Acid, Bases and Salts Class 10 Notes Science Chapter 2


1. Introduction: The term acid comes from the Latin term ‘acere’, which means “sour”. In everyday life, we come across many compounds that chemists classify as acids.
Bases are compounds that taste bitter, e.g., milk of magnesia.
Salts also have wide applications, e.g., ammonium chloride is used as an electrolyte in dry cells, sodium bicarbonate (baking powder) in the manufacture of glass etc.

2. Chemical properties of acids :
(i) Action of metals: Metals generally react with dilute acids to form their respective salts and liberate hydrogen gas.
Metal+ Acid—> Salt+ Hydrogen
(ii) Action with metal oxides (Basic oxides): Metal oxides are generally basic oxides. These oxides get neutralized when they react with acids. These reactions are mostly carried upon heating.
Basic oxide+ Acid —> Salt+ Water (Neutralisation reaction)
(iii) Action with metal carbonates and metal hydrogen carbonates: Acids react with carbonates and hydrogen carbonates to form their respective salts, water, and carbon dioxide gas.
Carbonate/bicarbonate + Acid—-> Salt + water + carbon dioxide

3. Chemical properties of bases:
(i) Reaction of metals with bases: Metals (e.g. Zn, Al, Sn) dissolve in NaOH (an alkali) to liberate hydrogen gas.

(ii) Action with acids: Bases combine with acids to form salt and water only. It is a neutralization reaction.
Base + Acid—->Salt+ Water

4. Strength of acids and bases : The strength of an acid or a base can be easily estimated by making use of universal indicator which is a mixture of several indicators. The universal indicator show different colours at different concentrations of hydrogen ions in solution

5. pH Scale: It is a scale that is used for measuring the H+ ion (Hydrogen ion) concentration of a solution.
The term pH stands for “potential” of “hydrogen”. It is the amount of hydrogen ions in a particular solution.
For acids: pH< 7 : For bases: pH> 7
For neutral substances: pH= 7

6. Importance of pH in daily life
(i) Blood pH: For proper functioning, our body needs to maintain a blood pH between 7.35 to 7.45. Values of blood pH greater than 7.8 or less than 6.8 often results in death.
(ii) Acid rain: When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into rivers, it lowers the pH of river water.
(iii) pH in our digestive system: We know that hydrochloric acid (HCl) produced in our stomach helps in digestion of food without harming stomach. However excess of acid causes indigestion and leads to pain as well as irritation. To get rid of this people use bases called “antacids”.
(iv) pH of the soil: For healthy growth, plants require a specific pH. Soils with high peat content or iron minerals or with rotting vegetation tend to become acidic and the soil pH can reach as low as 4.
(v) pH change as the cause of tooth decay: Tooth decay starts when the pH of the mouth is lower than 5.5.
(vi) Self defence by animals and plants through chemical warfare: Bee­ sting leaves an acid (formic acid or methanoic acid, HCOOH) which causes pain and irritation. The use of mild base like baking soda on the stung area gives relief

7. Salts: A salt is an ionic compound that dissociates to yield a positive ion other than hydrogen ion (H+) and a negative ion other than hydroxyl ion (OH-)
e.g. NaCl. Salts are formed by the reaction of acid and base which is also known as neutralisation reaction.
(i) Sodium hydroxide or Caustic soda: It is prepared on a commercial scale by the electrolysis of a strong solution of sodium chloride (NaCl) also called brine. The process is called the chlor-alkali process.
The overall reaction taking place is :

(a) Sodium hydroxide is mostly used as a base in the laboratory.
(b)It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soap, detergents, etc.

(ii) Baking soda or Sodium hydrogen carbonate (NaHCO3) : Sodium bicarbonate, commonly known as baking soda, is a chemical compound
with the formula NaHCO3. It is a salt composed of a sodium cation and a bicarbonate anion. The overall reaction taking place :

When NaHCO3 is heated, the following reaction occurs:

The above reaction occurs when baking soda is heated during cooking.
(a) In baking powder: The most practical use of baking soda is as a leavening agent in baking.
(b) As an antacid: Baking soda reacts with acid due to its alkaline nature and neutralizes acidity (i.e. acts as an antacid).
(c) In fire extinguishers: It is used in soda-acid fire extinguisher.

(iii) Washing soda or sodium carbonate: Sodium carbonate can be obtained by heating baking soda, recrystallisation of sodium carbonate gives washing soda. It is also a basic salt

(a)Sodium carbonate (washing soda) is used in glass, soap and paper industries.
(b)It is used for removing the permanent hardness of water.

(iv) Bleaching powder or Calcium hypochlorite: It is a chemical compound with the formula CaOCl2. It is a yellowish powder with smell of chlorine. It is widely used for water treatment and as a bleaching agent (bleaching powder).

(a) Calcium hypochlorite is used for the disinfection of drinking water or swimming pool water.

Plaster of paris is a white powder and on mixing with water it changes to gypsum once again, giving a hard solid mass.

(a) It is used for making moulds or casts for toys, pottery, ceramics etc.
(b) It is used in surgical bandages for setting fractured bones.

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