NCERT Class 10 • Science • Chapter "Acids Bases and Salts" | NCERT Class 7 • Science • Chapter "Acids Bases and Salts"

Acids, Bases and Indicators

An acid gives hydrogen ions in water while a base gives hydroxide ions. A base that dissolves in water is called an alkali. Their behaviour is tested with indicators, substances that change colour depending on whether the solution is acidic or basic.

Properties and Indicators

• Acids taste sour, turn blue litmus red, and are corrosive in concentrated form.
• Bases taste bitter, feel soapy, and turn red litmus blue.
• Natural indicators include litmus, turmeric, china rose and red cabbage extract.
• Synthetic indicators include methyl orange and phenolphthalein, which is colourless in acid and pink in base.
• Olfactory indicators such as onion and vanilla change their smell in acidic and basic media.
• Common acids are hydrochloric, sulphuric and nitric acid; common bases are sodium hydroxide, calcium hydroxide and ammonium hydroxide.
• Organic acids such as citric acid in lemon, acetic acid in vinegar and lactic acid in curd are weaker than mineral acids.

Reactions of Acids and Bases

• An acid reacts with a metal to give a salt and hydrogen gas, which burns with a pop sound.
• An acid reacts with a metal carbonate or bicarbonate to give salt, water and carbon dioxide, which turns lime water milky.
• An acid reacts with a metal oxide to give salt and water, which shows that metal oxides are basic.
• A base reacts with a non metal oxide to give salt and water, which shows that non metal oxides are acidic.
• Neutralisation is the reaction of an acid with a base to give salt and water.
• Acids and bases conduct electricity in solution because they furnish free ions.
• Acids show their acidic behaviour only in the presence of water, since water allows the hydrogen ion to be released.

Neutralisation — the reaction between an acid and a base that produces salt and water.