CBSE Class 10 Science Acids Bases and Salts Notes

Introduction to Acids, Bases and Salts

Acids, Bases and Salts is Chapter 2 of Class 10 Science and one of the most important chemistry chapters for board exams. This chapter builds on your Class 7 introduction and dives deep into the chemical nature of acids and bases, how they behave in water, how they react with each other and with metals, and how salts are formed and classified.

You encounter acids and bases every day — the citric acid in lemon juice, the acetic acid in vinegar, the hydrochloric acid in your stomach, and the sodium hydroxide in soap. Understanding their chemistry helps you explain these phenomena scientifically.

This chapter introduces the Arrhenius, Bronsted-Lowry, and Lewis theories, the pH scale, indicators, and the chemistry of important salts like NaCl, NaHCO3, Na2CO3, and bleaching powder.

Key Topics Covered

•         Acids and Bases — Definition, Properties, Examples

•         Chemical Properties: Reactions with Metals, Oxides, and Each Other

•         Indicators — Natural and Synthetic

•         The pH Scale and its Significance

•         Neutralisation Reaction and Salt Formation

•         Types of Salts and their Preparation

•         Important Salts: NaCl, NaHCO3, Na2CO3, Bleaching Powder, Plaster of Paris

•         Key Reactions and Board Exam Practice

1.  Acids — Definition, Properties and Examples

Acids are substances that produce hydrogen ions (H+) when dissolved in water. More precisely, they produce hydronium ions (H3O+) because the H+ immediately combines with water. The word 'acid' comes from the Latin acidus meaning sour.

HCl + H2O  ->  H3O+  +  Cl-

H2SO4 + 2H2O  ->  2H3O+  +  SO4(2-)

Properties of Acids

•         Sour taste (never taste acids in lab — dangerous!)

•         Turn blue litmus red and methyl orange pink/red.

•         React with metals to produce hydrogen gas: Zn + H2SO4 -> ZnSO4 + H2(gas)

•         React with bases (neutralisation) to form salt and water.

•         React with metal carbonates to produce CO2 gas: Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2(gas)

•         Conduct electricity in aqueous solution (they are electrolytes).

•         pH less than 7 in aqueous solution.

Common Acids and Their Sources

2.  Bases and Alkalis — Definition, Properties and Examples

Bases are substances that produce hydroxide ions (OH-) when dissolved in water. Bases that are soluble in water are called alkalis. All alkalis are bases but not all bases are alkalis.

NaOH  ->  Na+  +  OH-   (alkali)

Mg(OH)2  -->  MgO + H2O  (base, insoluble in water — not an alkali)

Properties of Bases

•         Bitter taste and soapy/slippery feel.

•         Turn red litmus blue and phenolphthalein pink.

•         React with acids (neutralisation) to form salt and water.

•         React with non-metal oxides (acidic oxides): 2NaOH + CO2 -> Na2CO3 + H2O

•         React with metals (some): 2NaOH + Zn -> Na2ZnO2 + H2(gas)

•         pH greater than 7 in aqueous solution.

Common Bases and Alkalis

3.  Indicators and the pH Scale

Indicators

An indicator is a substance that changes colour depending on whether it is in acidic or basic solution. Indicators help us identify whether a solution is acidic, basic, or neutral.

Olfactory indicators: Onion and clove oil lose their smell in basic solutions. Vanilla retains its smell in acid but loses it in base.

The pH Scale

The pH scale measures the concentration of hydrogen ions (H+) in a solution. It ranges from 0 to 14. Lower pH means more acidic; higher pH means more basic; pH 7 is neutral.

pH  =  -log[H+]

Importance of pH in Daily Life

•         Digestive system: Stomach maintains pH 1.5–3. Excess acid causes acidity; antacids (Mg(OH)2) neutralise it.

•         Tooth decay: Bacteria produce acid (pH < 5.5) which corrodes enamel. Toothpaste (basic) neutralises acid.

•         Soil pH: Most plants grow best at pH 6–7. Acidic soil is treated with slaked lime; alkaline soil with acids.

•         Bee sting: Acidic (formic acid) — treated with baking soda (NaHCO3, basic).

•         Wasp sting: Basic — treated with vinegar (acetic acid, acidic).

4.  Neutralisation and Chemical Reactions

Neutralisation Reaction

A neutralisation reaction occurs when an acid reacts with a base to form a salt and water. The H+ ions from the acid combine with the OH- ions from the base to form water. Heat is always released — neutralisation is exothermic.

Acid + Base  ->  Salt + Water   (Neutralisation)

H+ (aq) + OH- (aq)  ->  H2O (l)   (ionic equation)

HCl + NaOH  ->  NaCl + H2O    (common example)

H2SO4 + 2KOH  ->  K2SO4 + 2H2O

HNO3 + NaOH  ->  NaNO3 + H2O  (tick)

Reaction of Acids with Metals

Acids react with metals that are above hydrogen in the reactivity series to produce a salt and hydrogen gas. The gas burns with a 'pop' sound when a burning splint is brought near it.

Metal + Acid  ->  Salt + Hydrogen gas

Zn + H2SO4  ->  ZnSO4 + H2(gas) [pop sound]

Fe + 2HCl  ->  FeCl2 + H2(gas)

Mg + 2HCl  ->  MgCl2 + H2(gas) [vigorous]

Reaction of Acids with Metal Carbonates and Bicarbonates

Acids react with carbonates and bicarbonates to produce a salt, water, and carbon dioxide gas. CO2 turns lime water milky.

