Acids Bases and Salts - Previous Year Questions Solved Examples (Class 10 Science)
Acids, bases, and salts are essential concepts with industrial and practical applications. These questions test understanding of pH, neutralization, and sa
TL;DR: Acids, bases, and salts are essential concepts with industrial and practical applications. These questions test understanding of pH, neutralization, a…
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Acids, bases, and salts are essential concepts with industrial and practical applications. These questions test understanding of pH, neutralization, and sa
Acids Bases and Salts - Previous Year Questions — Solved Numerical Examples (Step by Step)
Example 1: What is pH? What is the pH of a neutral solution? Give examples of acidic and basic solutions. [3 marks]
Solution: pH is the measure of hydrogen ion concentration, defined as pH = -log[H+]. pH scale ranges from 0 to 14. A neutral solution has pH = 7, with equal concentration of H+ and OH- ions. Acidic solutions have pH < 7 (more H+ ions), examples: lemon juice (pH ~2.5), vinegar (pH ~3), tomato juice (pH ~4.1). Basic solutions have pH > 7 (more OH- ions), examples: ammonia solution (pH ~11), soap solution (pH ~8-9), milk of magnesia (pH ~10.5).
Example 2: Write balanced equations for: (a) Hydrochloric acid reacting with sodium hydroxide, (b) Sulphuric acid reacting with sodium carbonate. [3 marks]
Solution: (a) HCl + NaOH = NaCl + H2O. This is a neutralization reaction between strong acid and strong base producing a neutral salt. (b) H2SO4 + Na2CO3 = Na2SO4 + H2O + CO2. This reaction produces water, salt, and carbon dioxide gas which can be identified by limewater test (turns milky).
Example 3: What happens when sodium hydroxide solution is added to a solution of ferric chloride? Write the equation. [2 marks]
Solution: FeCl3 + 3NaOH = Fe(OH)3 + 3NaCl. A reddish-brown precipitate of ferric hydroxide forms. This is a double displacement reaction.
Example 4: Explain why an aqueous solution of sodium carbonate is basic in nature. [2 marks]
Solution: Sodium carbonate (Na2CO3) is a salt formed from a strong base (NaOH) and weak acid (H2CO3). In aqueous solution, carbonate ions undergo hydrolysis: CO3²- + H2O = HCO3- + OH-. This produces hydroxide ions, making the solution basic (pH > 7). This process is called hydrolysis of salt.
Example 5: What is the action of dilute hydrochloric acid on calcium carbonate? Write the equation. What gas is evolved? [3 marks]
Solution: CaCO3 + 2HCl = CaCl2 + H2O + CO2. The gas evolved is carbon dioxide. This gas can be identified by its action with limewater, which turns milky white due to formation of calcium carbonate precipitate. The equation for this is: CO2 + Ca(OH)2 = CaCO3 + H2O.
Example 6: What are the uses of common salt in industry? [2 marks]
Solution: Common salt (NaCl) has multiple industrial uses: (1) Production of caustic soda and chlorine through electrolysis, (2) Production of hydrochloric acid, (3) Food preservation and seasoning, (4) De-icing roads in winter, (5) Production of sodium carbonate and sodium bicarbonate.
Example 7: Explain the difference between a strong acid and a weak acid with examples. [3 marks]
Solution: Strong acids completely dissociate in water, producing maximum H+ ions. Examples: HCl, H2SO4, HNO3. pH is very low (< 2). Weak acids partially dissociate in water, establishing an equilibrium. Examples: acetic acid (CH3COOH), carbonic acid (H2CO3). pH is higher than strong acids of same concentration. Strong acids are better conductors of electricity than weak acids.
Tips
- Remember pH scale: 0-7 acidic, 7 neutral, 7-14 basic. pH = 7 is neutral only for pure water.
- Identify salt type: neutral salt (strong acid + strong base), acidic salt (strong acid + weak base), basic salt (weak acid + strong base).
- Use indicators to identify acids and bases: methyl orange (red in acid), phenolphthalein (pink in base).
- In titration problems, use the formula: n1M1V1 = n2M2V2 where n is the number of replaceable H+ or OH- ions.
Frequently Asked Questions
Why does the pH of rainwater decrease over time?
Rainwater absorbs CO2 from the atmosphere, forming weak carbonic acid (H2CO3), which lowers the pH to about 5.6, making it slightly acidic.
What is the difference between a salt and a neutral salt?
A salt is an ionic compound. A neutral salt is specifically formed from a strong acid and strong base (like NaCl), with pH = 7.
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