Key Concepts of Acids, Bases and Salts for Class 10 CBSE

Q: What is the main difference between an acid and a base?

Acids produce hydrogen ions in water, while bases produce hydroxide ions in water.

Short Introduction

The chapter Acids, Bases and Salts explains the chemical nature of substances around us. It covers acids, bases, indicators, pH scale, neutralisation reactions and important salts such as baking soda, washing soda, bleaching powder, plaster of Paris and caustic soda. This chapter is very important for CBSE board exams because it includes definitions, chemical equations, reasoning questions and application-based questions.

Chapter Overview

Acids

Acids produce hydrogen ions, H⁺, in aqueous solution and turn blue litmus red.

Bases

Bases produce hydroxide ions, OH⁻, in aqueous solution and turn red litmus blue.

Salts

Salts are ionic compounds formed mainly by neutralisation of acids and bases.

pH Scale

The pH scale measures acidic or basic strength from 0 to 14.

Neutralisation

Acid + Base → Salt + Water. This reaction is useful in daily life.

Important Salts

Caustic soda, baking soda, washing soda, bleaching powder and plaster of Paris are exam-important salts.

Important Keywords

Acid Base Salt Indicator Litmus Phenolphthalein Methyl orange Universal indicator pH scale Hydronium ion Hydroxide ion Neutralisation Amphoteric oxide Dilution Water of crystallisation Bleaching powder Baking soda Washing soda Plaster of Paris Caustic soda

Important Very Short Answer Questions

Q1. What is an acid?

Answer: An acid is a substance that produces H⁺ ions in aqueous solution. Example: HCl, H₂SO₄.

Q2. What is a base?

Answer: A base is a substance that produces OH⁻ ions in aqueous solution. Example: NaOH, KOH.

Q3. What is an alkali?

Answer: A base that dissolves in water is called an alkali. Example: sodium hydroxide.

Q4. Name the ion responsible for acidic nature.

Answer: Hydrogen ion, H⁺, or hydronium ion, H₃O⁺.

Q5. Name the ion responsible for basic nature.

Answer: Hydroxide ion, OH⁻.

Q6. What is the pH of a neutral solution?

Answer: The pH of a neutral solution is 7.

Q7. What is neutralisation?

Answer: The reaction between an acid and a base to form salt and water is called neutralisation.

Q8. What is the chemical formula of baking soda?

Answer: Sodium hydrogen carbonate, NaHCO₃.

Q9. What is the chemical formula of washing soda?

Answer: Na₂CO₃ · 10H₂O.

Q10. What is water of crystallisation?

Answer: Water molecules present in the fixed ratio in a salt crystal are called water of crystallisation.

Q11. Give one example of an amphoteric oxide.

Answer: Aluminium oxide, Al₂O₃, or zinc oxide, ZnO.

Q12. What colour does phenolphthalein show in a basic solution?

Answer: Pink.

Q13. What colour does methyl orange show in an acidic solution?

Answer: Red.

Q14. Which gas is released when acids react with metals?

Answer: Hydrogen gas.

Q15. Which salt is used for whitewashing?

Answer: Slaked lime, Ca(OH)₂, is used for whitewashing.

Short Answer Questions

Q1. Write two physical properties of acids.

Answer:

Acids are sour in taste.
Acids turn blue litmus solution red.
Many acids are corrosive in concentrated form.

Q2. Write two physical properties of bases.

Answer:

Bases are bitter in taste.
Bases feel soapy to touch.
Bases turn red litmus solution blue.

Q3. What happens when an acid reacts with a metal? Give equation.

Answer: Acids react with active metals to form salt and hydrogen gas.

Acid + Metal → Salt + Hydrogen

Zn + 2HCl → ZnCl₂ + H₂

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

Answer: An acid reacts with a metal carbonate to form salt, water and carbon dioxide gas.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Q5. How can carbon dioxide gas be tested?

