Class 10 Science Chapter 2 Question Answer | Acids, Bases and Salts | English Medium

Class 10 Science Chapter 2 Question Answer | Acids, Bases and Salts | English Medium | ASSEB

Class 10 Science Chapter 2 — Acids, Bases and Salts

Welcome to HSLC Guru. This article presents complete textbook question answers for Class 10 Science Chapter 2 “Acids, Bases and Salts” prescribed by the Assam State School Education Board (ASSEB). Students preparing for the HSLC examination will find a structured summary, important short and long answer questions, multiple choice questions, fill in the blanks, true/false statements and a chapter glossary — all crafted strictly according to the ASSEB syllabus.

Summary

Acids are substances that taste sour, turn blue litmus red and release hydrogen ions (H⁺) when dissolved in water. Common examples include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), acetic acid (in vinegar) and citric acid (in lemons). Bases taste bitter, feel soapy, turn red litmus blue and release hydroxide ions (OH^–) in aqueous solution. Sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)₂] and ammonium hydroxide (NH₄OH) are typical bases. Bases that are soluble in water are called alkalis. Acids react with metals to release hydrogen gas, with metal carbonates and bicarbonates to release carbon dioxide, and with bases to form salt and water in a neutralisation reaction.

The ionisation of acids and bases takes place only in the presence of water. Acids dissociate to produce hydronium ions (H₃O⁺) while bases dissociate to give OH^– ions. The strength of an acid or base depends on the extent of ionisation — strong acids and bases ionise completely whereas weak acids and bases ionise only partially. The pH scale, ranging from 0 to 14, measures the hydrogen ion concentration of a solution. A pH of 7 is neutral, less than 7 is acidic and greater than 7 is basic. Indicators such as litmus, methyl orange, phenolphthalein and natural indicators like turmeric and red cabbage extract help detect acidic or basic nature. Olfactory indicators like onion and clove change their smell on contact with acid or base.

pH plays a vital role in everyday life — our body works within a narrow pH range of 7.0 to 7.8, plants grow best in soils of suitable pH, tooth decay begins when mouth pH falls below 5.5, and bee stings (acidic) are treated with mild bases like baking soda. The reaction of acid and base producing salt and water is known as neutralisation: NaOH + HCl → NaCl + H₂O. Salts derived from strong acid and strong base are neutral, those from strong acid and weak base are acidic, and those from weak acid and strong base are basic.

Important salts in daily life include common salt (NaCl) — used in cooking and as raw material for chlor-alkali industry; washing soda (Na₂CO₃·10H₂O) — used in glass, soap and paper industries and to remove permanent hardness of water; baking soda (NaHCO₃) — used in cooking, antacids and fire extinguishers; bleaching powder [Ca(OCl)Cl] — used as disinfectant and bleaching agent; and plaster of Paris (CaSO₄·½H₂O) — used for plaster casts, decoration and construction. Crystalline salts contain a fixed number of water molecules called water of crystallisation, e.g., CuSO₄·5H₂O (blue vitriol), Na₂CO₃·10H₂O and CaSO₄·2H₂O (gypsum).

Textbook Questions and Answers

Very Short Answer Questions (1 mark)

Q1. What is an acid?

Answer: An acid is a substance which produces hydrogen ions (H⁺) in aqueous solution and turns blue litmus red.

Q2. Name an indicator obtained from lichen.

Answer: Litmus is the indicator obtained from lichen.

Q3. What is the pH of a neutral solution?

Answer: The pH of a neutral solution is 7.

Q4. What is the chemical name of washing soda?

Answer: Sodium carbonate decahydrate (Na₂CO₃·10H₂O).

Q5. Which gas is evolved when an acid reacts with a metal?

Answer: Hydrogen gas (H₂) is evolved.

Q6. What is the colour of phenolphthalein in an acidic solution?

Answer: Phenolphthalein remains colourless in an acidic solution.

Q7. Name the acid present in ant sting.

Answer: Formic acid (methanoic acid) is present in ant sting.

Q8. What is plaster of Paris chemically?

Answer: Calcium sulphate hemihydrate (CaSO₄·½H₂O).

Q9. Name a salt that contains water of crystallisation.

Answer: Copper sulphate pentahydrate (CuSO₄·5H₂O), commonly known as blue vitriol.

Q10. What is the pH range of human blood?

Answer: The pH of human blood ranges between 7.35 and 7.45 (about 7.0 to 7.8 within tolerance).

Short Answer Questions (2-3 marks)

Q1. Why does dry HCl gas not change the colour of dry blue litmus paper?

Answer: Dry HCl gas does not ionise to give H⁺ ions in the absence of water. Since H⁺ ions are responsible for acidic behaviour, dry HCl gas cannot turn blue litmus red. Only when water is present, HCl ionises to give H₃O⁺ ions which then change the colour of moist litmus paper.

Q2. What is neutralisation reaction? Give one example.

Answer: The reaction in which an acid reacts with a base to form salt and water is called a neutralisation reaction. In this reaction the H⁺ ions of the acid combine with the OH^– ions of the base to form water. Example: NaOH + HCl → NaCl + H₂O.

