Acids and Bases

Properties of Acids

• Corrosive ('burns' your skin)
• Sour taste (e.g. lemons, vinegar)
• Contains hydrogen ions (H⁺) when dissolved in water
• Has a pH less than 7
• Turns blue litmus paper to a red colour
• Reacts with bases to form salt and water
• Reacts with metals to form hydrogen gas
• Reacts with carbonates to form carbon dioxide, water and a salt

Examples of Acids

• Hydrochloric acid (HCl) in gastric juice
• Sulphuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Carbonic acid in soft drink (H₂CO₃)
• Uric acid in urine
• Ascorbic acid (Vitamin C) in fruit
• Citric acid in oranges and lemons
• Acetic acid in vinegar
• Tannic acid (in tea and wine)
• Tartaric acid (in grapes)

Properties of Bases and Alkalis

• Corrosive ('burns' your skin)
• Soapy feel
• Has a pH more than 7
• Turns red litmus paper to a blue colour
• Many alkalis (soluble bases) contain hydroxyl ions (OH^–)
• Reacts with acids to form salt and water

Examples of Bases and Alkalis

• Sodium hydroxide (NaOH) or caustic soda
• Calcium hydroxide ( Ca(OH)₂ ) or limewater
• Ammonium hydroxide (NH₄OH) or ammonia water
• Magnesium hydroxide ( Mg(OH)₂ ) or milk of magnesia
• Many bleaches, soaps, toothpastes and cleaning agents

The pH Scale

Common Acids	pH	Common Bases	pH
Hydrochloric acid Sulphuric acid Stomach juice Lemons Vinegar Apples Oranges Grapes Sour milk White bread Fresh milk	0.1 0.3 1-3 2.3 2.9 3.1 3.5 4 4.4 5.5 6.5	Human saliva Distilled water Blood plasma Eggs Seawater Borax Milk of magnesia Ammonia water Limewater Caustic soda	6-8 7 7.4 7.8 7.9 9.2 10.5 11.6 12.4 14

Indicators

An indicator, when added to an acid, a neutral substance or a base, will change different colours.

INDICATOR	COLOUR IN ACID	COLOUR IN NEUTRAL SOLUTION	COLOUR IN BASE
Litmus	red	purple	blue
Bromothymol Blue	yellow	blue	blue
Phenolphthalein	clear	clear	purple
Universal Indicator	red	Yellow-green	purple

Reactions with Acids

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