Answer: Carbon dioxide gas turns lime water milky due to the formation of calcium carbonate.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Q6. Why do acids show acidic behaviour only in aqueous solution?

Answer: Acids produce H⁺ ions only in the presence of water. These H⁺ ions combine with water to form H₃O⁺ ions, which are responsible for acidic behaviour.

Q7. What is dilution of an acid?

Answer: Dilution is the process of decreasing the concentration of acid by adding water. During dilution, heat is released because the process is highly exothermic.

Q8. Why should acid be added to water and not water to acid?

Answer: Mixing acid with water is highly exothermic. If water is added to concentrated acid, the heat produced may cause the mixture to splash. Therefore, acid should always be added slowly to water with constant stirring.

Q9. What are indicators? Give examples.

Answer: Indicators are substances that show different colours in acidic and basic solutions.

Natural indicator: Litmus, turmeric
Synthetic indicator: Phenolphthalein, methyl orange
Olfactory indicator: Onion, vanilla, clove oil

Q10. What are amphoteric oxides?

Answer: Oxides that react with both acids and bases to form salt and water are called amphoteric oxides. Examples: Al₂O₃ and ZnO.

Q11. Write the reaction of zinc oxide with acid and base.

Answer:

ZnO + 2HCl → ZnCl₂ + H₂O

ZnO + 2NaOH → Na₂ZnO₂ + H₂O

Q12. Why is pH important in daily life?

Answer: pH is important because it affects digestion, tooth decay, soil fertility, survival of aquatic life and proper functioning of the human body.

Long Answer Questions

Q1. Explain the chemical properties of acids with equations.

Answer: Important chemical properties of acids are:

Reaction with metals: Acids react with metals to form salt and hydrogen gas.

Zn + 2HCl → ZnCl₂ + H₂

Reaction with metal carbonates: Acids form salt, water and carbon dioxide.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Reaction with bases: Acids neutralise bases to form salt and water.

HCl + NaOH → NaCl + H₂O

Reaction with metal oxides: Acids react with metal oxides to form salt and water.

CuO + 2HCl → CuCl₂ + H₂O

Q2. Explain the chemical properties of bases with equations.

Answer: Important chemical properties of bases are:

Reaction with acids: Bases react with acids to form salt and water.

NaOH + HCl → NaCl + H₂O

Reaction with non-metal oxides: Bases react with acidic oxides to form salt and water.

2NaOH + CO₂ → Na₂CO₃ + H₂O

Reaction with some metals: Strong bases react with metals like zinc to produce hydrogen gas.

Zn + 2NaOH → Na₂ZnO₂ + H₂

Q3. Describe the pH scale and its importance.

Answer:

The pH scale measures the concentration of H⁺ ions in a solution.
It ranges from 0 to 14.
pH less than 7 indicates acidic solution.
pH equal to 7 indicates neutral solution.
pH greater than 7 indicates basic solution.
Lower pH means stronger acid and higher pH means stronger base.

Importance: pH helps in understanding digestion, tooth decay, soil treatment, plant growth and aquatic life.

Q4. Explain neutralisation reaction with real-life examples.

Answer: Neutralisation is the reaction between an acid and a base to form salt and water.

Acid + Base → Salt + Water

HCl + NaOH → NaCl + H₂O

Real-life examples:

Acidity in stomach: Antacids like milk of magnesia neutralise excess acid.
Bee sting: Bee sting contains acid and can be neutralised by mild base like baking soda.
Wasp sting: Wasp sting is basic and can be neutralised by vinegar.
Soil treatment: Acidic soil is treated with slaked lime.

Q5. Write the preparation, formula and uses of bleaching powder.

Answer:

Chemical name: Calcium oxychloride
Formula: CaOCl₂
Preparation: It is prepared by passing chlorine gas over dry slaked lime.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses:

Used for bleaching cotton and linen in textile industry.
Used for disinfecting drinking water.
Used as an oxidising agent in chemical industries.

Q6. Explain baking soda and washing soda with formula and uses.

