Class 10 Science Chapter 2 Question Answers - Acids, Bases and Salts

Q1: Explain the process of neutralization with the help of an example. Write the balanced chemical equation for the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH), identify the salt formed, and describe the pH of the resulting solution.
Ans: Neutralization is a chemical reaction between an acid and a base to form water and a salt. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), neutralization occurs.
Balanced Chemical Equation:
HCl + NaOH → NaCl + H₂O
Salt Formed:
The salt formed is sodium chloride (NaCl), commonly known as table salt.
Explanation: In the given reaction, hydrochloric acid (HCl), which is an acid, reacts with sodium hydroxide (NaOH), which is a base. The balanced equation shows that one molecule of HCl reacts with one molecule of NaOH to yield one molecule of NaCl (salt) and one molecule of water (H₂O). The hydrogen ion (H⁺) from the acid combines with the hydroxide ion (OH⁻) from the base to form water. The remaining sodium ion (Na⁺) from the base combines with the chloride ion (Cl⁻) from the acid to form sodium chloride, which is a neutral compound.
The resulting solution has a pH of 7, indicating that it is neutral. This is because the acidic and basic properties of the initial reactants cancel each other out during the neutralization process, resulting in a solution with equal concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻), which results in a neutral pH.

Q2: Explain the term 'pH' and its significance. Provide examples of substances with acidic, basic, and neutral pH values. Write the chemical reaction when an acid reacts with a metal. Take hydrochloric acid (HCl) and zinc (Zn) as an example and write the balanced chemical equation.
Ans: pH is a measure of the acidity or alkalinity of a solution. It is a scale that ranges from 0 to 14, where pH 7 represents neutrality, pH less than 7 indicates acidity, and pH greater than 7 indicates basicity. pH is significant as it helps us determine the nature of a solution and its potential for chemical reactions.
Examples:

• Substances with acidic pH: Lemon juice, vinegar.
• Substances with basic pH: Soap solution, baking soda solution.
• Substances with neutral pH: Pure water, table salt solution.

Balanced Chemical Equation for Acid-Metal Reaction:
HCl + Zn → ZnCl₂ + H₂
Explanation: When hydrochloric acid (HCl) reacts with zinc (Zn), an acid-metal reaction takes place. The balanced equation indicates that one molecule of HCl reacts with one atom of zinc to produce one molecule of zinc chloride (ZnCl₂) and one molecule of hydrogen gas (H₂). During the reaction, hydrogen ions (H⁺) from the acid displace the metal zinc (Zn) from its compound, resulting in the formation of zinc chloride and the evolution of hydrogen gas. This is a typical example of a single displacement reaction where a more reactive element (hydrogen in HCl) displaces a less reactive element (zinc) from its compound.

Q3: Describe the process of saponification. Write the balanced chemical equation for the saponification of ethanoic acid (CH₃COOH) with sodium hydroxide (NaOH), identify the products formed, and explain the cleansing action of soap.
Ans: Saponification is the process of making soap from fats or oils by hydrolyzing them in the presence of a strong alkali like sodium hydroxide (NaOH) or potassium hydroxide (KOH).
Balanced Chemical Equation for Saponification:
CH₃COOH + NaOH → CH₃COONa + H₂O
Products Formed:
The products formed are sodium acetate (CH₃COONa) and water (H₂O).
Explanation: In the saponification reaction, ethanoic acid (CH₃COOH), which is a type of fatty acid, reacts with sodium hydroxide (NaOH), a strong base. The balanced equation shows that one molecule of ethanoic acid reacts with one molecule of sodium hydroxide to yield one molecule of sodium acetate and one molecule of water. Sodium acetate is a type of salt.
The cleansing action of soap is due to its ability to form micelles. In a soap solution, soap molecules arrange themselves in micelles with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. When the soap solution comes in contact with oily or greasy substances, the hydrophobic tails of soap molecules interact with the oil molecules, surrounding them in micelles. These micelles are then easily washed away by water since their hydrophilic heads interact with water molecules. This enables soap to emulsify and remove dirt, oil, and grease from surfaces.

