Why some substances taste sour and others feel soapy
Acids taste sour and turn blue litmus red; lemon juice, vinegar and curd contain acids. Bases taste bitter, feel soapy and turn red litmus blue; baking soda, soap and milk of magnesia are bases. Indicators show whether a substance is acidic or basic. Besides litmus, natural indicators such as turmeric, which turns red in a base, and china rose petals are useful, and phenolphthalein turns pink in bases.
In water, acids produce hydrogen ions and bases produce hydroxide ions. The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is basic. When an acid reacts with a base, they neutralise each other to form a salt and water, for example HCl + NaOH → NaCl + H₂O. Acids also react with metals to give hydrogen gas and with carbonates to give carbon dioxide. Common salts include sodium chloride, washing soda, baking soda and plaster of Paris.
Why it matters
Antacids neutralise excess stomach acid, farmers add lime to acidic soil, and toothpaste is basic to fight acids that cause tooth decay. An ant sting is treated with a mild base like baking soda. Acid rain harms buildings and lakes, and our bodies work only within a narrow pH range. This topic leads into ionic equilibrium and pH calculations in Class 11.
Key terms
- Indicator
- A substance that changes colour to show whether a solution is acidic or basic.
- pH scale
- A scale from 0 to 14 that measures how acidic or basic a solution is.
- Neutralisation
- The reaction between an acid and a base that produces a salt and water.
- Salt
- An ionic compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.