Answer:

Salt	Chemical Name	Formula	Uses
Baking soda	Sodium hydrogen carbonate	NaHCO₃	Used in baking powder, antacids and soda-acid fire extinguishers.
Washing soda	Sodium carbonate decahydrate	Na₂CO₃ · 10H₂O	Used in glass, soap and paper industries and for removing permanent hardness of water.

Q7. What is plaster of Paris? Write its preparation and uses.

Answer:

Chemical name: Calcium sulphate hemihydrate
Formula: CaSO₄ · ½H₂O
Preparation: It is prepared by heating gypsum at 373 K.

CaSO₄ · 2H₂O → CaSO₄ · ½H₂O + 1½H₂O

Uses:

Used for making casts for fractured bones.
Used for making toys and decorative materials.
Used for making smooth surfaces on walls.

Important Salt Formation and Uses

Compound	Formula	Preparation / Formation	Important Use
Caustic soda	NaOH	Electrolysis of brine	Soap, paper, textile industry
Bleaching powder	CaOCl₂	Chlorine gas over dry slaked lime	Bleaching and disinfecting water
Baking soda	NaHCO₃	Produced from sodium chloride as raw material	Baking powder and antacid
Washing soda	Na₂CO₃ · 10H₂O	Recrystallisation of sodium carbonate	Removing permanent hardness of water
Plaster of Paris	CaSO₄ · ½H₂O	Heating gypsum at 373 K	Plaster casts and decorative items

Case-Study Based Questions

Case Study 1: Antacid and Stomach Acidity

Rohan often feels acidity after eating spicy food. His doctor advises him to take an antacid containing magnesium hydroxide.

Q1. Which acid is present in the stomach?

Answer: Hydrochloric acid, HCl.

Q2. Why does magnesium hydroxide give relief?

Answer: Magnesium hydroxide is basic. It neutralises excess hydrochloric acid in the stomach.

Q3. Write the neutralisation reaction.

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Case Study 2: Soil pH and Plant Growth

A farmer notices that his crops are not growing properly. A soil test shows that the soil is highly acidic.

Q1. What can be added to neutralise acidic soil?

Answer: Slaked lime, Ca(OH)₂, or quick lime, CaO, can be added.

Q2. Why is pH important for plants?

Answer: Plants require a suitable pH range for proper nutrient absorption and growth.

Q3. What type of reaction occurs when lime is added to acidic soil?

Answer: Neutralisation reaction.

Case Study 3: Tooth Decay

Tooth decay starts when the pH of the mouth falls below 5.5 due to acids produced by bacteria.

Q1. Why does tooth decay occur at low pH?

Answer: Acid dissolves the enamel of teeth when pH falls below 5.5.

Q2. Why are basic toothpastes useful?

Answer: Basic toothpastes neutralise excess acid in the mouth and prevent tooth decay.

Q3. Name the hardest substance in the human body.

Answer: Tooth enamel.

Case Study 4: Water of Crystallisation

Blue crystals of copper sulphate become white when heated. On adding water, the blue colour returns.

Q1. Why do blue copper sulphate crystals become white on heating?

Answer: They lose water of crystallisation and become anhydrous copper sulphate.

Q2. Write the formula of hydrated copper sulphate.

Answer: CuSO₄ · 5H₂O.

Q3. What does this experiment prove?

Answer: It proves the presence of water of crystallisation in copper sulphate crystals.

Critical Thinking Questions

Q1. A solution turns red litmus blue but does not dissolve in water completely. Is it a base or alkali?

Answer: It is a base. It cannot be called an alkali because alkalis are bases that dissolve in water.

Q2. Why is dry HCl gas unable to change the colour of dry blue litmus paper?

Answer: Dry HCl does not produce H⁺ ions without water. Therefore, it does not show acidic behaviour on dry litmus paper.

Q3. Two solutions have pH 3 and pH 5. Which one is more acidic and why?

