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: Neutralisation is a chemical reaction between an acid and a base that produces a salt and water. It involves the combination of hydrogen ions (H⁺) from the acid with hydroxide ions (OH⁻) from the base to form water.
Balanced Chemical Equation:
HCl + NaOH → NaCl + H₂O
Salt Formed:
The salt produced is sodium chloride (NaCl), commonly known as table salt.
Explanation: In this reaction one mole of hydrochloric acid reacts with one mole of sodium hydroxide to give one mole of sodium chloride and one mole of water. The H⁺ ion from HCl combines with the OH⁻ ion from NaOH to form H₂O. The remaining Na⁺ and Cl⁻ ions combine to form NaCl. If equal amounts (in moles) of a strong acid and a strong base are mixed, the resulting solution is neutral with a pH close to 7. The actual pH of the final solution depends on concentrations and the strengths of the acid and base, but for strong acid + strong base at stoichiometric amounts the solution is neutral.

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 numerical scale used to measure how acidic or basic a solution is. It roughly indicates the concentration of hydrogen ions (H⁺) in a solution. The pH scale commonly runs from 0 to 14: values below 7 are acidic, 7 is neutral and values above 7 are basic (alkaline). pH is important because many chemical and biological processes depend on the acidity or alkalinity of the environment.
Examples:

• Acidic: Lemon juice (pH ≈ 2), vinegar (pH ≈ 3)
• Neutral: Pure water (pH 7)
• Basic: Soap solution (pH > 7), baking soda solution (pH ≈ 8.5)

Chemical Reaction of Acid with Metal (general):
Acid + Metal → Salt + Hydrogen gas
Example with Zinc and Hydrochloric Acid (Balanced Equation):
Zn + 2HCl → ZnCl₂ + H₂
Explanation: Zinc reacts with hydrochloric acid to produce zinc chloride (a salt) and hydrogen gas. Two hydrogen ions from two HCl molecules combine to form one molecule of hydrogen gas (H₂), while zinc is converted to Zn²⁺ and pairs with chloride ions to form ZnCl₂. This type of reaction is a single-displacement reaction where the metal reacts with hydrogen ions from the acid to liberate hydrogen gas.

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 usually refers to the alkaline hydrolysis of fats or oils (which are esters of glycerol and long-chain fatty acids) to produce glycerol and the sodium salts of fatty acids (soap). A general saponification reaction for a triglyceride is:
Typical Saponification (conceptual):
Triglyceride + 3NaOH → Glycerol + 3 Soap (sodium salts of fatty acids)
Note on the Given Reaction:
The reaction CH₃COOH + NaOH → CH₃COONa + H₂O is a neutralisation reaction between ethanoic acid (acetic acid) and sodium hydroxide. It forms sodium acetate (CH₃COONa) and water, and is not the saponification of a fat. The equation is balanced as written.
Products Formed (for the given equation):
Sodium acetate (CH₃COONa) and water (H₂O).
Cleansing Action of Soap:
Soap molecules have two distinct parts: a long non-polar hydrocarbon tail that is hydrophobic (water-repelling) and a polar ionic head that is hydrophilic (water-attracting). In water, soap molecules form structures called micelles. The hydrophobic tails surround and trap oily dirt inside the micelle, while the hydrophilic heads face outward towards the water. This allows grease and oil to be emulsified and washed away with water, thereby cleaning the surface.

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 measures the acidity or alkalinity of a solution by indicating the concentration of hydrogen ions (H⁺). The pH scale typically ranges from 0 to 14.
pH Range:

• Acidic substances: pH < 7
• Neutral substances: pH = 7
• Basic (alkaline) substances: pH > 7

Chemical Species:

• Hydrogen ion: H⁺
• Hydroxide ion: OH⁻

Explanation: Because the pH scale is logarithmic, each unit change represents a tenfold change in hydrogen ion concentration. For example, a solution of pH 4 has ten times more H⁺ ions than a solution of pH 5. Acids increase the concentration of H⁺ ions in water, lowering the pH; bases increase the concentration of OH⁻ ions, which reduces the H⁺ concentration and raises the pH.