Acid + Metal Carbonate  ->  Salt + CO2(gas) + Water

Na2CO3 + 2HCl  ->  2NaCl + H2O + CO2(gas)

NaHCO3 + HCl  ->  NaCl + H2O + CO2(gas)

CaCO3 + 2HCl  ->  CaCl2 + H2O + CO2(gas)

Reaction of Acids with Metal Oxides

Metal oxides (basic oxides) react with acids to give a salt and water.

Metal Oxide + Acid  ->  Salt + Water

CuO + H2SO4  ->  CuSO4 + H2O  (blue solution formed)

MgO + 2HCl  ->  MgCl2 + H2O

5.  Salts — Formation, Types and Important Salts

A salt is an ionic compound formed when an acid reacts with a base (or a metal, metal oxide, or carbonate). Salts are composed of a metallic cation (positive ion) and an acidic anion (negative ion).

Types of Salts

Important Salts in Detail

Common Salt — Sodium Chloride (NaCl)

Common salt (table salt) is one of the most important industrial chemicals. It is obtained by evaporation of sea water or by mining rock salt deposits.

•         NaCl + H2O + CO2 + NH3 -> NH4HCO3 + NaCl  (Solvay process — produces NaHCO3)

•         Chlor-alkali process: Electrolysis of brine (NaCl solution) gives Cl2, H2, and NaOH.

2NaCl(aq) + 2H2O(l)  -electricity->  2NaOH(aq) + Cl2(g) + H2(g)

•         Products of chlor-alkali process: NaOH (soap, paper), Cl2 (disinfectant, PVC), H2 (fuel, margarine).

Sodium Hydroxide (NaOH) — Caustic Soda

•         Made by: Chlor-alkali process (electrolysis of brine).

•         Uses: Manufacture of soap, paper, artificial fibres, degreasing metals.

•         Reacts with non-metal oxides: 2NaOH + CO2 -> Na2CO3 + H2O

Baking Soda — Sodium Bicarbonate (NaHCO3)

Also called sodium hydrogen carbonate. Mildly alkaline in nature (pH slightly above 7).

•         Preparation: NaCl + H2O + CO2 + NH3 -> NH4Cl + NaHCO3

•         On heating: 2NaHCO3 -> Na2CO3 + H2O + CO2(gas)

•         Uses: Baking (CO2 makes bread/cake rise), antacid, soda water, fire extinguisher.

NaHCO3 is used in fire extinguishers because it releases CO2 gas when heated, which smothers fire.

Washing Soda — Sodium Carbonate (Na2CO3 . 10H2O)

Obtained by heating baking soda and then dissolving in water followed by recrystallisation. Also called soda ash (anhydrous form).

•         Preparation: 2NaHCO3 -> Na2CO3 + H2O + CO2 (then recrystallise with water)

•         Uses: Washing clothes (removes grease), glass/soap/paper manufacturing, water softening.

•         Na2CO3 is a basic salt: Na2CO3 + H2O -> 2NaOH + CO2 (hydrolyses to give alkaline solution).

Bleaching Powder — Ca(OCl)Cl

Bleaching powder is a mixture of calcium hypochlorite and calcium chloride. Made by passing chlorine gas over dry slaked lime.

Ca(OH)2 + Cl2  ->  CaOCl2 + H2O  (bleaching powder)

•         Uses: Bleaching cotton/linen/paper, disinfecting drinking water, making chloroform.

•         Active agent: The hypochlorite ion (OCl-) releases nascent oxygen which bleaches and disinfects.

Plaster of Paris — CaSO4 . (1/2)H2O

Made by heating gypsum (CaSO4 . 2H2O) at 100 degrees C. Sets hard when water is added (undergoes hydration).

CaSO4.2H2O  --(100 deg C)-->  CaSO4.(1/2)H2O + (3/2)H2O

CaSO4.(1/2)H2O + (3/2)H2O  ->  CaSO4.2H2O  (sets hard)

•         Uses: Setting broken bones (plaster casts), making toys/statues, fire-proofing, dentistry.

Plaster of Paris must be stored in a moisture-proof container — it absorbs water and sets hard, becoming useless.

6.  Key Reactions Summary Table

All important reactions from this chapter tested in CBSE board examinations:

7.  Board Exam Practice Questions

These questions cover all question types from CBSE Class 10 Science (Chapter 2) board examinations.

1 Mark Questions

1.       What is the colour of litmus in an acidic solution?

2.       Name the gas evolved when zinc reacts with dilute sulphuric acid.

3.       What is the pH of a neutral solution?

4.       Define an olfactory indicator. Give one example.

5.       Write the chemical name and formula of baking soda.

3 Mark Questions

1.       Explain the chlor-alkali process with a balanced chemical equation. Name the three products and their uses.

2.       Differentiate between acids and bases on the basis of: (a) taste (b) litmus test (c) pH value (d) effect on indicators.

3.       What is neutralisation? Give one example from daily life. Write the chemical equation for the neutralisation of HCl with NaOH.

4.       Explain the preparation and uses of bleaching powder with a balanced equation.

5 Mark Questions

1.       Explain the preparation, properties, and uses of washing soda (Na2CO3). How is it related to baking soda? Write all relevant equations.

2.       What is Plaster of Paris? How is it prepared from gypsum? Write the equation and mention its uses and one precaution for storage.

3.       With the help of chemical equations, explain the reactions of dilute HCl with: (a) Zinc metal (b) Sodium hydroxide (c) Calcium carbonate (d) Magnesium oxide (e) Sodium carbonate.