Answer: The solution with pH 3 is more acidic because lower pH means higher concentration of H⁺ ions.

Q4. A student adds water directly to concentrated sulphuric acid. Why is this dangerous?

Answer: The process is highly exothermic. Sudden heat production may cause acid to splash and cause burns.

Q5. Why does zinc oxide react with both HCl and NaOH?

Answer: Zinc oxide is amphoteric. It reacts with both acids and bases to form salt and water.

Q6. A salt solution has pH less than 7. What can you infer about the salt?

Answer: The salt may be formed from a strong acid and weak base, so its aqueous solution is acidic.

Q7. Why does baking soda make cakes soft and spongy?

Answer: Baking soda releases carbon dioxide gas on heating or reacting with acid. CO₂ bubbles make the cake soft and spongy.

Q8. Why should plaster of Paris be stored in a moisture-proof container?

Answer: Plaster of Paris absorbs moisture and changes into hard gypsum. Therefore, it must be protected from moisture.

Q9. Why does turmeric stain become reddish-brown when soap is rubbed on it?

Answer: Soap is basic in nature. Turmeric acts as a natural indicator and turns reddish-brown in basic medium.

Q10. Why can metal carbonates be used to test acids?

Answer: Acids react with metal carbonates to produce CO₂ gas, which turns lime water milky. This confirms acidic nature.

Previous Year Board Exam Pattern Questions

Q1. A compound X of sodium is used in the kitchen for making crispy pakoras. It is also used for curing acidity. Identify X and write its formula.

Answer: X is baking soda. Its chemical name is sodium hydrogen carbonate and its formula is NaHCO₃.

Q2. Write the chemical equation for the preparation of washing soda from sodium carbonate.

Answer:

Na₂CO₃ + 10H₂O → Na₂CO₃ · 10H₂O

Q3. Why does an aqueous solution of an acid conduct electricity?

Answer: Acids produce ions in water. These ions carry electric current, so aqueous solutions of acids conduct electricity.

Q4. Why is universal indicator better than litmus paper?

Answer: Litmus only tells whether a solution is acidic or basic. Universal indicator shows different colours for different pH values and helps to determine acidic or basic strength.

Q5. What is chlor-alkali process? Name the products formed.

Answer: The electrolysis of aqueous sodium chloride is called chlor-alkali process. The products formed are sodium hydroxide, chlorine gas and hydrogen gas.

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

Q6. Give two uses of bleaching powder.

Answer:

It is used for disinfecting drinking water.
It is used for bleaching cotton and linen in textile industry.

Olympiad Level Questions

Q1. Solution A has pH 2 and solution B has pH 4. How many times more acidic is A than B?

Answer: Each pH unit represents a tenfold change in H⁺ ion concentration. Difference = 2 units. Therefore, solution A is 10 × 10 = 100 times more acidic than solution B.

Q2. A metal oxide reacts with both hydrochloric acid and sodium hydroxide. Identify the type of oxide.

Answer: It is an amphoteric oxide. Example: Al₂O₃ or ZnO.

Q3. A salt solution turns red litmus blue. What type of salt can it be?

Answer: It may be a salt of strong base and weak acid, such as sodium carbonate.

Q4. Why is sodium hydrogen carbonate used in soda-acid fire extinguishers?

Answer: It reacts with acid to produce carbon dioxide gas. CO₂ does not support combustion and helps extinguish fire.

Q5. A colourless solution turns phenolphthalein pink. What will happen if HCl is added drop by drop?

Answer: The pink colour will gradually disappear because HCl neutralises the base present in the solution.

Q6. Why does NaOH solution feel slippery?

Answer: NaOH is a strong base and reacts with oils on the skin to form soap-like substances, so it feels slippery.

Assertion-Reason Questions

Choose the correct option:

A. Both Assertion and Reason are true and Reason is the correct explanation.

B. Both Assertion and Reason are true but Reason is not the correct explanation.

C. Assertion is true but Reason is false.

D. Assertion is false but Reason is true.