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 + H2O 

Na2CO3 + 2HCl→NaCl + Co2 + H2O 

Reaction of metal hydrogen carbonate with acid :-

All metal carbonates and hydrogen carbonates react with acid to form corresponding salt, carbon dioxide and water.

Metal hydrogen carbonate + acid  →salt + CO2 + H2O

Example :-

2NaHCO3 + H2SO4 → Na2SO4+CO2 + H2O

NaHCO3 + HCl → NaCl + CO2 + H2O

Neutralization reaction :-

Acids and bases react with each other to form salt and water. This reaction is called neutralization reaction.

Base + Acid → Salt + Water

Example :-

NaOH + HCl →NaCl + H2O 

2NaOH + H2SO4 →Na2SO4 + 2H2O

Reaction of metallic oxides with acids: -

Like the reaction of base and acid, metallic oxides react with acids to form salt and water.

Metal oxide + acid →salt + water

Na2O+2HCl → 2NaCl+H2O 

Reaction of non-metallic oxide with base :-

Bases react with non-metallic oxides to form salt and water

Base + non-metallic oxide →salt + water 

Example :-2NaOH + CO2 →Na2CO3 + H2O

2KOH+ SO3 →K2SO4 + H2O 

Dilution :-

On adding acid or base to water, the concentration of ion per unit volume decreases. This process is called dilution.

pH scale :-

The full form of pH is Potenz Hydronium. In this pH, P is the indicator. Potenz means power.

Def :- 1 The scale used to measure the concentration of hydrogen ions in a solution is called pH scale.

Def :- 2 The negative logarithm of the concentration of hydrogen ions is called pH scale.

Note :- The higher the concentration of hydrogen ions, the lower will be its pH.

⇒ For any neutral solution, the pH value will be 7.

⇒ If the solution has a pH value less than 7, then this solution will be acidic.

⇒ If the solution has a pH value more than 7, then this solution will be alkaline.

pH value of the following solutions :-

Solution pH value

Gastric juice 1.2

Lemon juice 2.2

Pure water 7

Blood 7.4

Milk of magnesia 10

Sodium hydroxide 14

Hydrochloric acid 0

Strong acid :

The acids which produce more number of H+ ions are called strong acids. 

Example :-HCl, H2SO4 

Weak acid :-

The acids which produce less number of H+ ions are called weak acids. 

Example :- CH3COOH 

Acid Rain:--

When the pH value of rainwater falls below 5.6, it is called acid rain.

Bad effects of acid rain:-

When acid rain water flows into a river, the pH value of the river water falls. Survival of aquatic organisms becomes difficult in such a river.

Due to acid rain, the Taj Mahal made of marble has started turning black. This is because calcium carbonate reacts with the acid present in the rain.

Importance of pH in daily life:-

• The pH value of our body remains active between 7.0 to 7.8.

• When the pH value of rainwater falls below 5.6, it is called acid rain.

• Plants require a specific pH value.

• Our stomach produces HCL. This acid helps in the digestion of our food without causing harm. In case of indigestion, a large amount of HCL is produced in our stomach. Due to which stomach pain and burning sensation is felt. To get rid of it, bases like "antacid" are used in our stomach.

• When the pH value of mouth is less than 5.5, the enamel of teeth starts corroding.

• The sting of a bee releases methanoic acid. Relief is obtained by using a weak base like baking soda on the stung part.

Substances made from common salt: -

Sodium hydroxide is also called caustic soda. Its chemical formula is NaOH. On passing electricity through an aqueous solution of sodium chloride, it disintegrates and produces sodium hydroxide. This is called "Chlor-alkali process".