Indicators
Indicators change colour in acids and bases.
Indicator | In acid | In base |
|---|---|---|
Blue litmus | turns red | stays blue |
Red litmus | stays red | turns blue |
Phenolphthalein | colourless | pink |
Methyl orange | red | yellow |
Turmeric | stays yellow | turns reddish-brown |
Olfactory indicators change smell: onion and vanilla lose their smell in a basic solution but not in an acidic one.
Chemical properties of acids and bases
1. With metals. Acids react with reactive metals to give a salt and hydrogen gas.
Zn + H₂SO₄ → ZnSO₄ + H₂
The gas burns with a "pop" sound, the test for hydrogen. Some bases, such as sodium hydroxide, also react with zinc to give hydrogen: 2NaOH + Zn → Na₂ZnO₂ + H₂.
2. With carbonates and hydrogencarbonates. Acids give a salt, carbon dioxide and water.
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
The CO₂ turns lime water milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Passing excess CO₂ makes it clear again, as soluble calcium hydrogencarbonate forms.
3. Neutralisation. An acid and a base react to give a salt and water.
NaOH + HCl → NaCl + H₂O
4. With metal oxides and non-metal oxides. Metal oxides are basic and react with acids (CuO + 2HCl → CuCl₂ + H₂O; the solution turns blue-green). Non-metal oxides are acidic and react with bases, which is why CO₂ reacts with lime water.
What makes an acid an acid?
All acids produce hydrogen ions, H⁺(aq), in water. H⁺ cannot exist alone; it combines with water to form hydronium ions, H₃O⁺. Bases produce hydroxide ions, OH⁻(aq). Bases that dissolve in water are called alkalis (NaOH, KOH, Ca(OH)₂).
That is why acid and base solutions conduct electricity: they contain ions. Glucose and alcohol solutions contain hydrogen but produce no ions, so they are not acids and don't conduct.
Dry HCl gas does not change the colour of dry litmus paper; acidic behaviour needs water.
Diluting acids. Mixing concentrated acid with water releases a lot of heat. Always add acid to water slowly, with stirring, never water to acid, which can make the mixture splash out and cause burns.
The pH scale
The pH scale, from 0 to 14, measures how strongly acidic or basic a solution is.
pH 7 is neutral (pure water).
pH below 7 is acidic; the lower the pH, the stronger the acid.
pH above 7 is basic; the higher the pH, the stronger the base.
Universal indicator shows different colours across the pH range. Strong acids such as HCl give many H⁺ ions; weak acids such as acetic acid give fewer.
Substance | Approximate pH |
|---|---|
Gastric juice | about 1.2 |
Lemon juice | about 2.2 |
Pure water | 7 |
Blood | about 7.4 |
Milk of magnesia | about 10 |
Sodium hydroxide solution | about 14 |
pH in everyday life
Our bodies work within a narrow pH range, 7.0 to 7.8.
Acid rain (pH below 5.6) makes rivers acidic and harms aquatic life.
Plants grow best in soil of a particular pH; farmers add lime to acidic soil.
Digestion: the stomach makes hydrochloric acid. Too much causes acidity, relieved by antacids such as milk of magnesia (magnesium hydroxide), a mild base.
Tooth decay starts when the mouth's pH falls below 5.5, as bacteria break down food left in the mouth. Toothpastes are generally basic.
Stings: a bee sting injects an acid (methanoic acid); rubbing mild base such as baking soda relieves it. Nettle leaves also inject methanoic acid; the dock plant growing nearby is a traditional remedy.
Salts
A salt is formed in a neutralisation reaction. Salts of a strong acid and strong base (NaCl) are neutral; of a strong acid and weak base (NH₄Cl) are acidic; of a weak acid and strong base (CH₃COONa, Na₂CO₃) are basic.
Salt | Formula | Made from | Uses |
|---|---|---|---|
Common salt | NaCl | sea water, rock salt | food; raw material for NaOH, baking soda, washing soda and bleaching powder |
Sodium hydroxide | NaOH | chlor-alkali process: electrolysis of brine gives NaOH, Cl₂ (at the anode) and H₂ (at the cathode) | soaps, paper, degreasing metals |
Bleaching powder | CaOCl₂ | chlorine on dry slaked lime | bleaching cotton and paper; disinfecting drinking water |
Baking soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ | baking powder (with tartaric acid), antacid, soda-acid fire extinguishers |
Washing soda | Na₂CO₃·10H₂O | recrystallising sodium carbonate | glass, soap and paper industries; removing permanent hardness of water |
Plaster of Paris | CaSO₄·½H₂O | heating gypsum (CaSO₄·2H₂O) at 373 K | plaster for fractures, toys, decorative ceilings |
Water of crystallisation is the fixed number of water molecules in one formula unit of a salt. Blue copper sulphate crystals, CuSO₄·5H₂O, turn white on heating as they lose this water, and turn blue again when water is added.
Baking soda on heating gives sodium carbonate, water and CO₂: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ makes cakes rise.
Plaster of Paris sets into hard gypsum when mixed with water. It must be stored in a moisture-proof container.
Summary
Acids turn blue litmus red and produce H⁺ in water; bases turn red litmus blue and produce OH⁻.
Acids react with metals (H₂), carbonates (CO₂) and bases (salt + water).
pH 7 is neutral; below 7 acidic, above 7 basic.
Common salt is the raw material for NaOH, bleaching powder, baking soda and washing soda.