CBSE Class 10 Science (2026–27)
Chapter 2: Acids, Bases and Salts
20 Important Questions and Answers
Q1. What are acids and bases? Give two examples of each.
Answer:
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. They generally have a sour taste and turn blue litmus red. Examples include hydrochloric acid (HCl) and sulfuric acid (H₂SO₄). Bases are substances that produce hydroxide ions (OH⁻) in aqueous solutions. They have a bitter taste and turn red litmus blue. Examples include sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)₂]. Acids and bases react with each other in a neutralization reaction to form salt and water. The strength of an acid or base depends on the degree of ionization in water. These substances play important roles in industry, agriculture, and daily life.
Q2. Why do acids show acidic behavior only in aqueous solutions?
Answer:
Acids exhibit acidic behavior only in aqueous solutions because they release hydrogen ions (H⁺) only when dissolved in water. Water helps in the ionization of acids, allowing the formation of hydronium ions (H₃O⁺), which are responsible for acidic properties. For example, dry hydrogen chloride gas does not change the color of dry litmus paper because it does not produce H⁺ ions. However, when dissolved in water, HCl forms H₃O⁺ ions and shows acidic characteristics. Thus, water is essential for the dissociation of acids and the expression of their acidic nature. Without water, acids cannot conduct electricity or show their typical reactions.
Q3. Explain the reaction of acids with metals with an example.
Answer:
Acids react with metals to produce salt and hydrogen gas. In this reaction, the metal displaces hydrogen from the acid. The hydrogen gas evolved can be identified by the characteristic ‘pop’ sound when a burning splint is brought near it. For example, when zinc reacts with dilute hydrochloric acid, zinc chloride and hydrogen gas are formed.
Equation:
Zn + 2HCl → ZnCl₂ + H₂↑
This reaction demonstrates the chemical property of acids. However, less reactive metals like copper and silver do not react with dilute acids. Such reactions are important in understanding metal reactivity and industrial applications involving acid-metal interactions.
Q4. What happens when acids react with metal carbonates?
Answer:
When acids react with metal carbonates, they produce salt, water, and carbon dioxide gas. The carbon dioxide gas released causes effervescence and can be identified by passing it through lime water, which turns milky due to the formation of calcium carbonate. For example, hydrochloric acid reacts with calcium carbonate as follows:
Equation:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
This reaction is commonly observed when marble chips react with dilute hydrochloric acid. The reaction is useful in laboratories for preparing carbon dioxide gas and demonstrates the characteristic property of acids reacting with carbonates and bicarbonates.
Q5. Define neutralization reaction with an example.
Answer:
A neutralization reaction is a chemical reaction in which an acid reacts with a base to form salt and water. During the reaction, hydrogen ions from the acid combine with hydroxide ions from the base to produce water. The resulting solution becomes neutral or nearly neutral.
Example:
HCl + NaOH → NaCl + H₂O
Here, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. Neutralization reactions are exothermic, meaning heat is released. These reactions are widely used in daily life, such as treating indigestion with antacids and neutralizing acidic soils in agriculture. They help maintain pH balance in many natural and industrial processes.
Q6. What is the pH scale? Why is it important?
Answer:
The pH scale is a numerical scale used to measure the acidity or basicity of a solution. It ranges from 0 to 14. A pH value less than 7 indicates an acidic solution, a value of 7 indicates a neutral solution, and a value greater than 7 indicates a basic solution. The pH scale helps compare the strength of acids and bases. It is important because many biological and chemical processes depend on a specific pH range. For example, human blood maintains a pH of about 7.4. Soil pH affects crop growth, and pH control is essential in industries, medicine, and environmental studies.
Q7. How does the concentration of hydrogen ions affect pH?
