Acids, Bases and Salts Notes Class 10

Acids, Bases and Salts Revision Notes CBSE Class 10

Acids

Acids are substances that release hydrogen ions (H⁺) when dissolved in water.

Properties of Acids

1. Sour taste
2. Turn blue litmus paper red
3. pH value less than 7
4. Conduct electricity in aqueous solution

Examples: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), Acetic acid (CH₃COOH).

Types of Acids

Strong Acids: Ionize almost completely in water (e.g., HCl, H₂SO₄, HNO₃).

Weak Acids: Ionize partially in water (e.g., CH₃COOH, H₂CO₃).

Chemical Properties of Acids

Reaction with Metals

Acids react with most metals above hydrogen in the reactivity series to produce a salt and hydrogen gas.

Example: 2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

Reaction with Metal Carbonates and Bicarbonates

Acids react with metal carbonates and bicarbonates to produce a salt, water, and carbon dioxide gas.

Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Reaction with Metal Oxides (Basic Oxides)

Acids react with basic oxides to form salt and water. This is a neutralization reaction.

Example: 2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l)

Reaction with Bases

Acids react with bases to form salt and water. This is a neutralization reaction.

Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Reaction with Water

Acids reacts with water to ionise as given below.

HCl + H₂O → H₃O⁺ + Cl⁻

Bases

Bases are substances that release hydroxide ions (OH⁻) when dissolved in water.

Properties of Bases

1. Bitter taste
2. Slippery feel
3. Turn red litmus paper blue
4. pH value greater than 7

Alkalis are bases that are soluble in water (e.g., NaOH, KOH).

Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Ammonium hydroxide (NH₄OH).

Types of Bases

Strong Bases: Ionize almost completely in water (e.g., NaOH, KOH).

Weak Bases: Ionize partially in water (e.g., NH₄OH, Ca(OH)₂).

Chemical Properties of Bases

Reaction with Non-metal Oxides (Acidic Oxides)

Bases react with acidic oxides to form salt and water.

Example: 2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l)

Reaction with Acids:

Bases react with acids to form salt and water (neutralization).

Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Reaction with Certain Metal Salts:

Some bases can precipitate insoluble metal hydroxides from salt solutions.

Example: 2NaOH(aq) + FeCl₃(aq) → Fe(OH)₃(s) + 3NaCl(aq)

Reaction with Water

Bases reacts with water to ionise as given below.

NaOH → Na⁺ + OH⁻

Salts

Salts are ionic compounds formed by the neutralisation reaction between an acid and a base.

Types of Salts

Neutral Salts: Formed from strong acid and strong base (e.g., NaCl, KNO₃). pH is approximately 7.

Acidic Salts: Formed from strong acid and weak base (e.g., NH₄Cl). pH is less than 7.

Basic Salts: Formed from weak acid and strong base (e.g., Na₂CO₃, CH₃COONa). pH is greater than 7.

General Reaction

Acid + Base → Salt + Water

Salts are generally neutral in nature (pH ≈ 7) and do not affect the litmus paper.

Examples: Sodium chloride (NaCl), Potassium nitrate (KNO₃), Calcium carbonate (CaCO₃).

Summary Table: Properties of Acids, Bases, and Salts

Property	Acids	Bases	Salts
Ions in Water	H+	OH−	Cations & Anions
Litmus Effect	Blue → Red	Red → Blue	No effect
pH	< 7	> 7	≈ 7
Taste	Sour	Bitter	Varies
Examples	HCl, H2SO4, CH3COOH	NaOH, KOH, NH4OH	NaCl, KNO3, CaCO3

Neutralisation Reaction

When an acid reacts with a base, they neutralise each other to form salt and water.

Example: NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O(l)

pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, ranging from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral.

Acids: pH < 7 (lower pH indicates stronger acid)
Bases: pH > 7 (higher pH indicates stronger base)
Neutral substances: pH = 7

Importance of pH in Everyday Life

Digestive System: pH in the stomach is highly acidic (1.5-3.5) for protein digestion.

Tooth Decay: Caused by the degradation of tooth enamel by acids produced by bacteria when pH in the mouth falls below 5.5.

Plant Growth: Specific pH ranges are optimal for the growth of different plants. Soil testing is crucial.

Animal Survival: Aquatic life is sensitive to changes in pH of water bodies (e.g., acid rain).

Chemical Properties and Reactions

Acids with Metals: Produce hydrogen gas and form salts.

Acids with Metal Oxides: Form salt and water.

Bases with Non-metal Oxides: Form salt and water.

Dilution: Adding water to acids or bases decreases their concentration and is an exothermic process (releases heat).

Water of Crystallization

The fixed number of water molecules chemically combined with one formula unit of a salt in its crystalline state. These water molecules give the crystals their characteristic shape and color.

Copper Sulphate: CuSO₄.5H₂O (blue)
Gypsum: CaSO₄⋅2H₂O
Washing Soda: Na₂CO₃⋅10H₂O
Ferrous Sulphate: FeSO₄⋅7H₂O (green)

Heating these salts removes the water of crystallization, often leading to a change in color and loss of crystalline structure.

Important Compounds and Their Preparation

Compound	Formula	Preparation/Use
Bleaching Powder	CaOCl2	By action of chlorine on dry slaked lime
Baking Soda	NaHCO3	Used in baking, releases COâ‚‚ on heating
Washing Soda	Na2CO3·10H2O	Used in cleaning, water softening
Plaster of Paris	CaSO4·½H2O	Prepared by heating gypsum, used in casts
Gypsum	CaSO4·2H2O	Used in cement and plaster manufacture
Slaked Lime	Ca(OH)2	Used in whitewashing

Common Salts and Their Uses/Preparation

Sodium Chloride (NaCl)

Preparation: Obtained from seawater by evaporation.
Uses: Common salt, raw material for other chemicals.

Sodium Hydroxide (NaOH - Caustic Soda)

Preparation: Electrolysis of aqueous solution of NaCl (brine) - Chlor-alkali process.
2NaCl(aq) + 2H₂O(l) --electricity→ 2NaOH(aq) + Cl₂(g) + H₂(g)
Uses: Soap and detergent manufacturing, paper industry.

Bleaching Powder (CaOCl₂- Calcium Oxychloride)

Preparation: Reaction of chlorine gas with dry slaked lime.
Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
Uses: Bleaching cotton, disinfectant, oxidizing agent.

Baking Soda (NaHCO₃- Sodium Hydrogen Carbonate):

Preparation: Solvay process.
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Uses: Antacid, baking (releases CO₂ on heating), fire extinguishers.

Washing Soda (Na₂CO₃⋅10H₂O - Sodium Carbonate Decahydrate):

Preparation: Heating baking soda, then recrystallization.
2NaHCO₃(s) --heat→ Na₂CO₃(s) + H₂O(l) + CO₂(g)
Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃⋅10H₂O(s)
Uses: Cleaning agent, glass, soap, and paper industries, removing permanent hardness of water.

Plaster of Paris (CaSO₄⋅ ½H₂O - Calcium Sulphate Hemihydrate):

Preparation: Heating gypsum (CaSO₄⋅2H₂O) to 100° C (373 K).
CaSO₄⋅2H₂O(s) --373K→ CaSO₄⋅ ½H₂O(s) + 1 ½H₂O(l)
Uses: Used by doctors for setting fractured bones, making casts, decorative materials, fire-proofing materials.

Note: If gypsum is heated above 100° C, all water of crystallization is lost, forming dead burnt plaster (CaSO₄), which does not set with water.