Chapter 2

Acids, Bases and Salts

Acids:- Acids are those substances which emit one or more H+ ions in aqueous solution. Acids are sour in taste. Acids turn blue litmus paper red.

Example:- HCl, HNO3, H2SO4, CH3CaOH 

Acids found in some natural substances:-

Alkalis:- Alkalis are bitter in taste. Alkalis turn red litmus paper blue. Alkalis are those substances which emit [OH- ] ions in aqueous solution.

Example:-- NaOH, KOH, Ca(OH)2, Mg (OH)2

Mg (OH)2 in removing acidity of stomach

Ca(OH)2 in removing acidity of soil

Indicator: Substances which indicate the presence of acid and alkali in the solution by change in colour or change in smell are called indicators.

Types of indicators:-

Natural indicators

Those which are obtained from natural sources are called natural indicators.

Example:- Litmus, turmeric, cabbage, petunia, geranium

Litmus solution is a purple coloured dye which is obtained from a plant of the thallophyta group.

Synthetic indicators

Those indicators which are prepared chemically are called synthetic indicators. Example: Methyl orange and phenolphthalein

Olfactory indicators

Substances whose odor changes in acidic or alkaline medium are called olfactory indicators.

Example: Onion, vanilla etc.

Properties of acids:-

* Acids are sour in taste.

* They dissolve in water and give H+ ions.

* They turn blue litmus paper red.

* Their aqueous solution conducts electricity.

Properties of bases:-

* They are bitter in taste.

* They dissolve in water and give OH- ions.

* They turn red litmus paper red.

* They feel like soap when touched.

Reaction of acids with metals:-

When acid reacts with metal, hydrogen gas is released. Metal + dilute acid → metal nitrate + hydrogen gas

Example :- 

  Zn + H2SO4 →ZNS04 + H2 

Fe + 2HCl → Fecl2+ H2 

Reaction of alkalis with metals :-

When alkalis react with metals, hydrogen gas is evolved.

Alkali + metal Metal oxide + H2 gas

Example :- 

Zn+ 2NaOH → Na2ZnO2 + H2 gas

[ Sodium zincate ]

2Al + 2NaOH →2NaAlO2 + 3H2 

Reaction of metal carbonates with acids :-

All metal carbonates react with acids to form corresponding salts, carbon dioxide and water.

Metal carbonate + acid → salt + C02 + H20

Example :- CaCO3 + 2HCl → CaCl2 + CO2 + H2