Q3. Why is the pH important in our digestive system?

Answer: Our stomach produces hydrochloric acid which helps in digestion of food without harming the stomach lining. When excess acid is produced it causes indigestion and pain. To neutralise this excess acid, we use mild bases called antacids such as milk of magnesia [Mg(OH)₂], which raise the pH of the stomach to a comfortable level and provide relief from acidity.

Q4. What happens when an acid reacts with a metal carbonate? Write a balanced equation.

Answer: When an acid reacts with a metal carbonate, salt, water and carbon dioxide gas are produced. The CO₂ evolved turns lime water milky. Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑.

Q5. Why does tooth decay start when the pH of mouth is lower than 5.5?

Answer: Tooth enamel is made up of calcium phosphate, the hardest substance of the body. It does not dissolve in water but is corroded when the pH of the mouth falls below 5.5. Bacteria present in the mouth break down food sugars to produce acids that lower mouth pH. Brushing teeth with a basic toothpaste neutralises this acid and prevents tooth decay.

Q6. Why is sodium hydrogen carbonate used in fire extinguishers?

Answer: When sodium hydrogen carbonate (baking soda) reacts with an acid in the fire extinguisher, large amounts of carbon dioxide gas are produced. CO₂ being heavier than air covers the burning material and cuts off oxygen supply, thereby extinguishing the fire. Reaction: 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂.

Long Answer Questions (5-6 marks)

Q1. Describe the chemical properties of acids with balanced equations.

Answer: Acids show the following important chemical properties:

Reaction with metals: Acids react with active metals to liberate hydrogen gas. Example: Zn + 2HCl → ZnCl₂ + H₂↑.
Reaction with metal carbonates: Acids react with carbonates to give salt, water and CO₂. Example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
Reaction with metal hydrogen carbonates: NaHCO₃ + HCl → NaCl + H₂O + CO₂.
Reaction with bases (neutralisation): HCl + NaOH → NaCl + H₂O.
Reaction with metal oxides: Metal oxides being basic in nature react with acids to form salt and water. Example: CuO + 2HCl → CuCl₂ + H₂O.
Effect on indicators: Acids turn blue litmus red, methyl orange to red and keep phenolphthalein colourless.

Q2. Explain the pH scale and its importance in everyday life.

Answer: The pH scale is a numerical scale ranging from 0 to 14 used to measure the hydrogen ion concentration of a solution. The ‘p’ in pH stands for “potenz” meaning power in German. A pH value of 7 indicates a neutral solution, less than 7 indicates an acidic solution, and more than 7 indicates a basic solution. The lower the pH the stronger the acid; the higher the pH the stronger the base.

Importance of pH in daily life:

In living systems: Living organisms can survive only within a narrow pH range. Human blood pH must remain between 7.35 and 7.45.
In digestion: Stomach produces HCl with pH about 1.2 for digestion. Antacids neutralise excess acid.
In agriculture: Plants grow well only in soils of optimum pH; too acidic or alkaline soils require treatment with lime or organic matter.
In tooth decay: When mouth pH falls below 5.5, enamel is corroded. Basic toothpaste neutralises this acid.
In self-defence by animals and plants: Bee stings (formic acid) and nettle leaves cause pain; rubbing baking soda or dock leaves provides relief.
In rain water: Rain water with pH less than 5.6 is acid rain, harmful to aquatic life and plants.

Q3. How is washing soda prepared from common salt? Mention its uses.

Answer: Washing soda is prepared from common salt by the Solvay process and recrystallisation:

Saturated brine solution (NaCl) is treated with ammonia and carbon dioxide to form sodium hydrogen carbonate: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl.
Sodium hydrogen carbonate so produced is heated to give sodium carbonate (anhydrous): 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
The anhydrous sodium carbonate is recrystallised with water to obtain washing soda: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.

Uses: (i) In glass, soap and paper industries; (ii) For removing permanent hardness of water; (iii) In manufacture of borax and sodium compounds; (iv) As a domestic cleansing agent for washing clothes.

Q4. Describe the manufacture and uses of bleaching powder.

Answer: Bleaching powder, chemically calcium oxychloride [Ca(OCl)Cl], is produced by passing chlorine gas over dry slaked lime [Ca(OH)₂]. The reaction is: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. The chlorine required is obtained from the chlor-alkali process — electrolysis of brine.

Uses of bleaching powder:

For bleaching cotton and linen in textile industry.
For bleaching wood pulp in paper industry.
For bleaching washed clothes in laundry.
As an oxidising agent in many chemical industries.
For disinfecting drinking water and swimming pools.
For making chloroform.

Q5. What is water of crystallisation? Explain with reference to copper sulphate and gypsum.

Answer: Water of crystallisation is the fixed number of water molecules chemically combined per formula unit of a salt in its crystalline form. These molecules give the salts their definite shape, colour and stability.

