CBSE Class 10th Science Conceptual Notes Chapter 2 – Acid Bases and Salts

Acid Bases and Salts

Acid

• Chemical Substance which turn blue litmus red & when dissolved in water , dissociate to give H+ ions
• Acids are sour in Taste
• Corrosive in nature

Base

• Chemical substance which turn red litmus blue & when dissolved in water , dissociate to give OH¯ ions .
• Bitter in Taste
• Feel soapy , soft to touch
• All bases are not soluble in water
• Bases which are soluble in water→Alkali

Salt

• Formed when an acid reacts with a base
• CaCl_{2 ,}KCl, Na₂SO₄ NaNO_{3 etc}

Indicators

Substance that are used to test whether a substance is acidic or basic or neutral in nature

Types of Indicators

1. Natural
2. Synthetic
3. Olfactory

Chemical Properties

Reaction of an acid with metals :-

Acid + Metal → salt + Hydrogen gas

1. Mg+2HCl →MgCl_{2 +}H_{2 (g)}
2. Zn + H₂SO₄ → ZnSO_{4 (aq)} + H_{2 (g)}
3. Cu(s) + HCl → No reaction

Reaction of a base with metals

1. 2NaOH + Zn → Na₂ZnO₂ + H₂

Reaction of an metal carbonates /Hydrogen carbonates with Acid

Metal Carbonates/Hydrogen carbonates + Acid → Salt +Co_{2 +} H₂O

1. Na₂Co₃ (s)+ 2HCl  (aq)→ 2NaCl (aq) + H₂O (l)+ CO_{2 (g)}
2. NaHCO₃ (s) + HCl (aq)→ NaCl (aq)+ H₂O  (l)+ CO_{2 (g)}

On passing the  CO₂ gas evolved through lime water

1. Ca(OH)_{2 (aq)}  + CO_{2 (g)}  → CaCO₃ ( s) + H₂O(l)
2. CaCO₃ (s) + H₂O (l)  + CO_2(g) → Ca(HCO₃)_{2 (aq) Soluble in water} 

Reactions of acids & bases / Neutralisation reaction

Base + Acid → Salt + water

Reaction of Metallic oxides with Acids

Metal Oxide + Acid → Salt + Water

Reaction of Non metallic oxide with Base

Non-Metal oxide + Base → Salt + water

Why do all acids and all bases have in common ?

• Acids or base solutions in water conducts electricity
• Acids or base dissociate into ions when added in water
• Separation of ions from the compound is not possible in the absence of water
• C₂H₅OH   C₆H₁₂O₆

1. HCl + H₂O → H₃O + Cl^–
2. NaOH + H₂O → Na⁺ + OH^–

Can we add water directly into an acid ?

• The process of dissolving an acid or a base in water is a highly exothermic one
• The acid must always be added slowly to water with constant stirring
• if water is added to a concentrated acid the heat generated may cause the mixture to splash out and cause burns

Dilution

• Mixing an acid or base with water results in decrease in the concentration of ions (H₃O+/OH^-)
• such a process is called dilution and the acid or the base is said to be diluted

Strength of an acid

• The more will be the H⁺ion dissociated the strength will be an acid
• Dissociation of H⁺ion in water depends on the strength of the bond between hydrogen and other component
• stronger will be the bond the lesser will be the dissociation and vice versa
• therefore HCl is more acidic than H₂SO₄

Strength of a base

• we already know that bases are the chemicals which dissociates OH^– into ion when dissolved in water
• Similarly the more will be the OH^–  ion dissociates the stronger will be the base
• The universal indicator shows different colours at different concentrations of hydrogen ions in a solution

Importance of pH in everyday life :-

Our body works within the pH range of 7.0 to 7.8

pH of the soil in our backyard

plants require a specific pH range for their healthy growth

• slightly basic →CaO/CaCO₃
• Slightly acidic → decayed organic matter

pH in Our digestive system

our stomach produces hydrochloric  acid which helps in the digestion of food without harming the stomach

1. Antacid-Milk of Magnesia
2. Baking Soda

pH change as the cause of tooth decay

Tooth decay starts when the pH of the mouth is lower than 5.5

Self defence by animals & plants through chemical welfare

Bee-Sting leaves an acid which causes pain & irritation. use of a mild base like baking soda on the stung area gives relief

pH of salts

• Salts of a strong acid & a strong base are neutral with pH value of 7. e.g-Nacl
• Salts of a strong acid & a weak base are acidic with pH value less than 7. e.g-CuCl₂
• salts of a strong base & weak acid are basic in nature with pH value more than 7. e,g- Na₂CO₃

Sodium Hydroxide

Method of Preparation- Chlor Alakli process

Uses

• For making soaps,  detergents
• for making papers , plasticides PVC , disinfectants

Bleaching water

• Produced by the action of chlorine or dry slaked lime  ⌈Ca(OH)_2⌉
• represented as CaOCl₂
• Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses

• for bleaching cotton & linen in the textile industries
• used as an oxidising agent in many chemical industries
• for disinfecting drinking water to make it free of germs

Baking Soda

• Chemical Name-sodium hydrogen carbonate ⌈NaHCO_3⌉
• Manufacture of baking soda -solvable process
• produced by using sodium chloride as one of the raw materials
• NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

Uses

• making of baking powder which is used in making cakes , fire extinguishes

washing Soda

• Re crystallisation of sodium carbonate gives washing soda . it is basic salt
• Na₂CO₃ + 10H₂O → Na₂CO_3.H₂O

Uses

• in paper industries , in soap industries as a cleaning agent for domestic purposes for removing permanent hardness of water

water of Crystallisation

• it means having a fixed number of modules present in one formula of a unit of salt
• caused by forming purified crystals from an aqueous solution
• e.g-hydrated copper sulphate CuSO₄ .5H₂O

Gypsum

• Chemical formula -CaSO_4.2H₂O
• it has two water molecules as water of crystallisation

Plaster of Paris

• Chemical formula -CaSO₄ 1/2H_2O
• On heating gypsum at 373K it loses water molecules &  becomes calcium sulphate hemihydrate

Heating of POP

• CaSO_4.2H₂O → CaSO_4.1/2H₂O + 3/2H₂O

Uses

• It is used as plaster for supporting fractured bones in the right position , to make sculptures & used as decorative in buildings a coating on wood and metal structures to avoid any fire accidents