Introduction
This chapter deals with the chemical properties of acids and bases, how they interact with metals, metal carbonates, and each other (neutralisation). You will learn about the pH scale for measuring acidity and basicity, the importance of pH in everyday life, and the preparation and properties of important salts like baking soda, washing soda, bleaching powder, and Plaster of Paris.
Acids and Bases: Properties and Reactions
Acids are substances that produce hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water. Bases produce hydroxide ions (OH-) in water. Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy, and turn red litmus blue. Important reactions of acids: (1) Acid + Metal -> Salt + Hydrogen gas (e.g., 2HCl + Zn -> ZnCl2 + H2). (2) Acid + Metal carbonate -> Salt + Water + CO2 (e.g., 2HCl + Na2CO3 -> 2NaCl + H2O + CO2). (3) Acid + Metal hydrogen carbonate -> Salt + Water + CO2. (4) Acid + Base -> Salt + Water (neutralisation). (5) Acid + Metal oxide -> Salt + Water. Bases react similarly with non-metal oxides.
Key Points
- •Acids produce H+ (or H3O+) in water; bases produce OH-
- •Acid + Metal -> Salt + H2 (hydrogen gas — pop test)
- •Acid + Carbonate/Bicarbonate -> Salt + H2O + CO2 (turns lime water milky)
- •Neutralisation: Acid + Base -> Salt + Water
- •All acids generate H+ only in aqueous solution (dry HCl gas has no effect on litmus)
- •Strong acids: HCl, H2SO4, HNO3; Weak acids: CH3COOH, H2CO3
Worked Example
Test for CO2: Pass the gas through lime water Ca(OH)2. Ca(OH)2 + CO2 -> CaCO3 (white precipitate, milky) Excess CO2: CaCO3 + H2O + CO2 -> Ca(HCO3)2 (soluble, turns clear again)
Watch Out
Remember: dry HCl gas does NOT change the colour of dry litmus paper. It only shows acidic behaviour in the presence of water. This is a very common board exam question.
The pH Scale
The pH scale measures the concentration of hydrogen ions in a solution on a scale from 0 to 14. pH stands for 'potenz des Wasserstoffs' (power of hydrogen). pH 7 is neutral (pure water). pH less than 7 is acidic (lower pH means more acidic). pH greater than 7 is basic/alkaline (higher pH means more basic). The pH scale is logarithmic — a change of 1 unit means a 10-fold change in H+ concentration. pH is crucial in everyday life: our stomach uses HCl (pH ~1.2) for digestion; blood must maintain pH 7.35-7.45; tooth enamel corrodes below pH 5.5; soil pH affects plant growth; and acid rain (pH < 5.6) damages buildings and aquatic life.
Key Points
- •pH 0-6: acidic; pH 7: neutral; pH 8-14: basic
- •Stomach HCl: pH ~1.2; Blood: pH 7.35-7.45
- •Tooth enamel corrodes below pH 5.5 (use basic toothpaste)
- •Bee sting is acidic (treat with baking soda); wasp sting is basic (treat with vinegar)
- •Acid rain: pH < 5.6 (due to SO2 and NO2 in atmosphere)
- •pH paper or universal indicator used to measure pH
Worked Example
Why does a person get stomach pain during indigestion? Excess HCl is produced in the stomach, lowering the pH. Antacids like Milk of Magnesia (Mg(OH)2) or baking soda (NaHCO3) are taken to neutralise the excess acid and raise the pH back to normal.
Watch Out
pH of blood is 7.35-7.45 (slightly basic). Any deviation leads to serious health issues. This fact appears in 1-mark MCQs frequently.
Salts: Preparation and Properties
A salt is formed when the H+ of an acid is replaced by a metal ion or ammonium ion. The pH of a salt solution depends on the strength of the acid and base that formed it: strong acid + strong base gives neutral salt (pH 7); strong acid + weak base gives acidic salt (pH < 7); weak acid + strong base gives basic salt (pH > 7). Salts can be classified as acidic, basic, or neutral based on this. A family of salts shares the same positive or negative ion (e.g., NaCl and Na2SO4 are both sodium salts; NaCl and KCl are both chloride salts).
