Properties of Acids & Bases
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
What are Acids? Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They have a pH less than 7. Common acids: Hydrochloric acid (HCl) - stomach acid, cleaning metals Sulfuric acid (H₂SO₄) - car batteries, fertilizers Nitric acid (HNO₃) - making fertilizers and explosives Ethanoic acid (CH₃COOH) - vinegar Citric acid (C₆H₈O₇) - found in citrus fruits What are Bases?
Bases are substances that neutralize acids. They can accept hydrogen ions (H⁺) or release hydroxide ions (OH⁻). Bases have a pH greater than 7. Alkalis are bases that are soluble in water. They release hydroxide ions (OH⁻) when dissolved in water.
Key point: All alkalis are bases, but not all bases are alkalis! Only soluble bases are called alkalis. Common bases and alkalis: Sodium hydroxide (NaOH) - alkali, used in soap making Potassium hydroxide (KOH) - alkali, used in batteries Calcium hydroxide (Ca(OH)₂) - alkali (slightly soluble), limewater Ammonia solution (NH₃(aq)) - alkali, cleaning products Copper oxide (CuO) - base (insoluble), NOT an alkali Zinc oxide (ZnO) - base (insoluble), NOT an alkali The pH Scale The pH scale measures how acidic or alkaline a solution is.
It ranges from 0 to 14. pH Value Type Examples 0-2 STRONG ACID Stomach acid (pH 1-2), concentrated HCl 3-6 WEAK ACID Vinegar (pH 3), rain water (pH 5-6) 7 NEUTRAL Pure water, sodium chloride solution 8-10 WEAK ALKALI Baking soda (pH 8-9), soap (pH 9-10) 11-14 STRONG ALKALI Bleach (pH 12-13), drain cleaner (pH 14) Properties of Acids Property Observation Taste Sour (NEVER taste chemicals in lab!) pH Less than 7 Litmus paper Turns blue litmus RED Universal indicator Red/orange/yellow (depending on strength) With metals Produces hydrogen gas (fizzing) acid + metal → salt + hydrogen With carbonates Produces CO₂ gas (fizzing) acid + carbonate → salt + water + CO₂ With bases Neutralization occurs acid + base → salt + water Electrical conductivity Conducts electricity (contains ions) Properties of Bases/Alkalis Property Observation Taste Bitter (NEVER taste chemicals in lab!) Feel Soapy/slippery (NEVER touch strong alkalis!) pH Greater than 7 Litmus paper Turns red litmus BLUE Universal indicator Blue/purple (depending on strength) With acids Neutralization occurs base + acid → salt + water With ammonium salts Produces ammonia gas (pungent smell) base + NH₄⁺ → NH₃ + H₂O Electrical conductivity Alkalis conduct electricity (contain ions) Reactions of Acids 1.
Acid + Metal → Salt + Hydrogen Example: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g) Observation: Fizzing, metal dissolves, test for H₂ with lit splint (squeaky pop) 2. Acid + Metal Carbonate → Salt + Water + Carbon Dioxide Example: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g) Observation: Fizzing, test for CO₂ with limewater (turns milky/cloudy) 3.
Acid + Base → Salt + Water (Neutralization) Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Observation: Temperature rises (exothermic), pH moves toward 7 4. Acid + Alkali → Salt + Water (Neutralization) Example: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l) Observation: Temperature rises (exothermic), pH moves toward 7 Reactions of Bases/Alkalis 1.
Base + Acid → Salt + Water (Neutralization) Example: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) Observation: Black solid dissolves, blue solution forms 2. Alkali + Ammonium Salt → Salt + Water + Ammonia Example: NaOH(aq) + NH₄Cl(s) → NaCl(aq) + H₂O(l) + NH₃(g) Observation: Pungent smell of ammonia, turns damp red litmus blue Neutralization Reactions Neutralization is the reaction between an acid and a base to produce a salt and water.
It is an exothermic reaction. General equation: Acid + Base → Salt + Water What happens during neutralization: H⁺ ions from the acid combine with OH⁻ ions from the base This forms water: H⁺ + OH⁻ → H₂O The remaining ions form a salt Heat is released (exothermic) pH moves toward 7 (neutral) Testing for Acids and Bases - Indicators Indicator In Acid In Neutral In Alkali Litmus RED Purple BLUE Methyl orange RED Orange YELLOW Phenolphthalein COLORLESS Colorless PINK Universal indicator Red/orange/yellow Green Blue/purple Universal indicator is the most useful as it shows a range of colors for different pH values, not just acid/neutral/alkali.
Common Salts and Their Uses Salt Formula Use Sodium chloride NaCl Table salt, food preservation Calcium carbonate CaCO₃ Making cement, neutralizing soil acidity Ammonium nitrate NH₄NO₃ Fertilizer Copper sulfate CuSO₄ Fungicide, electroplating Sodium hydrogencarbonate NaHCO₃ Baking soda, antacid Everyday Uses of Neutralization Neutralizing excess stomach acid: Stomach produces hydrochloric acid for digestion Too much acid causes indigestion/heartburn Antacids (bases like Mg(OH)₂, CaCO₃) neutralize excess acid Treating acidic soil: Some soils become too acidic for crops Add lime (calcium oxide CaO or calcium hydroxide Ca(OH)₂) Neutralizes acid, improves crop growth Treating bee and wasp stings: Bee stings are acidic → treat with alkali (baking soda) Wasp s…
Interactive revision notes, videos and practice questions load below.