sodium carbonate belongs to the carbonate family due to its anion $(CO_3^{2-})$.

Notes: Industrial Salt Process:

• Manufacture of Sodium Chloride: Sodium chloride is obtained mainly from sea water or rock salt.
• Process: The sea water is evaporated in large ponds to obtain salt. In the case of rock salt, it is mined and purified.
• Electrolysis: Sodium chloride is subjected to electrolysis in the industrial process to produce chlorine gas, hydrogen gas, and sodium hydroxide.

Salt Family Classification:

1. Normal Salts: Formed by the neutralization of an acid with a base (e.g., NaCl).
2. Acidic Salts: Formed from the neutralization of a strong acid with a weak base (e.g., NH4Cl).
3. Basic Salts: Formed from the neutralization of a weak acid with a strong base (e.g., Na2CO3).
4. Double Salts: Contain two different salts (e.g., Mohr’s salt).

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Key Concept: Industrial Application, Complex Reaction Analysis

b) $\text{CaO}$
[Solution Description] Calcium oxide $(\text{CaO})$ reacts with hydrochloric acid according to the following equation:

$\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$

The product $\text{CaCl}_2$ is known for its hygroscopic properties and is commonly used as a drying agent.

Other oxides like $\text{Fe}_2\text{O}_3$, MgO, and $\text{Al}_2\text{O}_3$ do not form such useful products in this context.

Notes:

• Industrial Applications of Acids and Bases:
  • Sulfuric Acid (H₂SO₄): Used in manufacturing fertilizers, detergents, and in petroleum refining.
  • Nitric Acid (HNO₃): Used in the production of fertilizers, explosives, and dyes.
  • Hydrochloric Acid (HCl): Used in cleaning metals, preparation of food products, and in chemical industries.
  • Sodium Hydroxide (NaOH): Used in soap making, paper production, and water treatment.
• Complex Reaction Analysis:
  • Reactions in industries often involve neutralization, precipitation, and redox processes.
  • Acid-Base Titration helps determine the concentration of unknown acids/bases.
  • Buffer Solutions maintain stable pH in various chemical processes.

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Key Concept: Basic Conductivity

b) $\text{Na}^+ \quad \text{OH}^-$

[Solution Description] Sodium hydroxide dissociates in water to produce $\text{Na}^+ \quad \text{OH}^-$ ions, both contributing to conductivity.

Notes:

• Conduction of Electricity: Substances that conduct electricity in their aqueous solution are called electrolytes.
• Electrolytes: Acids, bases, and salts are electrolytes as they dissociate into ions in water, allowing electric current to flow.
• Ions: The conductivity of a solution depends on the number of ions present. More ions lead to higher conductivity.
• Acids: Acids like HCl dissociate to form H⁺ and Cl⁻ ions.
• Bases: Bases like NaOH dissociate to form Na⁺ and OH⁻ ions.
• Salts: Salts like NaCl dissociate to form Na⁺ and Cl⁻ ions.
• Strong vs Weak Electrolytes: Strong electrolytes dissociate completely, while weak electrolytes dissociate partially.

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Key Concept: Hydrogen Ion Production, Dilution Effects

b) 0.7

[Solution Description]

First calculate the initial concentration of $\text{H}^+$ ions before dilution using $c_1v_1 = c_2v_2$ where $c_1$ is the initial concentration and $v_1$ the initial volume:

$1 \times 100 = c_2 \times 500$

Solving for $c_2$:

$c_2 = \frac{100}{500} = 0.2 \, \text{M}$

The $\text{pH}$ is given by:

$\text{pH} = -\log_{10}(c_2)$

Calculate:

$\text{pH} = -\log_{10}(0.2) = 0.69897 \approx 0.7$

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Key Concept: pH Scale Range

c) 10
[Solution Description] On the pH scale, a value greater than 7 indicates a basic solution.

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Key Concept: Litmus Paper Use

c) Turns red
[Solution Description] Lemon juice is acidic in nature. Therefore, when blue litmus paper is dipped into lemon juice, it turns red indicating the presence of an acidic solution.

Notes:

Key Concept: Metal Carbonate Reaction

c) Assertion is true, but Reason is false.

[Solution Description]

In the given assertion, when sodium carbonate $(\text{Na}_2\text{CO}_3)$ reacts with hydrochloric acid (HCl), it forms sodium chloride (NaCl), carbon dioxide $(CO_2),$ and water $(H_2O).$ This reaction can be written as:

$\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$

which is true.

The reason states that metal hydrogencarbonates do not react with acids, which is false because metal hydrogencarbonates do indeed react with acids to form a salt, carbon dioxide, and water. An example reaction is:

$\text{NaHCO}_3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$

Hence, the correct answer is option (c).

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Key Concept: pH and Chemical Reactions, pH and Universal Indicator

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description] In this question, we examine the relationship between the effect of pH on chemical reaction rates and the role of universal indicators. Both assertion and reason are true statements; the rate of many chemical reactions is influenced by pH, as some reactions proceed faster or slower in acidic conditions compared to neutral ones. Meanwhile, the reason correctly explains why universal indicators show different colors: they contain multiple dyes that react distinctively across the pH spectrum. However, the reason does not directly explain the concept mentioned in the assertion about the impact of pH on reaction rates. Thus, while both statements are true, the reason is not a direct explanation for the assertion.

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Key Concept: pH Range

c) Basic
[Solution Description]
A pH of 12 is greater than 7, placing it in the range of basic solutions. Therefore, the solution is basic in nature.

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