Acids, Bases and Salts: pH Scale and Common Salts

Acids, bases and salts is the chemistry chapter that gives exams the largest number of one line facts: which acid is in which fruit, what an indicator turns to, what the pH of a substance is, and which household chemical has which formula. The definitions come first, but the marks lie in the tables.

What acids and bases are

Arrhenius defined an acid as a substance that gives hydrogen ions in water and a base as one that gives hydroxide ions. The wider Bronsted and Lowry definition calls an acid a proton donor and a base a proton acceptor, while Lewis called an acid an electron pair acceptor and a base an electron pair donor. A base that dissolves in water is called an alkali, so all alkalis are bases but not every base is an alkali. Acids taste sour, turn blue litmus red and conduct electricity in solution because of free ions. Bases taste bitter, feel soapy and turn red litmus blue. Strong acids such as hydrochloric, sulphuric and nitric acid ionise almost completely, while weak acids such as acetic, carbonic and citric acid ionise only partly; the same difference separates sodium hydroxide from ammonium hydroxide.

Indicators

Onion and vanilla are olfactory indicators, whose smell is destroyed by a base, and litmus itself is obtained from lichens.

Reactions worth memorising

• Acid with an active metal gives a salt and hydrogen gas, which burns with a pop sound
• Acid with a metal carbonate or hydrogen carbonate gives a salt, water and carbon dioxide, which turns lime water milky
• Acid with a base gives salt and water; this is neutralisation
• A metal oxide with an acid gives salt and water, so metal oxides are basic
• A non-metal oxide with a base gives salt and water, so non-metal oxides are acidic

The pH scale

The pH scale, proposed by Sorensen, runs from 0 to 14 and measures the concentration of hydrogen ions; pH is the negative logarithm of that concentration. A value below 7 is acidic, 7 is neutral and above 7 is basic, and each step of one unit means a tenfold change, so a solution of pH 3 has a thousand times more hydrogen ions than one of pH 6. Human blood is close to 7.4 and stays there within narrow limits. Gastric juice in the stomach is strongly acidic because of hydrochloric acid, which helps digestion and kills germs; too much of it causes acidity, treated with an antacid such as milk of magnesia, which is magnesium hydroxide. Tooth enamel, made of calcium phosphate, begins to corrode when the pH in the mouth falls below about 5.5, which is why toothpastes are mildly basic. Rain water with a pH below about 5.6 is called acid rain. Most crops need a soil pH near neutral, and an acidic soil is treated with quick lime or slaked lime.

Acids in everyday substances

Salts and the important ones

A salt is formed when the hydrogen of an acid is replaced by a metal, and its nature follows its parents. A strong acid with a strong base gives a neutral salt such as sodium chloride; a strong acid with a weak base gives an acidic salt such as ammonium chloride; a weak acid with a strong base gives a basic salt such as sodium acetate. Common salt, sodium chloride, is the raw material of the chlor alkali process, in which electricity passed through brine gives sodium hydroxide, chlorine and hydrogen.

Bleaching powder is made by passing chlorine over dry slaked lime. Plaster of Paris is made by heating gypsum, calcium sulphate dihydrate, to about 373 kelvin, that is a hundred degrees celsius; heating it further destroys its setting property. Adding water to plaster of Paris turns it back into gypsum, which is why it sets hard. Water of crystallisation is the fixed number of water molecules in a crystal, seen in washing soda, gypsum and blue vitriol, which is copper sulphate pentahydrate.