CBSE Notes Class 10 Science Chapter 2 - Acids Bases and Salts

9.0Detailed CBSE Class 10 Science Chapter 2 – Key Notes

1. Acids
The term acid is derived from the Latin word “acidus”, meaning sour in taste. An acid is a substance that, when dissolved in water, ionizes to release hydrogen ions H⁺ (aq) in solution.
Example:
HCl (aq) → H⁺ (aq) + Cl⁻ (aq)

2. Bases
According to Arrhenius theory, substances that produce hydroxide ions OH⁻ in aqueous solutions are called bases. Bases that completely dissolve in water are known as alkalis. Bases are bitter in taste and feel soapy to touch.
Example:
NaOH (aq) → Na⁺ (aq) + OH⁻ (aq)

3. Indicators
It is a chemical which indicates the nature of solution by changing its colour/odour. Olfactory indicators: Indicators giving different odour in acidic and basic medium are called olfactory indicators. E.g : Onion, vanilla essence, and clove oil loses their smell in bases but does not change their smell in acids.

Types of Indicators

4. Chemical Properties of Acids and Bases

Reaction of Metals with Acids
Example: Zn (s) + dil. H₂SO₄ → ZnSO₄ (aq) + H₂ (g)

Reaction of Metals with Bases
Example: Zn (s) + 2NaOH (aq) → Na₂ZnO₂ (aq) + H₂ (g)

Reaction of Metal Carbonates with Acids
Example: Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)

Ca(OH)₂ + CO₂(g) → CaCO₃(s)+ H₂O (l)

On passing excess CO₂ , the solution becomes clear due to conversion of insoluble CaCO₃ into soluble calcium hydrogen carbonate. CaCO₃(s) +  H₂O(l) + CO₂(g)→Ca(HCO₃)₂ (aq.)

Reaction of Metal Oxides with Acids

Whenever a solution of acid is treated with a solution of base, salt and water are formed as the products. This reaction is known as neutralisation reaction.

Acid + Base→ Salt + Water

Example :

HNO₃(aq.) + KOH(aq.) → KNO₃(aq.) + H₂O(l)

Reaction of Metal Oxides with Acids

A metal oxide reacts with dilute acids to form salt and water.

Metal oxide + Acid→Salt + Water
CuO + 2HCl → CuCl₂ + H₂O

Reaction of Non-Metal Oxides with Bases

Base react with non-metallic oxide to form their respective

salts and water.

Base + Non-metallic oxide → Salt + Water
Example : 2NaOH (aq) + CO₂ (g) → Na₂CO₃ (aq) + H₂O (l)

5. Comparison Between Acids and Bases

Acids

• Acids are sour in taste.
• Properties of acids are due to the presence of (H⁺ ) ion in water solution.
• Acid turns blue litmus to red.
• Acid reacts with active metals like Na, K, Ca and Zn to give hydrogen gas

Bases

• Bases are bitter in taste.
• Properties of bases are due to the presence of OH⁻ water solution.
• Base turns red litmus to blue.
• Base does not react with metals except with Zn, Al and Sn.

Q. What happens when water is mixed with an acid or a base?

Ans.
During dilution, concentration of ions (H₃O⁺ / OH⁻) per unit volume decreases. This process is generally exothermic in nature, which produces heat. During dilution small quantity of acid or base should be added in water with stirring.Heat evolved in this case will be quite slow.
As electric current is carried through the solution by ions, this shows that acids dissociate in the solution to produce H⁺(aq) ions but substances like glucose and alcohol do not dissociate to give H⁺ ions and hence are not acidic.Thus, H⁺ ions are responsible for the acidic properties.

pH Scale

S.P.L. Sorenson, a Danish chemist in 1909 introduced the concept of measuring the concentration of hydrogen ions [H⁺(aq.)] in a particular solution. The ‘p’ in pH stands for ‘potenz’ in German, meaning power.

On the pH scale we can measure pH from “0” (very acidic) to 14 (very alkaline).

• Solutions with pH = 0–3 are strongly acidic, with pH = 3–5 are moderately acidic while with pH = 5–7 are weakly acidic.

• Solutions with pH = 7–9 are weakly basic, with pH = 9–12 are moderately basic while with pH = 12–14 are strongly basic.

If pH = 7, then the solution is neutral.

Importance of pH in everyday life

● Soil Treatment :
Acidic soil can be neutralized by farmers spray it with calcium oxide (quick lime) or calcium hydroxide (slaked lime) or basic soil neutralized by adding, a large amount of organic manure is added.

● Indigestion :
Acidity in stomach can be neutralised by taking an edible base like baking soda.

● Factory Wastes :
The wastes of many factories contain acids. These wastes are treated with lime which neutralises acids.

