Acids, Bases, and Indicators

• Sour and bitter tastes in food are due to acids and bases.
• Acids and bases can neutralize each other's effects.
• Acids are sour and turn blue litmus red; bases are bitter and turn red litmus blue.
• Litmus is a natural indicator; turmeric is another.
• Curry stains turn reddish-brown with soap (basic), then yellow again with water.
• Methyl orange and phenolphthalein are synthetic indicators.

Litmus

• Litmus solution is a purple dye from lichen (Thallophyta division).
• Litmus is purple when neither acidic nor basic.
• Red cabbage leaves, turmeric, Hydrangea, Petunia, and Geranium petals indicate acidity/basicity.
• These are acid-base indicators or simply indicators.

Acids and Bases in the Laboratory (Activity 2.1)

• Solutions to collect: Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO3), acetic acid (CH3COOH), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)2], and ammonium hydroxide (NHOH).
• Indicators change color depending on whether a substance is acidic or basic.
• Olfactory indicators change odor in acidic or basic media

Olfactory indicators (Activity 2.2)

• Finely chopped onions in a plastic bag with cloth strips can test for acids/bases after being refrigerated overnight.
• Dilute HCl on one strip and dilute NaOH on another.
• Vanilla, onion, and clove can be used as olfactory indicators based on odor changes.

Reaction of Acids and Bases with Metals (Activity 2.3)

• Dilute H₂SO₄ reacts with zinc granules, releasing gas.
• The evolved gas forms bubbles in soap solution and burns with a pop sound when a burning candle is brought near. This confirms the gas is hydrogen.
• Repeat with acids (HCl, HNO3, CH3COOH); observations may vary.
• Metal + Acid → Salt + Hydrogen gas
• The metal displaces hydrogen atoms in acids, forming a salt.

Sodium Hydroxide and Metals

• 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) produces sodium zincate and hydrogen gas.
• Not all metals react.

Metal Carbonates and Hydrogencarbonates with Acids (Activity 2.5)

• Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
• NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
• CO₂ gas turns lime water milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
• Excess CO₂ leads to CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) (soluble).
• Metal carbonate/Hydrogencarbonate + Acid → Salt + Carbon dioxide + Water.
• Limestone, chalk, and marble are different forms of calcium carbonate.

Acid-Base Reactions

• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
• Neutralization of a base by acid, and vice versa.
• Base + Acid → Salt + Water

Reaction of Metallic Oxides with Acids (Activity 2.7)

• Metal oxide + Acid → Salt + Water
• CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
• Copper oxide dissolves in hydrochloric acid.
• The solution turns blue-green (copper(II) chloride).
• Metallic oxides + Acids → Salts + Water
• Metallic oxides are basic oxides.
• Calcium hydroxide (a base) reacts with carbon dioxide to produce a salt and water.
• Non-metallic oxides are acidic.

Acids and Common Properties

• Acids produce hydrogen gas when reacting with metals.
• Acids contain H⁺ ions (cations), where Cl- in HCl, NO3- in HNO3, SO42- in H2SO4, CH3COO- in CH3COOH.
• Acids produce H+(aq) responsible for acidic properties.
• H+ ions are responsible for acidity, but require water to seperate.
• HCl + H₂O → H₃O⁺ + Cl-
• H⁺ + H₂O → H₃O⁺ (hydronium ion)
• Acids produce hydronium ions (H₃O⁺) or H+(aq) in water.
• Bases release hydroxide ions
• NaOH(s) + H₂O → Na+(aq) + OH-(aq)
• KOH(s) + H₂O → K+(aq) + OH- (aq)
• Mg(OH)₂(s) + H₂O → Mg²+(aq) + 2OH⁻(aq)

Indicators

• Bases generate hydroxide (OH-) ions in water.
• Soluble bases are alkalis.
• Acid + Base → Salt + Water
• HX + MOH → MX + HOH
• H+(aq) + OH-(aq) → H₂O(l)

Acid and Base Dilution (Activity 2.10)

• Dissolving acid/base in water is highly exothermic.
• Always add acid slowly to water, stirring constantly, to avoid splashes/burns.
• Mixing acid/base with water decreases H3O+/OH- concentration per volume (dilution).

pH Scale

• Universal indicators distinguish acids/bases through color changes at H⁺ ion concentrations.
• pH scale measures hydrogen ion (H⁺) concentration, ranging from 0 (acidic) to 14 (alkaline).
• 'p' in pH means 'potenz' (power) in German.
• pH indicates a solution's acidity or basicity.
• hydronium ion concentration is inversely correlated to the pH value.
• Neutral solution is pH 7
• Acidic solution is below pH 7.
• pH value increases from 7 to 14 = increase in OH- ion concentration or strength of alkali.
• Universal indicator paper measures pH.

pH Levels

• Gastric juice approx pH 1.2.
• Lemon juice approx pH 2.2.
• Pure water approx pH 7.4.
• Milk of magnesia approx pH 10.
• Sodium hydroxide solution approx pH 14.
• Acid/base strength depends on H⁺/OH⁻ ions produced.
• Acids producing more H⁺ ions are strong; less results in weak acids.

pH in Everyday Life

• Body functions optimally between pH 7.0-7.8.
• Organisms require narrow pH range.
• Acid rain: Rainwater with pH < 5.6.
• Acid rain lowers river pH affecting aquatic life.