Copper sulphate (CuSO₄·5H₂O): The blue colour and crystalline shape of copper sulphate are due to five water molecules associated with each formula unit. On heating, it loses water and becomes white anhydrous copper sulphate. When water is added back, the blue colour returns: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O.

Gypsum (CaSO₄·2H₂O): Each unit of calcium sulphate is associated with two molecules of water. On heating gypsum at 373 K, it loses 1.5 water molecules and forms plaster of Paris (CaSO₄·½H₂O), used in setting fractured bones and decoration: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O.

Additional Questions

Multiple Choice Questions (MCQ)

Q1. The chemical formula of baking soda is —
(a) Na₂CO₃ (b) NaHCO₃ (c) Na₂CO₃·10H₂O (d) NaCl
Answer: (b) NaHCO₃.

Q2. Which has the highest pH value?
(a) Lemon juice (b) Pure water (c) Sodium hydroxide (d) Vinegar
Answer: (c) Sodium hydroxide.

Q3. Tooth enamel is made up of —
(a) Calcium carbonate (b) Calcium sulphate (c) Calcium phosphate (d) Calcium chloride
Answer: (c) Calcium phosphate.

Q4. Bleaching powder is prepared from —
(a) Quick lime (b) Slaked lime (c) Lime water (d) Limestone
Answer: (b) Slaked lime.

Q5. The acid present in tomato is —
(a) Tartaric acid (b) Lactic acid (c) Oxalic acid (d) Citric acid
Answer: (c) Oxalic acid.

Q6. Phenolphthalein in basic solution gives —
(a) Red (b) Pink (c) Colourless (d) Yellow
Answer: (b) Pink.

Q7. Plaster of Paris is produced by heating gypsum at —
(a) 273 K (b) 373 K (c) 473 K (d) 573 K
Answer: (b) 373 K.

Q8. Common salt is —
(a) Acidic (b) Basic (c) Neutral (d) Amphoteric
Answer: (c) Neutral.

Q9. Which of the following is an olfactory indicator?
(a) Litmus (b) Methyl orange (c) Onion (d) Phenolphthalein
Answer: (c) Onion.

Q10. The number of water molecules in blue vitriol is —
(a) 2 (b) 5 (c) 7 (d) 10
Answer: (b) 5.

Fill in the Blanks

Q1. Acids release ________ ions in water.
Answer: hydronium (H₃O⁺).

Q2. A pH value of less than 7 indicates ________ solution.
Answer: acidic.

Q3. Plaster of Paris is chemically known as ________.
Answer: calcium sulphate hemihydrate.

Q4. The acid present in lemon is ________.
Answer: citric acid.

Q5. The chemical formula of bleaching powder is ________.
Answer: Ca(OCl)Cl or CaOCl₂.

True / False

Q1. Bases turn red litmus blue.
Answer: True.

Q2. Dry HCl gas turns dry blue litmus paper red.
Answer: False.

Q3. A solution with pH 14 is strongly acidic.
Answer: False.

Q4. Sodium hydrogen carbonate is used in fire extinguishers.
Answer: True.

Q5. Anhydrous copper sulphate is blue in colour.
Answer: False.

Glossary

Term	Meaning
Acid	A substance which produces H⁺ ions in aqueous solution and turns blue litmus red.
Base	A substance which produces OH^– ions in aqueous solution and turns red litmus blue.
Alkali	A water-soluble base such as NaOH or KOH.
Indicator	A substance used to detect the acidic or basic nature of a solution by colour change.
Olfactory indicator	An indicator whose smell changes in acidic or basic medium, e.g., onion, clove.
Neutralisation	The reaction between an acid and a base to form salt and water.
pH scale	A scale from 0 to 14 used to measure the H⁺ ion concentration of a solution.
Salt	A compound formed by the reaction of an acid with a base.
Water of crystallisation	The fixed number of water molecules chemically combined per formula unit of a salt in its crystalline form.
Washing soda	Sodium carbonate decahydrate, Na₂CO₃·10H₂O.
Baking soda	Sodium hydrogen carbonate, NaHCO₃, used in cooking and antacids.
Plaster of Paris	Calcium sulphate hemihydrate, CaSO₄·½H₂O, used in plaster casts and decoration.
Bleaching powder	Calcium oxychloride, Ca(OCl)Cl, used as bleaching and disinfecting agent.
Chlor-alkali process	Electrolysis of brine to produce sodium hydroxide, chlorine and hydrogen.
Antacid	A mild base used to neutralise excess acid in the stomach.
Acid rain	Rain water having pH less than 5.6, harmful to aquatic life and crops.
Strong acid	An acid that ionises completely in water, e.g., HCl, H₂SO₄, HNO₃.
Weak acid	An acid that ionises only partially in water, e.g., acetic acid, carbonic acid.
Strong base	A base that ionises completely in water, e.g., NaOH, KOH.
Weak base	A base that ionises only partially in water, e.g., NH₄OH.
Brine	A concentrated aqueous solution of common salt (NaCl) used as raw material in the chlor-alkali industry.
Hydronium ion	The H₃O⁺ ion formed when an acid donates H⁺ to a water molecule.