● Stings of Ants and Bees :
To reduce the effect of formic acid (sting of ants and bees) is rubbed with a soap or baking soda solution or calamine which contains zinc carbonate.

● Preserving Milk for a Short Period :
To prevent the ill effect of lactic acid the milk vendor adds a small amount of baking soda to the milk.

● Prevention of Tooth Decay :
To prevent tooth decay, it is advised to clean the mouth and use toothpastes which are generally basic, for cleaning the teeth.

Salts

Salts are the ionic compounds consisting of two parts, one part carrying a positive charge called positive ion or cation and the other part carrying a negative charge called a negative ion or anion.

pH of salts

The nature of acid and base produced when salt reacts with water, the salts can be classified into the following four types.

• NaCl + H₂O → HCl + NaOH (Acids and base are strong. Salt is strong, pH = 7)
• NH₄Cl + H₂O → HCl + NH₄OH (Acid is strong. Salt is acidic. pH < 7)
• Na₂CO₃ + H₂O → H₂CO₃ + NaOH (Base is strong. Salt is basic. pH > 7)
• CH₃COONH₄ + H₂O → CH₃COOH + NH₄OH (Acid and base are weak. Salt is neutral. pH ≈ 7)

Chemicals from common salt

(I) Sodium hydroxide

(II) Bleaching powder

Chemically, bleaching powder is generally represented by the formula, CaOCl₂ (called, calcium oxychloride).

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses of bleaching powder

(a) For bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry.
(b) As an oxidising agent in many chemical industries.
(c) For disinfecting drinking water to make it free of germs.

Note : Bleaching powder is also known as chloride of lime.

(III) Baking soda (NaHCO₃)

Baking soda is prepared by Solvay process or ammonia-soda process.

NaCl(aq.) + H₂O(ℓ) + CO₂(g) + NH₃(g) → NH₄Cl(aq.) + NaHCO₃(s)

Uses of sodium hydrogen carbonate (NaHCO₃)

(a) For making baking powder which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid like tartaric acid.

(b) As an ingredient in antacids.

(c) It is used in soda-acid fire extinguisher.

(IV) Washing Soda

The chemical formula of washing soda is Na₂CO₃·10H₂O (sodium carbonate decahydrate).

Washing soda is manufactured by Solvay process. The reactions involved are.

Washing soda (Na₂CO₃·10H₂O) is obtained by recrystallisation from a saturated solution of soda ash (Na₂CO₃).

Uses of washing soda

(a) For washing clothes (laundry purposes).

(b) Washing soda is used for softening hard water.

(c) Sodium carbonate is used for the manufacture of many important compounds, such as borax (Na₂B₄O₇), hypo (Na₂S₂O₃·5H₂O), etc.

(d) Sodium carbonate is also used in paper, glass, soap and paint industries.

Crystallization of water

It is the fixed number of water molecules present in one formula unit of a crystalline salt.

Example : Blue vitriol CuSO₄·5H₂O, Green vitriol FeSO₄·7H₂O, Gypsum CaSO₄·2H₂O

Plaster of Paris (POP)

Plaster of Paris is hemihydrate (hemi means half and hydrate means water) of calcium sulphate. Its molecular formula is CaSO₄·1/2H₂O or (CaSO₄)₂·H₂O.

Plaster of Paris is obtained by heating gypsum (CaSO₄·2H₂O) in a kiln at 373K.

2[CaSO₄·2H₂O] (s) ⟶ (CaSO₄)₂·H₂O(s) + 3H₂O(g)
$2[{\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}](\text{s})\underset{\text{100°C}}{\overset{\text{Heat}}{\to }}({\text{CaSO}}_4{)}_2\cdot {\text{H}}_2\text{O}(\text{s})+3{\text{H}}_2\text{O}(\text{g})$
${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}(\text{s})\stackrel{\text{373K}}{\to }{\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}(\text{s})+\frac{3}{2}{\text{H}}_2\text{O}(l)$

10.0Benefits of CBSE Notes for Class 10 Science Chapter 2: Acids Bases and Salts

• Understanding Natural Phenomena: You can understand phenomena like acid rain and the chemical basis of stings from insects and plants.
• Preparation for Higher Studies: A strong grasp of this chapter is crucial for further studies in chemistry and related science fields.
• Development of Scientific Temper: It encourages a logical and analytical approach to understanding the properties and reactions of chemical substances.
• Awareness of Industrial Processes: You gain insight into the use of acids, bases, and salts in various industries like manufacturing soaps, detergents, and fertilisers.
• Improved Performance in Exams: Thorough understanding of this chapter and its concepts will significantly contribute to scoring well in your Class 10 exams.