Q4: Explain the term "pH" and its significance in determining the acidic or basic nature of a substance. Provide the pH range for acidic, neutral, and basic substances, along with the chemical formula of hydrogen ions and hydroxide ions.
Ans: The term "pH" stands for "potential of hydrogen" and is a measure of the concentration of hydrogen ions (H⁺) in a solution. It is used to indicate the acidic or basic nature of a substance on a scale ranging from 0 to 14. The significance of pH lies in its ability to quantify the level of acidity or alkalinity in a solution.
pH Range:

• Acidic substances have a pH less than 7.
• Neutral substances have a pH of 7.
• Basic (alkaline) substances have a pH greater than 7.

Chemical Formula:

• Hydrogen ions (H⁺) are responsible for acidity in a solution.
• Hydroxide ions (OH⁻) are responsible for basicity in a solution.

Explanation: The pH scale is logarithmic, meaning that each unit on the scale represents a tenfold difference in the concentration of hydrogen ions. For example, a solution with pH 4 has 10 times higher concentration of H⁺ ions compared to a solution with pH 5. Acids release hydrogen ions in water, increasing the concentration of H⁺ ions and leading to a lower pH value. Bases, on the other hand, release hydroxide ions, which react with H⁺ ions to form water, resulting in a higher pH value.
The chemical formula of hydrogen ions is H⁺, indicating a single positively charged hydrogen ion. The chemical formula of hydroxide ions is OH⁻, representing one negatively charged hydroxide ion.

Q5: Describe the process of neutralization with the help of a balanced chemical equation. Explain how the pH of the solution changes during the neutralization reaction, and state any two common uses of neutralization reactions in daily life.
Ans: Neutralization is a chemical reaction between an acid and a base to form salt and water. The balanced chemical equation for the neutralization reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is as follows:
HCl + NaOH → NaCl + H₂O
During the neutralization reaction, hydrogen ions (H⁺) from the acid react with hydroxide ions (OH⁻) from the base to form water (H₂O). The remaining ions, sodium (Na⁺) and chloride (Cl⁻), combine to form salt (NaCl).
pH Change: Initially, the acid solution has a low pH (acidic) due to the presence of excess H⁺ ions. The base solution, on the other hand, has a high pH (basic) due to the presence of OH⁻ ions. As the neutralization reaction progresses, the H⁺ ions from the acid combine with the OH⁻ ions from the base to form water. This reduces the concentration of both H⁺ and OH⁻ ions, leading to a decrease in the acidity of the solution. The pH gradually moves towards 7, indicating neutrality.
Common Uses of Neutralization Reactions:

• Antacid tablets: Neutralization reactions are used to relieve stomach acidity. Antacid tablets contain bases like magnesium hydroxide or calcium carbonate that neutralize excess stomach acid and provide relief from heartburn.
• Soil treatment: Agricultural lime, which is a base, is used to neutralize acidic soils. This improves soil fertility and promotes healthy plant growth.

Q6: Explain the difference between a strong acid and a weak acid. Provide examples of each, their ionization behavior in water, and the conductivity of their solutions.
Ans: Difference between Strong Acid and Weak Acid:

• Strength of Ionization:
  • Strong Acid: A strong acid is one that completely ionizes in water, releasing a high concentration of hydrogen ions (H⁺).
  • Weak Acid: A weak acid is one that only partially ionizes in water, releasing a lower concentration of hydrogen ions (H⁺).
• Conductivity:
  • Strong Acid Solution: The solution of a strong acid is a good conductor of electricity due to the presence of a high concentration of ions (H⁺ and anions).
  • Weak Acid Solution: The solution of a weak acid is a poor conductor of electricity due to the lower concentration of ions.

Examples:

• Strong Acid: Hydrochloric acid (HCl) is a strong acid. In water, it dissociates completely into hydrogen ions (H⁺) and chloride ions (Cl⁻):
  HCl → H⁺ + Cl⁻
• Weak Acid: Acetic acid (CH₃COOH) is a weak acid. It only partially ionizes in water:
  CH₃COOH ⇌ H⁺ + CH₃COO⁻

Explanation: Strong acids have a tendency to completely dissociate into ions in water, resulting in a high concentration of H⁺ ions. Consequently, their solutions are good conductors of electricity. Weak acids, on the other hand, only partially dissociate, leading to a lower concentration of H⁺ ions and poorer conductivity. The difference in ionization behavior is due to differences in the extent of bond breaking within the acid molecules.