Answer:
The pH of a solution depends on the concentration of hydrogen ions (H⁺). As the concentration of hydrogen ions increases, the pH value decreases, making the solution more acidic. Conversely, a lower concentration of hydrogen ions results in a higher pH value, indicating a less acidic or more basic solution. For example, a solution with pH 2 is much more acidic than one with pH 5. Since pH is measured on a logarithmic scale, a small change in pH represents a significant change in hydrogen ion concentration. Thus, pH is an effective way to express acidity and alkalinity quantitatively.
Q8. What is the importance of pH in our digestive system?
Answer:
The digestive system requires an appropriate pH for proper functioning. The stomach secretes hydrochloric acid, which maintains a pH of about 1–3. This acidic environment helps digest food and kills harmful bacteria entering with food. Sometimes excess acid production causes indigestion and irritation. In such cases, antacids like magnesium hydroxide are used to neutralize excess acid. Enzymes involved in digestion also work effectively within specific pH ranges. Therefore, maintaining the correct pH is essential for efficient digestion and overall health. Disturbance in pH balance may lead to digestive disorders and discomfort.
Q9. Why does tooth decay occur when the pH of the mouth falls below 5.5?
Answer:
Tooth decay occurs when bacteria present in the mouth break down food particles, especially sugars, and produce acids. These acids lower the pH in the mouth. When the pH falls below 5.5, the enamel, which is made of calcium phosphate, starts dissolving. Continuous exposure to acidic conditions weakens the enamel and leads to cavities. Proper dental hygiene, regular brushing, and using toothpaste help maintain the pH above the critical level. Toothpaste is generally basic in nature and neutralizes excess acid in the mouth. Thus, maintaining a proper pH helps protect teeth from decay and damage.
Q10. Explain the preparation of sodium hydroxide by the chlor-alkali process.
Answer:
Sodium hydroxide is prepared by the chlor-alkali process, which involves the electrolysis of an aqueous solution of sodium chloride (brine). During electrolysis, chlorine gas is produced at the anode, hydrogen gas at the cathode, and sodium hydroxide remains in the solution.
Equation:
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Sodium hydroxide produced by this process is widely used in making soaps, detergents, paper, and artificial fibers. Chlorine is used for water purification and PVC production, while hydrogen is used in fuel and chemical industries. This process is economically important because it produces three valuable products simultaneously.
Q11. What are indicators? Name two natural indicators.
Answer:
Indicators are substances that show different colors in acidic and basic solutions, helping identify the nature of the solution. They are commonly used in laboratories and industries to determine pH. Natural indicators are obtained from plants and naturally occurring substances. Two common natural indicators are litmus and turmeric. Litmus is extracted from lichens and turns red in acidic solutions and blue in basic solutions. Turmeric remains yellow in acidic solutions but turns reddish-brown in basic solutions. Indicators are important because they provide a simple and effective method for distinguishing between acids, bases, and neutral substances.
Q12. What is baking soda? Mention its uses.
Answer:
Baking soda is sodium hydrogen carbonate (NaHCO₃). It is a mild base and is commonly used in households and industries. When heated, it decomposes to form sodium carbonate, carbon dioxide, and water.
Equation:
2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
The carbon dioxide released helps bread and cakes rise, making them soft and fluffy. Baking soda is also used in antacids to relieve acidity and indigestion. Additionally, it is used in soda-acid fire extinguishers. Due to its versatile properties, baking soda is an important chemical in daily life and commercial applications.
Q13. What is washing soda? Write its uses.
Answer:
Washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O). It is obtained by heating baking soda and then crystallizing the resulting sodium carbonate. Washing soda is an important industrial chemical with several uses. It is used in the manufacture of glass, soap, and paper. It helps remove permanent hardness of water by precipitating calcium and magnesium salts. It is also used as a cleaning agent in homes and laundries. Due to its alkaline nature, washing soda effectively removes grease and stains. Its water-softening property makes it valuable in both domestic and industrial applications.
Q14. How is bleaching powder prepared and what are its uses?
Answer:
Bleaching powder is prepared by passing chlorine gas over dry slaked lime [Ca(OH)₂].
