The Ions in Acids & Alkalis
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
Acids and Hydrogen Ions (H⁺) Acids release hydrogen ions (H⁺) when dissolved in water. These H⁺ ions are responsible for the characteristic properties of acids. Examples of acid dissociation: HCl(aq) → H⁺(aq) + Cl⁻(aq) H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq) HNO₃(aq) → H⁺(aq) + NO₃⁻(aq) Important: The H⁺ ions are what make a solution acidic!
Without H⁺ ions, there would be no acid properties. What Actually Happens to H⁺ Ions in Water? H⁺ ions don't actually exist alone in water. They immediately attach to water molecules to form hydronium ions (H₃O⁺) .
H⁺(aq) + H₂O(l) → H₃O⁺(aq) For simplicity: In IGCSE Chemistry, we usually write H⁺(aq) rather than H₃O⁺(aq), but they mean the same thing - a hydrogen ion in aqueous solution. Alkalis and Hydroxide Ions (OH⁻) Alkalis release hydroxide ions (OH⁻) when dissolved in water.
These OH⁻ ions are responsible for the characteristic properties of alkalis. Examples of alkali dissociation: NaOH(aq) → Na⁺(aq) + OH⁻(aq) KOH(aq) → K⁺(aq) + OH⁻(aq) Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH⁻(aq) Important: The OH⁻ ions are what make a solution alkaline!
Without OH⁻ ions, there would be no alkaline properties. Ammonia - A Special Case Ammonia (NH₃) is an alkali, but it doesn't contain OH⁻ ions in its formula. How does it work? When ammonia dissolves in water: NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq) It reacts with water to produce OH⁻ ions!
This is why ammonia solution is alkaline - it produces OH⁻ ions when dissolved in water, even though ammonia itself doesn't contain OH⁻. How H⁺ Ions Cause Acid Properties The H⁺ ions are responsible for ALL the characteristic reactions of acids: Reaction Role of H⁺ Ions Equation With metals H⁺ ions are reduced to H₂ gas 2H⁺ + Mg → Mg²⁺ + H₂ With carbonates H⁺ ions react with CO₃²⁻ to form CO₂ 2H⁺ + CO₃²⁻ → H₂O + CO₂ With bases H⁺ ions react with OH⁻ to form water H⁺ + OH⁻ → H₂O Indicator color H⁺ ions change indicator colors Litmus red, universal indicator red/orange/yellow How OH⁻ Ions Cause Alkaline Properties The OH⁻ ions are responsible for ALL the characteristic reactions of alkalis: Reaction Role of OH⁻ Ions Equation With acids OH⁻ ions react with H⁺ to form water OH⁻ + H⁺ → H₂O With ammonium salts OH⁻ ions react with NH₄⁺ to release NH₃ OH⁻ + NH₄⁺ → NH₃ + H₂O Indicator color OH⁻ ions change indicator colors Litmus blue, universal indicator blue/purple Feel OH⁻ ions react with oils in skin (saponification) Gives soapy/slippery feel Neutralization in Terms of Ions Neutralization is the reaction between H⁺ ions (from acid) and OH⁻ ions (from alkali) to form water.
Ionic equation for neutralization: H⁺(aq) + OH⁻(aq) → H₂O(l) This is the SAME for ALL acid-alkali neutralizations! Full example: Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Ionic equation (showing all ions): H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l) Net ionic equation (removing spectator ions): H⁺(aq) + OH⁻(aq) → H₂O(l) Spectator ions are ions that don't change during the reaction (Na⁺ and Cl⁻ in this example).
They can be removed to show the essential reaction. Testing for H⁺ and OH⁻ Ions We can test for the presence of H⁺ and OH⁻ ions using indicators: Ion Present Indicator Test Result H⁺ ions (acid) Add blue litmus paper Turns RED H⁺ ions (acid) Add universal indicator Red/orange/yellow (pH OH⁻ ions (alkali) Add red litmus paper Turns BLUE OH⁻ ions (alkali) Add universal indicator Blue/purple (pH > 7) pH and H⁺ Ion Concentration The pH is a measure of the concentration of H⁺ ions in a solution.
pH Value H⁺ Ion Concentration Type Low pH (0-2) HIGH concentration of H⁺ ions STRONG acid Medium pH (3-6) MODERATE concentration of H⁺ ions WEAK acid pH = 7 EQUAL H⁺ and OH⁻ concentrations (very low) NEUTRAL Medium pH (8-10) LOW concentration of H⁺ ions MODERATE concentration of OH⁻ ions WEAK alkali High pH (11-14) VERY LOW concentration of H⁺ ions HIGH concentration of OH⁻ ions STRONG alkali Key relationship: As H⁺ concentration increases, pH decreases (more acidic).
As OH⁻ concentration increases, pH increases (more alkaline). Why Dry Acids Don't Show Acid Properties Pure, dry acids (like solid citric acid crystals) do NOT show acidic properties until dissolved in water.
Why? Dry acids don't release H⁺ ions H⁺ ions can only form when the acid dissolves in water Without H⁺ ions, there are no acid properties Water is needed to ionize the acid molecules Experiment: Dry HCl gas doesn't turn dry blue litmus paper red.
But if you add a drop of water to the litmus paper first, then HCl gas WILL turn it red (because HCl dissolves in water to form H⁺ ions). Common Ion Examples Acid/Alkali Formula Ions Produced Hydrochloric acid HCl H⁺ + Cl⁻ Sulfuric acid H₂SO₄ 2H⁺ + SO₄²⁻ Nitric acid HNO₃ H⁺ + NO₃⁻ Ethanoic acid CH₃COOH H⁺ + CH₃COO⁻ Sodium hydroxide NaOH Na⁺ + OH⁻ Potassium hydroxide KOH K⁺ + OH⁻ Calcium hydroxide Ca(OH)₂ Ca²⁺ + 2OH⁻ Ammonia solution NH₃ NH₄⁺ + OH⁻ (from reaction with water) Universal Indicator and pH Universal indicator changes color depending on the concentrat…
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