Q7: Explain the term "pH" and its scale. Provide examples of acidic, basic, and neutral substances along with their pH values. Write the chemical formula for hydrochloric acid and sodium hydroxide.
Ans: pH is a measure of the acidity or basicity of a solution. It is defined as the negative logarithm (base 10) of the concentration of hydrogen ions (H⁺) in the solution. The pH scale ranges from 0 to 14, where a pH of 7 is considered neutral, pH values less than 7 indicate acidity, and pH values greater than 7 indicate basicity.
Examples of Substances and Their pH Values:

• Acidic: Lemon juice (pH ~ 2), vinegar (pH ~ 3), stomach acid (pH ~ 1-3)
• Neutral: Pure water (pH 7)
• Basic: Baking soda solution (pH ~ 8.5), milk of magnesia (pH ~ 10.5), bleach (pH ~ 12)

Chemical Formulas:

• Hydrochloric acid: HCl
• Sodium hydroxide: NaOH

Q8: Discuss the concept of the self-ionization of water and the formation of hydronium and hydroxide ions. Write the balanced chemical equation for the self-ionization of water, and explain how it leads to the development of acidic and basic solutions. Define the terms "pH" and "pOH."
Ans: The self-ionization of water refers to the process in which water molecules act as both acids and bases, producing hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) through a reversible chemical reaction.
Balanced Chemical Equation for Self-Ionization of Water:
2H₂O ⇌ H₃O⁺ + OH⁻
In this reaction, two water molecules react to form a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). The hydronium ion is responsible for the acidic properties of a solution, while the hydroxide ion is responsible for the basic properties.
Development of Acidic and Basic Solutions: In an acidic solution, the concentration of hydronium ions (H₃O⁺) is higher than that of hydroxide ions (OH⁻). In a basic solution, the concentration of hydroxide ions (OH⁻) is higher than that of hydronium ions (H₃O⁺). A neutral solution has an equal concentration of hydronium and hydroxide ions.
pH and pOH: pH is a measure of the concentration of hydronium ions (H₃O⁺) in a solution, and pOH is a measure of the concentration of hydroxide ions (OH⁻) in a solution. The relationship between pH and pOH is given by the equation: pH + pOH = 14.

Q9: Explain the action of antacids in the stomach to provide relief from acidity. Provide examples of commonly used antacids, their chemical compositions, and the reactions that occur when they react with stomach acid. Write the balanced chemical equation for the reaction of calcium carbonate with hydrochloric acid.
Ans: Antacids are substances that neutralize excess stomach acid to provide relief from acidity and heartburn. They work by reacting with the acidic gastric juices to form neutral products, thereby raising the pH of the stomach.
Examples of Antacids and Reactions:

• Calcium carbonate (CaCO₃): CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
• Sodium bicarbonate (NaHCO₃): NaHCO₃ + HCl → NaCl + H₂O + CO₂
• Magnesium hydroxide (Mg(OH)₂): Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Balanced Chemical Equation for Calcium Carbonate Reaction:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
In this reaction, calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water, and carbon dioxide. The carbon dioxide gas produced can lead to belching, which helps relieve the feeling of fullness and discomfort caused by excess stomach acid.

Q10: Explain the terms "acid rain" and "soil alkalinity." Describe the environmental impact of acid rain on buildings, vegetation, and aquatic ecosystems. Write the chemical formula for sulfuric acid and calcium hydroxide.
Ans: Acid Rain: Acid rain refers to rainwater that has become acidic due to the presence of pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) in the atmosphere. These pollutants are released from human activities, particularly the burning of fossil fuels. When sulfur dioxide and nitrogen oxides react with water vapor in the atmosphere, they form sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which are then carried by raindrops to the ground.
Soil Alkalinity: Soil alkalinity refers to the pH of soil being higher than 7, indicating a basic or alkaline nature. It can result from the presence of basic substances such as calcium hydroxide (Ca(OH)₂) or magnesium hydroxide (Mg(OH)₂) in the soil.
Environmental Impact of Acid Rain:

• Buildings: Acid rain can corrode buildings and statues made of limestone and marble, as these materials react with sulfuric acid and nitric acid, leading to the deterioration of structures.
• Vegetation: Acid rain damages plant leaves, inhibiting photosynthesis and affecting plant growth. It can also leach essential nutrients from the soil, further harming plants.
• Aquatic Ecosystems: Acid rain lowers the pH of lakes, rivers, and streams, harming aquatic life. Fish and other aquatic organisms become stressed or die due to the increased acidity of the water.

Chemical Formulas:

• Sulfuric acid: H₂SO₄
• Calcium hydroxide: Ca(OH)₂