Acids, Bases and Salts — Notes

Acids taste sour, turn blue litmus red and give hydrogen ions, $\ce{H+}$, in water. Bases taste bitter, feel soapy, turn red litmus blue and give hydroxide ions, $\ce{OH-}$, in water. Never taste or touch laboratory chemicals.

Indicators show by a change of colour (or smell) whether a solution is acidic or basic.

• Litmus (from lichens; purple when neutral): acid → red, base → blue.
• Synthetic indicators: methyl orange (red in acid, yellow in base); phenolphthalein (colourless in acid and neutral solution, pink in base).
• Natural indicators: turmeric (reddish-brown in base, no change in acid), red cabbage extract, hibiscus (gudhal) petals.
• Olfactory indicators change smell: onion, vanilla essence and clove oil lose their smell in a base but keep it in an acid.

Natural sources of acids: citric acid (lemon, orange), tartaric acid (tamarind), lactic acid (curd), acetic acid (vinegar), methanoic acid (ant and nettle stings).

Metal oxides are basic oxides (they react with acids); non-metal oxides are acidic oxides (they react with bases).

Lime water test: carbon dioxide turns lime water milky (insoluble $\ce{CaCO3}$); with excess carbon dioxide it clears again (soluble $\ce{Ca(HCO3)2}$).

• All acids contain hydrogen, but not every hydrogen compound is an acid: glucose and alcohol solutions do not conduct electricity and are not acidic.
• Acid solutions conduct electricity because they contain ions. The $\ce{H+}$ ion cannot exist alone; it joins water to form the hydronium ion, $\ce{H3O+}$.
• Dry hydrogen chloride gas does not change dry blue litmus; acidic behaviour needs water.
• Bases give $\ce{OH-}$ ions in water. Bases that dissolve in water are alkalis (e.g. $\ce{NaOH}$, $\ce{KOH}$, $\ce{NH4OH}$); not all bases dissolve.
• Dilution (mixing an acid or base with water) lowers the concentration of ions per unit volume. It is highly exothermic: always add acid to water slowly with constant stirring. Adding water to concentrated acid can make the mixture boil and splash, and the container may crack.

• pH is a scale from 0 to 14 that measures how acidic or basic a solution is: 7 neutral, below 7 acidic, above 7 basic.
• The higher the concentration of $\ce{H+}$ ions, the lower the pH. Moving from 7 up to 14, the concentration of $\ce{OH-}$ ions increases.
• Universal indicator is a mixture of indicators that shows a different colour at different pH values, so it gives an approximate pH.
• Strong acids (hydrochloric, sulphuric, nitric) give many $\ce{H+}$ ions; weak acids (acetic, citric, carbonic) give few, at the same concentration. Strong bases: sodium and potassium hydroxide; weak base: ammonium hydroxide.
• Strong (degree of ionisation) is not the same as concentrated (amount of acid per volume of solution).

• Plants and animals: living things survive only in a narrow pH range; the human body works at about pH 7.0 to 7.8. Acid rain (pH below 5.6) makes rivers and lakes acidic and harms aquatic life.
• Soil: plants need a suitable soil pH. Acidic soil is treated with slaked lime or powdered limestone; basic soil with organic matter.
• Digestion: the stomach makes hydrochloric acid to help digestion. Excess acid causes pain and irritation; antacids such as milk of magnesia (magnesium hydroxide), mild bases, neutralise it.
• Tooth decay starts when the pH in the mouth falls below 5.5. Bacteria turn sugar and food particles into acids, which corrode enamel (calcium hydroxyapatite, the hardest substance in the body). Basic toothpastes neutralise the acid.
• Self-defence by animals and plants: bee stings leave an acid; ants and nettle hairs inject methanoic acid. A mild base such as baking soda gives relief; the dock plant is a traditional remedy for nettle stings.

• A salt is formed when an acid reacts with a base. The positive part comes from the base; the negative part from the acid. Salts sharing a positive or negative part form a family (sodium salts; chloride salts).
• pH of salt solutions: strong acid + strong base → neutral ($\ce{NaCl}$, $\ce{K2SO4}$); strong acid + weak base → acidic ($\ce{NH4Cl}$); weak acid + strong base → basic ($\ce{Na2CO3}$, $\ce{NaHCO3}$, $\ce{CH3COONa}$).
• Water of crystallisation is the fixed number of water molecules in one formula unit of a salt. Blue $\ce{CuSO4.5H2O}$ turns white (anhydrous) on heating and blue again when water is added. Gypsum is $\ce{CaSO4.2H2O}$; washing soda is $\ce{Na2CO3.10H2O}$.

Common salt is obtained by evaporating sea water and by mining rock salt.

Sodium hydroxide (chlor-alkali process): electrolysis of brine gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide near the cathode. Uses — hydrogen: fuels, ammonia for fertilisers, hardening oils; chlorine: water treatment, PVC, disinfectants, pesticides, bleaching powder; HCl: cleaning steel, medicines; NaOH: soaps, detergents, paper, artificial fibres, degreasing metals.

Bleaching powder ($\ce{CaOCl2}$): chlorine on dry slaked lime. Uses: bleaching cotton, linen, wood pulp and laundry; oxidising agent; disinfecting drinking water.

Baking soda ($\ce{NaHCO3}$): from salt, water, carbon dioxide and ammonia. A mild non-corrosive base. On heating gives sodium carbonate, water and carbon dioxide. Baking powder = baking soda + mild edible acid (tartaric acid). Uses: baking, antacids, soda-acid fire extinguishers.

Washing soda ($\ce{Na2CO3.10H2O}$): recrystallised sodium carbonate. Uses: glass, soap and paper industries; making borax; cleaning; removing permanent hardness of water.

Plaster of Paris ($\ce{CaSO4.1/2H2O}$): gypsum heated at 373 K. Sets with water into hard gypsum. Uses: casts for fractures, toys, decorative materials, smoothing surfaces. Store in moisture-proof containers.