Acids, Bases and Salts

Acids and bases

• Indicators: litmus (from lichens), turmeric (turns red with bases), China rose (gudhal), red cabbage, phenolphthalein, methyl orange. Olfactory indicators (onion, vanilla, clove) change smell.
• Alkali: a base soluble in water (NaOH, KOH). All alkalis are bases, but not all bases are alkalis (copper oxide).
• Strong acids: HCl, H2SO4, HNO3. Weak acids: acetic acid, carbonic acid, citric acid. Strong bases: NaOH, KOH.
• Concentrated acid should be added to water slowly, never the reverse (heat is released).

Reactions

• Acid + metal -> salt + hydrogen (pops with a lighted splinter): Zn + H2SO4 -> ZnSO4 + H2.
• Acid + metal carbonate or bicarbonate -> salt + water + CO2 (turns lime water milky): CaCO3 + 2HCl -> CaCl2 + H2O + CO2.
• Acid + base -> salt + water (neutralisation): HCl + NaOH -> NaCl + H2O.
• Acid + metal oxide -> salt + water; base + non-metal oxide -> salt + water (so non-metal oxides are acidic and metal oxides are basic).
• Acids and bases conduct electricity in water because of ions.
• Dilution reduces the concentration of ions per unit volume.

The pH scale

• pH (S. P. L. Sorensen) measures the hydrogen-ion concentration: 0 to 14. 7 = neutral; below 7 is acidic; above 7 is basic. Each unit is ten times in concentration. A universal indicator gives colours for the pH.
• Common pH values: gastric juice 1.5 to 3, lemon juice about 2.2, vinegar about 3, tomato about 4, black coffee about 5, milk about 6.5, pure water 7, blood about 7.4, sea water about 8, baking soda about 8.3, milk of magnesia about 10, household ammonia about 11, sodium hydroxide about 14.
• Acid rain has pH below 5.6. Tooth decay begins when pH in the mouth falls below 5.5; toothpaste (basic) neutralises acid.
• Antacids (milk of magnesia, sodium bicarbonate) neutralise excess stomach acid (HCl).
• Ant and nettle stings contain methanoic (formic) acid; rub baking soda or calamine.
• Soil pH: lime is added to acidic soil; organic manure to alkaline soil.

Acids in daily life

Aqua regia (3 parts HCl : 1 part HNO3) dissolves gold and platinum.

Important salts

• Water of crystallisation is the fixed number of water molecules in a salt crystal.
• Salts of strong acid + strong base are neutral (pH 7); strong acid + weak base are acidic; weak acid + strong base are basic (washing soda pH about 11).
• Hard water has dissolved calcium and magnesium salts; it does not lather well. Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (chlorides, sulphates) by washing soda or ion-exchange.
• Baking powder = baking soda + a mild edible acid (tartaric acid).