Equation:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
The chemical name of bleaching powder is calcium oxychloride. It is widely used for disinfecting drinking water and swimming pools. It is also used as a bleaching agent in the textile and paper industries. Due to its ability to release chlorine, it acts as a strong disinfectant and germicide. Bleaching powder is useful in maintaining sanitation and preventing waterborne diseases. Its bleaching and disinfecting properties make it an important compound in public health and industrial processes.
Q15. What is Plaster of Paris? How is it prepared?
Answer:
Plaster of Paris (POP) is calcium sulfate hemihydrate with the chemical formula CaSO₄·½H₂O. It is prepared by heating gypsum (CaSO₄·2H₂O) at about 373 K.
Equation:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
Plaster of Paris is a white powder that hardens when mixed with water. It is used for making statues, decorative items, chalks, and casts for fractured bones. Due to its quick-setting property, it is widely used in construction and medical fields. The material is lightweight, easy to mold, and economical, making it highly useful for artistic and practical applications.
Q16. Differentiate between strong acids and weak acids.
Answer:
Strong acids completely ionize in water, producing a large number of hydrogen ions. Weak acids ionize only partially and release fewer hydrogen ions. Strong acids have lower pH values and conduct electricity more effectively than weak acids. Examples of strong acids include hydrochloric acid (HCl) and nitric acid (HNO₃). Examples of weak acids include acetic acid and carbonic acid. The strength of an acid depends on the degree of ionization rather than its concentration. Understanding the difference between strong and weak acids is important for predicting their chemical behavior, industrial applications, and effects on living organisms.
Q17. Why should acids not be stored in metal containers?
Answer:
Acids should not be stored in metal containers because they react with many metals to produce salt and hydrogen gas. This reaction can corrode the container and weaken its structure. The hydrogen gas produced is highly flammable and may create safety hazards. For example, hydrochloric acid reacts with zinc to produce zinc chloride and hydrogen gas. Continuous reaction can lead to leakage and contamination. Therefore, acids are usually stored in glass, plastic, or specially coated containers that are resistant to corrosion. Proper storage ensures safety, prevents accidents, and maintains the purity of the acid.
Q18. What are universal indicators? How are they useful?
Answer:
A universal indicator is a mixture of several indicators that shows different colors at different pH values. Unlike litmus, which only indicates whether a solution is acidic or basic, a universal indicator provides an approximate pH value. It changes color from red in strongly acidic solutions to violet in strongly basic solutions. By comparing the observed color with a standard pH chart, the pH of a solution can be estimated. Universal indicators are useful in laboratories, industries, agriculture, and environmental monitoring. They provide a quick and reliable method for determining the strength of acids and bases.
Q19. What happens when a base is dissolved in water?
Answer:
When a base is dissolved in water, it dissociates to produce hydroxide ions (OH⁻). These ions are responsible for the basic nature of the solution. For example, sodium hydroxide dissolves in water to form sodium ions and hydroxide ions.
Equation:
NaOH → Na⁺ + OH⁻
The concentration of hydroxide ions determines the strength of the base. Basic solutions turn red litmus blue and have a pH greater than 7. They can conduct electricity because of the presence of ions. The dissolution process may release heat, making it an exothermic reaction. Bases are widely used in cleaning products, medicines, and industrial processes.
Q20. Explain the role of salts in everyday life.
Answer:
Salts are ionic compounds formed by the reaction of acids and bases. They have numerous applications in daily life. Common salt (NaCl) is used in food preservation and seasoning. Baking soda is used in cooking and antacids, while washing soda is used for cleaning and softening hard water. Bleaching powder is used for water purification and disinfection. Plaster of Paris is used in construction, art, and medicine. Different salts have specific physical and chemical properties that make them useful in industries, agriculture, healthcare, and household activities. Thus, salts play a vital role in improving human life and supporting various industrial processes.