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: Neutralisation is the reaction between an acid and a base to form a salt and water.
Example (Balanced Equation):
HCl + NaOH → NaCl + H₂O
How pH Changes:
At the start, an acid solution has excess H⁺ ions (low pH) and a base has excess OH⁻ ions (high pH). During neutralisation, H⁺ ions combine with OH⁻ ions to form water, reducing the concentrations of both ions. As the reaction proceeds, the pH of the mixture moves toward 7. When a strong acid and a strong base are mixed in equal molar amounts, the final solution is typically neutral (pH ≈ 7). If amounts are not stoichiometric or if weak acid/base are involved, the final pH may be acidic or basic.
Two Common Uses:

• Antacid tablets: Contain bases such as magnesium hydroxide or calcium carbonate that neutralise excess stomach acid and give relief from heartburn.
• Soil treatment: Lime (calcium carbonate or calcium hydroxide) is used to neutralise acidic soils, improving fertility and crop 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:

• Extent of ionisation:
  • Strong acid: Ionises almost completely in water, producing a high concentration of H⁺ ions. Example: Hydrochloric acid (HCl): HCl → H⁺ + Cl⁻
  • Weak acid: Ionises only partially in water; an equilibrium exists between undissociated acid and ions. Example: Acetic acid (CH₃COOH): CH₃COOH ⇌ H⁺ + CH₃COO⁻
• Conductivity:
  • Solution of a strong acid: Good conductor of electricity because it contains a large number of ions.
  • Solution of a weak acid: Poor conductor of electricity because it produces fewer ions.

Explanation: The difference in conductivity and chemical behaviour stems from how completely the acid molecules break into ions in water. A strong acid gives many free ions that carry electric current, while a weak acid gives relatively few ions.

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 hydrogen ion concentration. The pH scale usually runs from 0 to 14: lower values are acidic, 7 is neutral and higher values are basic.
Examples and Approximate 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-ionisation (autoionisation) of water is the process in which two water molecules react to form a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻). This process is reversible and establishes an equilibrium in pure water.
Balanced Equation:
2H₂O ⇌ H₃O⁺ + OH⁻
How Acidic and Basic Solutions Develop:
In pure water the concentrations of H₃O⁺ and OH⁻ are equal. If an acid is added, it increases the concentration of H₃O⁺ and the solution becomes acidic. If a base is added, it increases OH⁻ concentration and the solution becomes basic. The relative amounts of these ions determine whether a solution is acidic or basic.
pH and pOH:
pH measures the concentration of hydronium (or hydrogen) ions and is defined as pH = -log[H₃O⁺]. pOH measures the hydroxide ion concentration: pOH = -log[OH⁻]. At 25 °C, pH + pOH = 14. For neutral pure water at 25 °C, [H₃O⁺] = [OH⁻] = 1.0 × 10⁻⁷ M, so pH = 7 and pOH = 7.

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 basic substances that neutralise excess hydrochloric acid in the stomach, raising the pH and providing relief from heartburn and indigestion. They react with HCl to form a salt, water and sometimes carbon dioxide.
Examples 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

Explanation: These reactions neutralise excess stomach acid. When carbonate or bicarbonate antacids react with HCl, carbon dioxide gas is produced; this can cause belching and rapid relief. Hydoxide antacids form water and a salt without generating much gas. The result is a reduction in acidity and relief from discomfort.

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 is precipitation (rain, snow, fog or dew) that has become acidic due to air pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases, mainly from burning fossil fuels, react with water vapour in the atmosphere to form acids such as sulfuric acid and nitric acid, which then fall with precipitation.
Soil Alkalinity: Soil alkalinity means the soil has a pH greater than 7 and is basic. It can result from the presence of alkaline substances such as calcium hydroxide (Ca(OH)₂) or from high concentrations of carbonate and bicarbonate ions.
Environmental Impacts of Acid Rain:

• Buildings and monuments: Acid rain corrodes limestone, marble and other calcareous stones, causing surface loss and structural damage.
• Vegetation: Acid rain can damage leaves, reduce photosynthesis, leach essential nutrients from the soil and weaken plants, making them more vulnerable to disease and harsh weather.
• Aquatic ecosystems: Acid rain lowers the pH of lakes and streams, which can harm or kill fish and other aquatic organisms. It can also mobilise toxic metals (for example aluminium) from soils into water bodies, further damaging aquatic life.

Chemical Formulas:

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