Soil pH (Activity 2.12)

• Plants need specific pH range.
• To determine ideal soil pH, collect soil samples, shake them with water, filter, and test filtrate pH with universal indicator paper.

pH in Digestive System

• Stomach produces hydrochloric acid to digest of food .
• Indigestion: too much acid production.
• Antacids (bases) neutralize excess acid; magnesium hydroxide (milk of magnesia) is a mild base.

pH and Tooth Decay

• Tooth decay begins when mouth pH drops below 5.5.
• Tooth enamel (calcium hydroxyapatite) corrodes at pH < 5.5.
• Bacteria degrades sugars, producing acids.
• Cleaning teeth and basic toothpastes neutralize acid.

Animal Self Defense

• Honeybee stings injects acid, use baking soda(mild base relieves symptoms).
• Nettle leaves inject methanoic acid, causing burning pain.

Occurring Acids

• Vinegar contains acetic acid.
• Orange contains citric acid.
• Sour milk(curd) contains lactic acid.

Family of Salts

• Salts with same +ve or -ve radicals belong to a family.
• e.g. NaCl and Na₂SO₄ are sodium salts. NaCl and KCl are chloride salts.

pH of Salts (Activity 2.14)

• Collect salt samples (NaCl, KNO₃, AlCl₃, ZnSO₄, CuSO₄, CH₃COONa, Na₂CO₃, NaHCO₃).
• Test their solubility in distilled water.
• Test the action of solutions on litmus and find pH using pH paper.
• Strong acid + strong base = neutral (pH 7).
• Strong acid + weak base = acidic pH < 7.
• Weak acid + strong base = basic pH > 7.

Common Salt

• NaCl is made by combination of HCl(Hydrochloric Acid) and NaOH(Sodium Hydroxide)
• Important raw material
• Sodium hydroxide, baking soda, washing soda, bleaching powder are made from common salt
• Seawater contains dissolved salts; NaCl separated.
• Rock salt are large crystals often brown from impurities.
• Rock salt formed when seas dried up.
• Used in Dandi March

Sodium Hydroxide

• Sodium chloride (brine) electrolysis forms sodium hydroxide (chlor-alkali process).
• 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
• Chlorine gas (Cl₂) at anode, hydrogen gas (H₂) at cathode, NaOH near cathode.

Bleaching Powder

• Chlorine + dry slaked lime = bleaching powder [Ca(OCl)₂].
• 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O

Bleaching Powder Uses

• Bleaching in textile and paper industries.
• Oxidizing agent.
• Disinfecting drinking water.

Baking Soda

• Baking soda (sodium hydrogencarbonate/NaHCO₃) is made from NaCl.
• NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
• NaHCO₃ neutralizes excess acid.
• 2NaHCO₃ (heat) → Na₂CO₃ + H₂O + CO₂
• Baking powder: NaHCO₃ + mild acid (tartaric acid).
• NaHCO3 + H⁺ → CO₂ + H₂O + Sodium salt of acid
• CO₂ causes softness and rising.

Washing Soda

• Another chemical from sodium chloride is Washing soda: Na₂CO₃·10H₂O
• Na₂CO₃·10H₂O is made when you heat baking soda, then you recrystalise
• Used in glass, soap and paper industries.
• Used to make borax and is also a cleaning agent for domestic purposes.
• Softens hard water.
• Used in glass, paper, and soap.
• Used in borax production and domestic cleaning.
• Removes permanent hardness of water.

Salt Crystals

• Heat copper sulphate crystals = water removal = white salt.
• Add water: blue color returns.
• Water of crystallisation = fixed water molecules in one salt.
• Hydrated copper sulphate: CuSO₄·5H₂O

Gypsum and Plaster of Paris

• Gypsum (CaSO₄·2H₂O) has water of crystallisation.
• Heat gypsum (373 K) = calcium sulphate hemihydrate (CaSO₄·½H₂O) = Plaster of Paris.
• Plaster of Paris + water = hard gypsum.
• Doctors use for casts, made in toys