Key Points
- •Salt = acid H+ replaced by metal/ammonium ion
- •Strong acid + strong base = neutral salt (NaCl, pH 7)
- •Strong acid + weak base = acidic salt (NH4Cl, pH < 7)
- •Weak acid + strong base = basic salt (Na2CO3, pH > 7)
- •Common salt (NaCl) is the raw material for many important chemicals
- •Rock salt is mined; sea salt from evaporation
Watch Out
When asked about pH of salt solutions, think about which acid and base formed it. NaCl (strong+strong) = 7; NH4Cl (strong+weak) < 7; Na2CO3 (weak+strong) > 7.
Important Chemical Compounds from Common Salt
Several industrially important compounds are derived from common salt (NaCl). (1) Sodium hydroxide (NaOH): produced by the chlor-alkali process — electrolysis of brine (concentrated NaCl solution). At the anode, chlorine gas is produced; at the cathode, hydrogen gas is produced; and NaOH forms near the cathode. (2) Baking soda (NaHCO3): used in baking (releases CO2 on heating), as antacid, and in fire extinguishers. (3) Washing soda (Na2CO3.10H2O): prepared by heating baking soda and recrystallising; used in glass, soap, and paper industries, and as a cleaning agent. (4) Bleaching powder (CaOCl2): made by passing Cl2 through dry Ca(OH)2; used for water purification and bleaching. (5) Plaster of Paris (CaSO4.1/2H2O): made by heating gypsum (CaSO4.2H2O) at 373K; used for casts, moulds, and toys.
Key Points
- •Chlor-alkali: 2NaCl(aq) + 2H2O -> 2NaOH(aq) + Cl2(g) + H2(g)
- •Baking soda: NaHCO3; used as antacid and in baking (releases CO2 when heated)
- •Baking powder = baking soda + tartaric acid (acid neutralises bitter taste of Na2CO3)
- •Washing soda: Na2CO3.10H2O (has 10 water of crystallisation)
- •Bleaching powder: Ca(OH)2 + Cl2 -> CaOCl2 + H2O
- •POP: CaSO4.2H2O --(373K)--> CaSO4.1/2H2O + 3/2H2O
- •POP sets: CaSO4.1/2H2O + 3/2H2O -> CaSO4.2H2O (exothermic, expands)
Worked Example
Why does Plaster of Paris harden when mixed with water? CaSO4.1/2H2O + 3/2H2O -> CaSO4.2H2O (gypsum) The half-hydrated calcium sulphate absorbs water and re-forms gypsum crystals. The process is exothermic and the gypsum crystals interlock, causing the paste to harden.
Watch Out
Water of crystallisation is the most tested concept from this section. Gypsum has 2 (CaSO4.2H2O), washing soda has 10 (Na2CO3.10H2O), copper sulphate has 5 (CuSO4.5H2O — blue crystals turn white when heated).
Quick Summary
- ✓Acids produce H+ in water; bases produce OH-; neutralisation gives salt + water
- ✓pH scale: 0-14; pH 7 is neutral; below 7 acidic; above 7 basic
- ✓pH of blood (7.4), stomach (1.2), and tooth decay threshold (5.5) are important values
- ✓Chlor-alkali process: electrolysis of brine gives NaOH, Cl2, and H2
- ✓Baking soda (NaHCO3): antacid, baking; Washing soda (Na2CO3.10H2O): cleaning
- ✓Bleaching powder (CaOCl2): water purification; POP (CaSO4.1/2H2O): moulds and casts
- ✓Water of crystallisation: fixed number of water molecules in crystal structure
- ✓Salt pH depends on strength of parent acid and base
Key Formulas
Chlor-alkali: 2NaCl + 2H2O -> 2NaOH + Cl2 + H2
Bleaching powder: Ca(OH)2 + Cl2 -> CaOCl2 + H2O
POP preparation: CaSO4.2H2O --(373K)--> CaSO4.1/2H2O + 3/2H2O
POP setting: CaSO4.1/2H2O + 3/2H2O -> CaSO4.2H2O
Baking soda: 2NaHCO3 --(heat)--> Na2CO3 + H2O + CO2
Acid + Metal -> Salt + H2
Acid + Carbonate -> Salt + H2O + CO2
Ready to practice?
Test your understanding with questions